Tuesday, November 18, 2008
Sunday, November 9, 2008
TOP FIVE COMPUTER VIRUSES
Top 5 Computer Viruses
They can wipe out your hard drive. They can crash servers and send shock waves around the cyber world. And they can embarrass you. Here are the biggest computer viruses in cyber history, so far:
1. Brain (1986)
This is the one that started it all. Created by two Pakistani brothers, the Brain virus is widely considered to be the first PC computer virus ever. It wasn't particularly virulent. It was a boot sector infector, and was accompanied by the creators' names and phone numbers. The brothers swore the intention was not malicious, but the genie was out of the bottle; the Brain virus became the model for numerous viruses and other malware for years to come.
2. Melissa (1999)
The Melissa virus was one of the first viruses to be spread via e-mail, crashing corporate networks by taking advantage of the increasingly popular Microsoft Outlook mail client. The virus was spread by an infected attachment to the email, and automatically sent itself to the first 50 names in the user's address book, making it the first virus to move from one computer to another on its own. The amount of email passing through servers quickly increased exponentially, forcing shutdowns and business stoppages. The Melissa virus was also noteworthy because its creator was caught and sent to prison.
3. ILOVEYOU (2000)
This is the virus that made us all afraid to open attachments. ILOVEYOU was one of the first to trick users into opening the file that actually activated the virus. The ILOVEYOU worm masqueraded as a love letter sent via e-mail, but instead was a computer script that sent copies of itself to users' Microsoft Outlook address book entries, overwrote and deleted computer files, modified Internet Explorer pages, and even interfered with Registry keys. It caused billions of dollars worth of damages, and is still considered one of the worst worms ever.
4. Klez (2001)
Yet another e-mail virus, Klez makes the list because it pioneered "spoofing," the trick of making it seem as if an email comes from someone other than the actual sender. Thanks to Klez, you can't trust that an email that appears to be from your mom is actually from your mom; a spammer may have "spoofed" the From field. Klez spreads itself using open networks and e-mail. Unlike other viruses, it doesn’t require Microsoft Outlook to spread itself – and while it isn't a particularly malicious virus, it can corrupt files and interfere with some software. Klez is also notoriously hard to exterminate, and versions of it continue to turn up fairly regularly.
5. Benjamin (2002)
Benjamin is significant in virus history because it’s the first that spread itself via a file-sharing program, in this case, Kazaa. It tricked users into thinking they were downloading media files. Instead, they downloaded the Benjamin virus. Benjamin created its own folder on the user's computer, filled the folder with replicated versions of itself, and then made itself available for sharing to other Kazaa users. Benjamin's primary effects were to overload a user's hard drive and eventually to slow down networks, but aside from that it wasn’t particularly malevolent.
BONUS: THE ANNA KOURNIKOVA VIRUS (2001)
The Anna Kournikova virus didn't really cause any major harm or data loss, but it did cause embarrassment. It was spread by users clicking on an attachment that promised to be a hot picture of the tennis star. Naturally, opening the attachment unleashed the virus, which sent itself to all of the contacts in the user's Microsoft Outlook address book. Server overload caused business stoppages, and numerous users were forced to admit that they found the promise of an Anna Kournikova image impossible to resist.
To learn more about malware, viruses, spyware and Trojan horses, watch VideoJug's series of informative videos with computer security expert "Hacker X."
They can wipe out your hard drive. They can crash servers and send shock waves around the cyber world. And they can embarrass you. Here are the biggest computer viruses in cyber history, so far:
1. Brain (1986)
This is the one that started it all. Created by two Pakistani brothers, the Brain virus is widely considered to be the first PC computer virus ever. It wasn't particularly virulent. It was a boot sector infector, and was accompanied by the creators' names and phone numbers. The brothers swore the intention was not malicious, but the genie was out of the bottle; the Brain virus became the model for numerous viruses and other malware for years to come.
2. Melissa (1999)
The Melissa virus was one of the first viruses to be spread via e-mail, crashing corporate networks by taking advantage of the increasingly popular Microsoft Outlook mail client. The virus was spread by an infected attachment to the email, and automatically sent itself to the first 50 names in the user's address book, making it the first virus to move from one computer to another on its own. The amount of email passing through servers quickly increased exponentially, forcing shutdowns and business stoppages. The Melissa virus was also noteworthy because its creator was caught and sent to prison.
3. ILOVEYOU (2000)
This is the virus that made us all afraid to open attachments. ILOVEYOU was one of the first to trick users into opening the file that actually activated the virus. The ILOVEYOU worm masqueraded as a love letter sent via e-mail, but instead was a computer script that sent copies of itself to users' Microsoft Outlook address book entries, overwrote and deleted computer files, modified Internet Explorer pages, and even interfered with Registry keys. It caused billions of dollars worth of damages, and is still considered one of the worst worms ever.
4. Klez (2001)
Yet another e-mail virus, Klez makes the list because it pioneered "spoofing," the trick of making it seem as if an email comes from someone other than the actual sender. Thanks to Klez, you can't trust that an email that appears to be from your mom is actually from your mom; a spammer may have "spoofed" the From field. Klez spreads itself using open networks and e-mail. Unlike other viruses, it doesn’t require Microsoft Outlook to spread itself – and while it isn't a particularly malicious virus, it can corrupt files and interfere with some software. Klez is also notoriously hard to exterminate, and versions of it continue to turn up fairly regularly.
5. Benjamin (2002)
Benjamin is significant in virus history because it’s the first that spread itself via a file-sharing program, in this case, Kazaa. It tricked users into thinking they were downloading media files. Instead, they downloaded the Benjamin virus. Benjamin created its own folder on the user's computer, filled the folder with replicated versions of itself, and then made itself available for sharing to other Kazaa users. Benjamin's primary effects were to overload a user's hard drive and eventually to slow down networks, but aside from that it wasn’t particularly malevolent.
BONUS: THE ANNA KOURNIKOVA VIRUS (2001)
The Anna Kournikova virus didn't really cause any major harm or data loss, but it did cause embarrassment. It was spread by users clicking on an attachment that promised to be a hot picture of the tennis star. Naturally, opening the attachment unleashed the virus, which sent itself to all of the contacts in the user's Microsoft Outlook address book. Server overload caused business stoppages, and numerous users were forced to admit that they found the promise of an Anna Kournikova image impossible to resist.
To learn more about malware, viruses, spyware and Trojan horses, watch VideoJug's series of informative videos with computer security expert "Hacker X."
Infection strategies
Infection strategies
This section does not cite any references or sources.
Please help improve this section by adding citations to reliable sources. Unverifiable material may be challenged and removed. (January 2008)
In order to replicate itself, a virus must be permitted to execute code and write to memory. For this reason, many viruses attach themselves to executable files that may be part of legitimate programs. If a user tries to start an infected program, the virus' code may be executed first. Viruses can be divided into two types, on the basis of their behavior when they are executed. Nonresident viruses immediately search for other hosts that can be infected, infect these targets, and finally transfer control to the application program they infected. Resident viruses do not search for hosts when they are started. Instead, a resident virus loads itself into memory on execution and transfers control to the host program. The virus stays active in the background and infects new hosts when those files are accessed by other programs or the operating system itself.
Nonresident viruses
Nonresident viruses can be thought of as consisting of a finder module and a replication module. The finder module is responsible for finding new files to infect. For each new executable file the finder module encounters, it calls the replication module to infect that file.
Resident viruses
Resident viruses contain a replication module that is similar to the one that is employed by nonresident viruses. However, this module is not called by a finder module. Instead, the virus loads the replication module into memory when it is executed and ensures that this module is executed each time the operating system is called to perform a certain operation. For example, the replication module can be called each time the operating system executes a file. In this case, the virus infects every suitable program that is executed on the computer.
Resident viruses are sometimes subdivided into a category of fast infectors and a category of slow infectors. Fast infectors are designed to infect as many files as possible. For instance, a fast infector can infect every potential host file that is accessed. This poses a special problem to anti-virus software, since a virus scanner will access every potential host file on a computer when it performs a system-wide scan. If the virus scanner fails to notice that such a virus is present in memory, the virus can "piggy-back" on the virus scanner and in this way infect all files that are scanned. Fast infectors rely on their fast infection rate to spread. The disadvantage of this method is that infecting many files may make detection more likely, because the virus may slow down a computer or perform many suspicious actions that can be noticed by anti-virus software. Slow infectors, on the other hand, are designed to infect hosts infrequently. For instance, some slow infectors only infect files when they are copied. Slow infectors are designed to avoid detection by limiting their actions: they are less likely to slow down a computer noticeably, and will at most infrequently trigger anti-virus software that detects suspicious behavior by programs. The slow infector approach does not seem very successful, however.
Vectors and hosts
Viruses have targeted various types of transmission media or hosts. This list is not exhaustive:
* Binary executable files (such as COM files and EXE files in MS-DOS, Portable Executable files in Microsoft Windows, and ELF files in Linux)
* Volume Boot Records of floppy disks and hard disk partitions
* The master boot record (MBR) of a hard disk
* General-purpose script files (such as batch files in MS-DOS and Microsoft Windows, VBScript files, and shell script files on Unix-like platforms).
* Application-specific script files (such as Telix-scripts)
* Documents that can contain macros (such as Microsoft Word documents, Microsoft Excel spreadsheets, AmiPro documents, and Microsoft Access database files)
* Cross-site scripting vulnerabilities in web applications
* Arbitrary computer files. An exploitable buffer overflow, format string, race condition or other exploitable bug in a program which reads the file could be used to trigger the execution of code hidden within it. Most bugs of this type can be made more difficult to exploit in computer architectures with protection features such as an execute disable bit and/or address space layout randomization.
PDFs, like HTML, may link to malicious code.[citation needed]
In operating systems that use file extensions to determine program associations (such as Microsoft Windows), the extensions may be hidden from the user by default. This makes it possible to create a file that is of a different type than it appears to the user. For example, a executable may be created named "picture.png.exe", in which the user sees only "picture.png" and therefore assumes that this file is an image and most likely is safe.
Methods to avoid detection
In order to avoid detection by users, some viruses employ different kinds of deception. Some old viruses, especially on the MS-DOS platform, make sure that the "last modified" date of a host file stays the same when the file is infected by the virus. This approach does not fool anti-virus software, however, especially those which maintain and date Cyclic redundancy checks on file changes.
Some viruses can infect files without increasing their sizes or damaging the files. They accomplish this by overwriting unused areas of executable files. These are called cavity viruses. For example the CIH virus, or Chernobyl Virus, infects Portable Executable files. Because those files had many empty gaps, the virus, which was 1 KB in length, did not add to the size of the file.
Some viruses try to avoid detection by killing the tasks associated with antivirus software before it can detect them.
As computers and operating systems grow larger and more complex, old hiding techniques need to be updated or replaced. Defending a computer against viruses may demand that a file system migrate towards detailed and explicit permission for every kind of file access.
Avoiding bait files and other undesirable hosts
A virus needs to infect hosts in order to spread further. In some cases, it might be a bad idea to infect a host program. For example, many anti-virus programs perform an integrity check of their own code. Infecting such programs will therefore increase the likelihood that the virus is detected. For this reason, some viruses are programmed not to infect programs that are known to be part of anti-virus software. Another type of host that viruses sometimes avoid is bait files. Bait files (or goat files) are files that are specially created by anti-virus software, or by anti-virus professionals themselves, to be infected by a virus. These files can be created for various reasons, all of which are related to the detection of the virus:
* Anti-virus professionals can use bait files to take a sample of a virus (i.e. a copy of a program file that is infected by the virus). It is more practical to store and exchange a small, infected bait file, than to exchange a large application program that has been infected by the virus.
* Anti-virus professionals can use bait files to study the behavior of a virus and evaluate detection methods. This is especially useful when the virus is polymorphic. In this case, the virus can be made to infect a large number of bait files. The infected files can be used to test whether a virus scanner detects all versions of the virus.
* Some anti-virus software employs bait files that are accessed regularly. When these files are modified, the anti-virus software warns the user that a virus is probably active on the system.
Since bait files are used to detect the virus, or to make detection possible, a virus can benefit from not infecting them. Viruses typically do this by avoiding suspicious programs, such as small program files or programs that contain certain patterns of 'garbage instructions'.
A related strategy to make baiting difficult is sparse infection. Sometimes, sparse infectors do not infect a host file that would be a suitable candidate for infection in other circumstances. For example, a virus can decide on a random basis whether to infect a file or not, or a virus can only infect host files on particular days of the week.
Stealth
Some viruses try to trick anti-virus software by intercepting its requests to the operating system. A virus can hide itself by intercepting the anti-virus software’s request to read the file and passing the request to the virus, instead of the OS. The virus can then return an uninfected version of the file to the anti-virus software, so that it seems that the file is "clean". Modern anti-virus software employs various techniques to counter stealth mechanisms of viruses. The only completely reliable method to avoid stealth is to boot from a medium that is known to be clean.
Self-modification
Most modern antivirus programs try to find virus-patterns inside ordinary programs by scanning them for so-called virus signatures. A signature is a characteristic byte-pattern that is part of a certain virus or family of viruses. If a virus scanner finds such a pattern in a file, it notifies the user that the file is infected. The user can then delete, or (in some cases) "clean" or "heal" the infected file. Some viruses employ techniques that make detection by means of signatures difficult but probably not impossible. These viruses modify their code on each infection. That is, each infected file contains a different variant of the virus.
Encryption with a variable key
A more advanced method is the use of simple encryption to encipher the virus. In this case, the virus consists of a small decrypting module and an encrypted copy of the virus code. If the virus is encrypted with a different key for each infected file, the only part of the virus that remains constant is the decrypting module, which would (for example) be appended to the end. In this case, a virus scanner cannot directly detect the virus using signatures, but it can still detect the decrypting module, which still makes indirect detection of the virus possible. Since these would be symmetric keys, stored on the infected host, it is in fact entirely possible to decrypt the final virus, but that probably isn't required, since self-modifying code is such a rarity that it may be reason for virus scanners to at least flag the file as suspicious.
An old, but compact, encryption involves XORing each byte in a virus with a constant, so that the exclusive-or operation had only to be repeated for decryption. It is suspicious code that modifies itself, so the code to do the encryption/decryption may be part of the signature in many virus definitions.
Polymorphic code
Polymorphic code was the first technique that posed a serious threat to virus scanners. Just like regular encrypted viruses, a polymorphic virus infects files with an encrypted copy of itself, which is decoded by a decryption module. In the case of polymorphic viruses however, this decryption module is also modified on each infection. A well-written polymorphic virus therefore has no parts which remain identical between infections, making it very difficult to detect directly using signatures. Anti-virus software can detect it by decrypting the viruses using an emulator, or by statistical pattern analysis of the encrypted virus body. To enable polymorphic code, the virus has to have a polymorphic engine (also called mutating engine or mutation engine) somewhere in its encrypted body. See Polymorphic code for technical detail on how such engines operate.
Some viruses employ polymorphic code in a way that constrains the mutation rate of the virus significantly. For example, a virus can be programmed to mutate only slightly over time, or it can be programmed to refrain from mutating when it infects a file on a computer that already contains copies of the virus. The advantage of using such slow polymorphic code is that it makes it more difficult for anti-virus professionals to obtain representative samples of the virus, because bait files that are infected in one run will typically contain identical or similar samples of the virus. This will make it more likely that the detection by the virus scanner will be unreliable, and that some instances of the virus may be able to avoid detection.
Metamorphic code
To avoid being detected by emulation, some viruses rewrite themselves completely each time they are to infect new executables. Viruses that use this technique are said to be metamorphic. To enable metamorphism, a metamorphic engine is needed. A metamorphic virus is usually very large and complex. For example, W32/Simile consisted of over 14000 lines of Assembly language code, 90% of which is part of the metamorphic engine.[7]
Vulnerability and countermeasures
The vulnerability of operating systems to viruses
Just as genetic diversity in a population decreases the chance of a single disease wiping out a population, the diversity of software systems on a network similarly limits the destructive potential of viruses.
This became a particular concern in the 1990s, when Microsoft gained market dominance in desktop operating systems and office suites. The users of Microsoft software (especially networking software such as Microsoft Outlook and Internet Explorer) are especially vulnerable to the spread of viruses. Microsoft software is targeted by virus writers due to their desktop dominance, and is often criticized for including many errors and holes for virus writers to exploit. Integrated and non-integrated Microsoft applications (such as Microsoft Office) and applications with scripting languages with access to the file system (for example Visual Basic Script (VBS), and applications with networking features) are also particularly vulnerable.
Although Windows is by far the most popular operating system for virus writers, some viruses also exist on other platforms. Any operating system that allows third-party programs to run can theoretically run viruses. Some operating systems are less secure than others. Unix-based OS's (and NTFS-aware applications on Windows NT based platforms) only allow their users to run executables within their protected space in their own directories.
An Internet based research revealed that there were cases when people willingly pressed a particular button to download a virus. A security firm F-Secure ran a half year advertising campaign on Google AdWords which said "Is your PC virus-free? Get it infected here!". The result was 409 clicks.[8]
As of 2006, there are relatively few security exploits[9] targeting Mac OS X (with a Unix-based file system and kernel). The number of viruses for the older Apple operating systems, known as Mac OS Classic, varies greatly from source to source, with Apple stating that there are only four known viruses, and independent sources stating there are as many as 63 viruses. It is safe to say that Macs are less likely to be targeted because of low market share and thus a Mac-specific virus could only infect a small proportion of computers (making the effort less desirable). Virus vulnerability between Macs and Windows is a chief selling point, one that Apple uses in their Get a Mac advertising.[10]
Windows and Unix have similar scripting abilities, but while Unix natively blocks normal users from having access to make changes to the operating system environment, older copies of Windows such as Windows 95 and 98 do not. In 1997, when a virus for Linux was released – known as "Bliss" – leading antivirus vendors issued warnings that Unix-like systems could fall prey to viruses just like Windows.[11] The Bliss virus may be considered characteristic of viruses – as opposed to worms – on Unix systems. Bliss requires that the user run it explicitly (so it is a trojan), and it can only infect programs that the user has the access to modify. Unlike Windows users, most Unix users do not log in as an administrator user except to install or configure software; as a result, even if a user ran the virus, it could not harm their operating system. The Bliss virus never became widespread, and remains chiefly a research curiosity. Its creator later posted the source code to Usenet, allowing researchers to see how it worked.[12]
The role of software development
Because software is often designed with security features to prevent unauthorized use of system resources, many viruses must exploit software bugs in a system or application to spread. Software development strategies that produce large numbers of bugs will generally also produce potential exploits.
Anti-virus software and other preventive measures
Many users install anti-virus software that can detect and eliminate known viruses after the computer downloads or runs the executable. There are two common methods that an anti-virus software application uses to detect viruses. The first, and by far the most common method of virus detection is using a list of virus signature definitions. This works by examining the content of the computer's memory (its RAM, and boot sectors) and the files stored on fixed or removable drives (hard drives, floppy drives), and comparing those files against a database of known virus "signatures". The disadvantage of this detection method is that users are only protected from viruses that pre-date their last virus definition update. The second method is to use a heuristic algorithm to find viruses based on common behaviors. This method has the ability to detect viruses that anti-virus security firms have yet to create a signature for.
Some anti-virus programs are able to scan opened files in addition to sent and received e-mails 'on the fly' in a similar manner. This practice is known as "on-access scanning." Anti-virus software does not change the underlying capability of host software to transmit viruses. Users must update their software regularly to patch security holes. Anti-virus software also needs to be regularly updated in order to prevent the latest threats.
One may also minimise the damage done by viruses by making regular backups of data (and the Operating Systems) on different media, that are either kept unconnected to the system (most of the time), read-only or not accessible for other reasons, such as using different file systems. This way, if data is lost through a virus, one can start again using the backup (which should preferably be recent). A notable exception to this rule is the Gammima virus, which propagates via infected removable media (specifically flash drives) [13] [14]. If a backup session on optical media like CD and DVD is closed, it becomes read-only and can no longer be affected by a virus (so long as a virus or infected file was not copied onto the CD/DVD). Likewise, an Operating System on a bootable can be used to start the computer if the installed Operating Systems become unusable. Another method is to use different Operating Systems on different file systems. A virus is not likely to affect both. Data backups can also be put on different file systems. For example, Linux requires specific software to write to NTFS partitions, so if one does not install such software and uses a separate installation of MS Windows to make the backups on an NTFS partition, the backup should remain safe from any Linux viruses. Likewise, MS Windows can not read file systems like ext3, so if one normally uses MS Windows, the backups can be made on an ext3 partition using a Linux installation.
Recovery methods
Once a computer has been compromised by a virus, it is usually unsafe to continue using the same computer without completely reinstalling the operating system. However, there are a number of recovery options that exist after a computer has a virus. These actions depend on severity of the type of virus.
Virus removal
One possibility on Windows Me, Windows XP and Windows Vista is a tool known as System Restore, which restores the registry and critical system files to a previous checkpoint. Often a virus will cause a system to hang, and a subsequent hard reboot will render a system restore point from the same day corrupt. Restore points from previous days should work provided the virus is not designed to corrupt the restore files or also exists in previous restore points [15]. Some viruses, however, disable system restore and other important tools such as Task Manager and Command Prompt. An example of a virus that does this is CiaDoor.
Administrators have the option to disable such tools from limited users for various reasons. The virus modifies the registry to do the same, except, when the Administrator is controlling the computer, it blocks all users from accessing the tools. When an infected tool activates it gives the message "Task Manager has been disabled by your administrator.", even if the user trying to open the program is the administrator.
Users running a Microsoft operating system can go to Microsoft's website to run a free scan, if they have their 20-digit registration number.
Operating system reinstallation
Reinstalling the operating system is another approach to virus removal. It involves simply reformatting the OS partition and installing the OS from its original media, or imaging the partition with a clean backup image (taken with Ghost or Acronis for example).
This method has the benefits of being simple to do, can be faster than running multiple anti-virus scans, and is guaranteed to remove any malware. Downsides include having to reinstall all other software as well as the operating system. User data can be backed up by booting off of a Live CD or putting the hard drive into another computer and booting from the other computer's operating system (though care must be taken not to transfer the virus to the new computer).
This section does not cite any references or sources.
Please help improve this section by adding citations to reliable sources. Unverifiable material may be challenged and removed. (January 2008)
In order to replicate itself, a virus must be permitted to execute code and write to memory. For this reason, many viruses attach themselves to executable files that may be part of legitimate programs. If a user tries to start an infected program, the virus' code may be executed first. Viruses can be divided into two types, on the basis of their behavior when they are executed. Nonresident viruses immediately search for other hosts that can be infected, infect these targets, and finally transfer control to the application program they infected. Resident viruses do not search for hosts when they are started. Instead, a resident virus loads itself into memory on execution and transfers control to the host program. The virus stays active in the background and infects new hosts when those files are accessed by other programs or the operating system itself.
Nonresident viruses
Nonresident viruses can be thought of as consisting of a finder module and a replication module. The finder module is responsible for finding new files to infect. For each new executable file the finder module encounters, it calls the replication module to infect that file.
Resident viruses
Resident viruses contain a replication module that is similar to the one that is employed by nonresident viruses. However, this module is not called by a finder module. Instead, the virus loads the replication module into memory when it is executed and ensures that this module is executed each time the operating system is called to perform a certain operation. For example, the replication module can be called each time the operating system executes a file. In this case, the virus infects every suitable program that is executed on the computer.
Resident viruses are sometimes subdivided into a category of fast infectors and a category of slow infectors. Fast infectors are designed to infect as many files as possible. For instance, a fast infector can infect every potential host file that is accessed. This poses a special problem to anti-virus software, since a virus scanner will access every potential host file on a computer when it performs a system-wide scan. If the virus scanner fails to notice that such a virus is present in memory, the virus can "piggy-back" on the virus scanner and in this way infect all files that are scanned. Fast infectors rely on their fast infection rate to spread. The disadvantage of this method is that infecting many files may make detection more likely, because the virus may slow down a computer or perform many suspicious actions that can be noticed by anti-virus software. Slow infectors, on the other hand, are designed to infect hosts infrequently. For instance, some slow infectors only infect files when they are copied. Slow infectors are designed to avoid detection by limiting their actions: they are less likely to slow down a computer noticeably, and will at most infrequently trigger anti-virus software that detects suspicious behavior by programs. The slow infector approach does not seem very successful, however.
Vectors and hosts
Viruses have targeted various types of transmission media or hosts. This list is not exhaustive:
* Binary executable files (such as COM files and EXE files in MS-DOS, Portable Executable files in Microsoft Windows, and ELF files in Linux)
* Volume Boot Records of floppy disks and hard disk partitions
* The master boot record (MBR) of a hard disk
* General-purpose script files (such as batch files in MS-DOS and Microsoft Windows, VBScript files, and shell script files on Unix-like platforms).
* Application-specific script files (such as Telix-scripts)
* Documents that can contain macros (such as Microsoft Word documents, Microsoft Excel spreadsheets, AmiPro documents, and Microsoft Access database files)
* Cross-site scripting vulnerabilities in web applications
* Arbitrary computer files. An exploitable buffer overflow, format string, race condition or other exploitable bug in a program which reads the file could be used to trigger the execution of code hidden within it. Most bugs of this type can be made more difficult to exploit in computer architectures with protection features such as an execute disable bit and/or address space layout randomization.
PDFs, like HTML, may link to malicious code.[citation needed]
In operating systems that use file extensions to determine program associations (such as Microsoft Windows), the extensions may be hidden from the user by default. This makes it possible to create a file that is of a different type than it appears to the user. For example, a executable may be created named "picture.png.exe", in which the user sees only "picture.png" and therefore assumes that this file is an image and most likely is safe.
Methods to avoid detection
In order to avoid detection by users, some viruses employ different kinds of deception. Some old viruses, especially on the MS-DOS platform, make sure that the "last modified" date of a host file stays the same when the file is infected by the virus. This approach does not fool anti-virus software, however, especially those which maintain and date Cyclic redundancy checks on file changes.
Some viruses can infect files without increasing their sizes or damaging the files. They accomplish this by overwriting unused areas of executable files. These are called cavity viruses. For example the CIH virus, or Chernobyl Virus, infects Portable Executable files. Because those files had many empty gaps, the virus, which was 1 KB in length, did not add to the size of the file.
Some viruses try to avoid detection by killing the tasks associated with antivirus software before it can detect them.
As computers and operating systems grow larger and more complex, old hiding techniques need to be updated or replaced. Defending a computer against viruses may demand that a file system migrate towards detailed and explicit permission for every kind of file access.
Avoiding bait files and other undesirable hosts
A virus needs to infect hosts in order to spread further. In some cases, it might be a bad idea to infect a host program. For example, many anti-virus programs perform an integrity check of their own code. Infecting such programs will therefore increase the likelihood that the virus is detected. For this reason, some viruses are programmed not to infect programs that are known to be part of anti-virus software. Another type of host that viruses sometimes avoid is bait files. Bait files (or goat files) are files that are specially created by anti-virus software, or by anti-virus professionals themselves, to be infected by a virus. These files can be created for various reasons, all of which are related to the detection of the virus:
* Anti-virus professionals can use bait files to take a sample of a virus (i.e. a copy of a program file that is infected by the virus). It is more practical to store and exchange a small, infected bait file, than to exchange a large application program that has been infected by the virus.
* Anti-virus professionals can use bait files to study the behavior of a virus and evaluate detection methods. This is especially useful when the virus is polymorphic. In this case, the virus can be made to infect a large number of bait files. The infected files can be used to test whether a virus scanner detects all versions of the virus.
* Some anti-virus software employs bait files that are accessed regularly. When these files are modified, the anti-virus software warns the user that a virus is probably active on the system.
Since bait files are used to detect the virus, or to make detection possible, a virus can benefit from not infecting them. Viruses typically do this by avoiding suspicious programs, such as small program files or programs that contain certain patterns of 'garbage instructions'.
A related strategy to make baiting difficult is sparse infection. Sometimes, sparse infectors do not infect a host file that would be a suitable candidate for infection in other circumstances. For example, a virus can decide on a random basis whether to infect a file or not, or a virus can only infect host files on particular days of the week.
Stealth
Some viruses try to trick anti-virus software by intercepting its requests to the operating system. A virus can hide itself by intercepting the anti-virus software’s request to read the file and passing the request to the virus, instead of the OS. The virus can then return an uninfected version of the file to the anti-virus software, so that it seems that the file is "clean". Modern anti-virus software employs various techniques to counter stealth mechanisms of viruses. The only completely reliable method to avoid stealth is to boot from a medium that is known to be clean.
Self-modification
Most modern antivirus programs try to find virus-patterns inside ordinary programs by scanning them for so-called virus signatures. A signature is a characteristic byte-pattern that is part of a certain virus or family of viruses. If a virus scanner finds such a pattern in a file, it notifies the user that the file is infected. The user can then delete, or (in some cases) "clean" or "heal" the infected file. Some viruses employ techniques that make detection by means of signatures difficult but probably not impossible. These viruses modify their code on each infection. That is, each infected file contains a different variant of the virus.
Encryption with a variable key
A more advanced method is the use of simple encryption to encipher the virus. In this case, the virus consists of a small decrypting module and an encrypted copy of the virus code. If the virus is encrypted with a different key for each infected file, the only part of the virus that remains constant is the decrypting module, which would (for example) be appended to the end. In this case, a virus scanner cannot directly detect the virus using signatures, but it can still detect the decrypting module, which still makes indirect detection of the virus possible. Since these would be symmetric keys, stored on the infected host, it is in fact entirely possible to decrypt the final virus, but that probably isn't required, since self-modifying code is such a rarity that it may be reason for virus scanners to at least flag the file as suspicious.
An old, but compact, encryption involves XORing each byte in a virus with a constant, so that the exclusive-or operation had only to be repeated for decryption. It is suspicious code that modifies itself, so the code to do the encryption/decryption may be part of the signature in many virus definitions.
Polymorphic code
Polymorphic code was the first technique that posed a serious threat to virus scanners. Just like regular encrypted viruses, a polymorphic virus infects files with an encrypted copy of itself, which is decoded by a decryption module. In the case of polymorphic viruses however, this decryption module is also modified on each infection. A well-written polymorphic virus therefore has no parts which remain identical between infections, making it very difficult to detect directly using signatures. Anti-virus software can detect it by decrypting the viruses using an emulator, or by statistical pattern analysis of the encrypted virus body. To enable polymorphic code, the virus has to have a polymorphic engine (also called mutating engine or mutation engine) somewhere in its encrypted body. See Polymorphic code for technical detail on how such engines operate.
Some viruses employ polymorphic code in a way that constrains the mutation rate of the virus significantly. For example, a virus can be programmed to mutate only slightly over time, or it can be programmed to refrain from mutating when it infects a file on a computer that already contains copies of the virus. The advantage of using such slow polymorphic code is that it makes it more difficult for anti-virus professionals to obtain representative samples of the virus, because bait files that are infected in one run will typically contain identical or similar samples of the virus. This will make it more likely that the detection by the virus scanner will be unreliable, and that some instances of the virus may be able to avoid detection.
Metamorphic code
To avoid being detected by emulation, some viruses rewrite themselves completely each time they are to infect new executables. Viruses that use this technique are said to be metamorphic. To enable metamorphism, a metamorphic engine is needed. A metamorphic virus is usually very large and complex. For example, W32/Simile consisted of over 14000 lines of Assembly language code, 90% of which is part of the metamorphic engine.[7]
Vulnerability and countermeasures
The vulnerability of operating systems to viruses
Just as genetic diversity in a population decreases the chance of a single disease wiping out a population, the diversity of software systems on a network similarly limits the destructive potential of viruses.
This became a particular concern in the 1990s, when Microsoft gained market dominance in desktop operating systems and office suites. The users of Microsoft software (especially networking software such as Microsoft Outlook and Internet Explorer) are especially vulnerable to the spread of viruses. Microsoft software is targeted by virus writers due to their desktop dominance, and is often criticized for including many errors and holes for virus writers to exploit. Integrated and non-integrated Microsoft applications (such as Microsoft Office) and applications with scripting languages with access to the file system (for example Visual Basic Script (VBS), and applications with networking features) are also particularly vulnerable.
Although Windows is by far the most popular operating system for virus writers, some viruses also exist on other platforms. Any operating system that allows third-party programs to run can theoretically run viruses. Some operating systems are less secure than others. Unix-based OS's (and NTFS-aware applications on Windows NT based platforms) only allow their users to run executables within their protected space in their own directories.
An Internet based research revealed that there were cases when people willingly pressed a particular button to download a virus. A security firm F-Secure ran a half year advertising campaign on Google AdWords which said "Is your PC virus-free? Get it infected here!". The result was 409 clicks.[8]
As of 2006, there are relatively few security exploits[9] targeting Mac OS X (with a Unix-based file system and kernel). The number of viruses for the older Apple operating systems, known as Mac OS Classic, varies greatly from source to source, with Apple stating that there are only four known viruses, and independent sources stating there are as many as 63 viruses. It is safe to say that Macs are less likely to be targeted because of low market share and thus a Mac-specific virus could only infect a small proportion of computers (making the effort less desirable). Virus vulnerability between Macs and Windows is a chief selling point, one that Apple uses in their Get a Mac advertising.[10]
Windows and Unix have similar scripting abilities, but while Unix natively blocks normal users from having access to make changes to the operating system environment, older copies of Windows such as Windows 95 and 98 do not. In 1997, when a virus for Linux was released – known as "Bliss" – leading antivirus vendors issued warnings that Unix-like systems could fall prey to viruses just like Windows.[11] The Bliss virus may be considered characteristic of viruses – as opposed to worms – on Unix systems. Bliss requires that the user run it explicitly (so it is a trojan), and it can only infect programs that the user has the access to modify. Unlike Windows users, most Unix users do not log in as an administrator user except to install or configure software; as a result, even if a user ran the virus, it could not harm their operating system. The Bliss virus never became widespread, and remains chiefly a research curiosity. Its creator later posted the source code to Usenet, allowing researchers to see how it worked.[12]
The role of software development
Because software is often designed with security features to prevent unauthorized use of system resources, many viruses must exploit software bugs in a system or application to spread. Software development strategies that produce large numbers of bugs will generally also produce potential exploits.
Anti-virus software and other preventive measures
Many users install anti-virus software that can detect and eliminate known viruses after the computer downloads or runs the executable. There are two common methods that an anti-virus software application uses to detect viruses. The first, and by far the most common method of virus detection is using a list of virus signature definitions. This works by examining the content of the computer's memory (its RAM, and boot sectors) and the files stored on fixed or removable drives (hard drives, floppy drives), and comparing those files against a database of known virus "signatures". The disadvantage of this detection method is that users are only protected from viruses that pre-date their last virus definition update. The second method is to use a heuristic algorithm to find viruses based on common behaviors. This method has the ability to detect viruses that anti-virus security firms have yet to create a signature for.
Some anti-virus programs are able to scan opened files in addition to sent and received e-mails 'on the fly' in a similar manner. This practice is known as "on-access scanning." Anti-virus software does not change the underlying capability of host software to transmit viruses. Users must update their software regularly to patch security holes. Anti-virus software also needs to be regularly updated in order to prevent the latest threats.
One may also minimise the damage done by viruses by making regular backups of data (and the Operating Systems) on different media, that are either kept unconnected to the system (most of the time), read-only or not accessible for other reasons, such as using different file systems. This way, if data is lost through a virus, one can start again using the backup (which should preferably be recent). A notable exception to this rule is the Gammima virus, which propagates via infected removable media (specifically flash drives) [13] [14]. If a backup session on optical media like CD and DVD is closed, it becomes read-only and can no longer be affected by a virus (so long as a virus or infected file was not copied onto the CD/DVD). Likewise, an Operating System on a bootable can be used to start the computer if the installed Operating Systems become unusable. Another method is to use different Operating Systems on different file systems. A virus is not likely to affect both. Data backups can also be put on different file systems. For example, Linux requires specific software to write to NTFS partitions, so if one does not install such software and uses a separate installation of MS Windows to make the backups on an NTFS partition, the backup should remain safe from any Linux viruses. Likewise, MS Windows can not read file systems like ext3, so if one normally uses MS Windows, the backups can be made on an ext3 partition using a Linux installation.
Recovery methods
Once a computer has been compromised by a virus, it is usually unsafe to continue using the same computer without completely reinstalling the operating system. However, there are a number of recovery options that exist after a computer has a virus. These actions depend on severity of the type of virus.
Virus removal
One possibility on Windows Me, Windows XP and Windows Vista is a tool known as System Restore, which restores the registry and critical system files to a previous checkpoint. Often a virus will cause a system to hang, and a subsequent hard reboot will render a system restore point from the same day corrupt. Restore points from previous days should work provided the virus is not designed to corrupt the restore files or also exists in previous restore points [15]. Some viruses, however, disable system restore and other important tools such as Task Manager and Command Prompt. An example of a virus that does this is CiaDoor.
Administrators have the option to disable such tools from limited users for various reasons. The virus modifies the registry to do the same, except, when the Administrator is controlling the computer, it blocks all users from accessing the tools. When an infected tool activates it gives the message "Task Manager has been disabled by your administrator.", even if the user trying to open the program is the administrator.
Users running a Microsoft operating system can go to Microsoft's website to run a free scan, if they have their 20-digit registration number.
Operating system reinstallation
Reinstalling the operating system is another approach to virus removal. It involves simply reformatting the OS partition and installing the OS from its original media, or imaging the partition with a clean backup image (taken with Ghost or Acronis for example).
This method has the benefits of being simple to do, can be faster than running multiple anti-virus scans, and is guaranteed to remove any malware. Downsides include having to reinstall all other software as well as the operating system. User data can be backed up by booting off of a Live CD or putting the hard drive into another computer and booting from the other computer's operating system (though care must be taken not to transfer the virus to the new computer).
History of Computer Viruses
The Creeper virus was first detected on ARPANET, the forerunner of the Internet in the early 1970s.[1] It propagated via the TENEX operating system and could make use of any connected modem to dial out to remote computers and infect them. It would display the message "I'M THE CREEPER : CATCH ME IF YOU CAN.". It is possible that the Reaper program, which appeared shortly after and sought out copies of the Creeper and deleted them, may have been written by the creator of the Creeper in a fit of regret.[1]
A common misconception is that a program called "Rother J" was the first computer virus to appear "in the wild" — that is, outside the single computer or lab where it was created, but that claim is false. See the Timeline of notable computer viruses and worms for other earlier viruses. It was however the first virus to infect computers "in the home". Written in 1982 by Richard Skrenta, it attached itself to the Apple DOS 3.3 operating system and spread by floppy disk.[2] This virus was originally a joke, created by a high school student and put onto a game on floppy disk. On its 50th use the Elk Cloner virus would be activated, infecting the computer and displaying a short poem beginning "Elk Cloner: The program with a personality".
The first PC virus in the wild was a boot sector virus called (c)Brain[3], created in 1986 by the Farooq Alvi Brothers, operating out of Lahore, Pakistan. The brothers reportedly created the virus to deter pirated copies of software they had written. However, analysts have claimed that the Ashar virus, a variant of Brain, possibly predated it based on code within the virus.[original research?]
Before computer networks became widespread, most viruses spread on removable media, particularly floppy disks. In the early days of the personal computer, many users regularly exchanged information and programs on floppies. Some viruses spread by infecting programs stored on these disks, while others installed themselves into the disk boot sector, ensuring that they would be run when the user booted the computer from the disk, usually inadvertently. PCs of the era would attempt to boot first from a floppy if one had been left in the drive. This was the most successful infection strategy until floppy disks fell from favour, making boot sector viruses the most common in the wild[4].
Traditional computer viruses emerged in the 1980s, driven by the spread of personal computers and the resultant increase in BBS and modem use, and software sharing. Bulletin board driven software sharing contributed directly to the spread of Trojan horse programs, and viruses were written to infect popularly traded software. Shareware and bootleg software were equally common vectors for viruses on BBS's.[citation needed] Within the "pirate scene" of hobbyists trading illicit copies of retail software, traders in a hurry to obtain the latest applications and games were easy targets for viruses.[original research?]
Since the mid-1990s, macro viruses have become common. Most of these viruses are written in the scripting languages for Microsoft programs such as Word and Excel. These viruses spread in Microsoft Office by infecting documents and spreadsheets. Since Word and Excel were also available for Mac OS, most of these viruses were able to spread on Macintosh computers as well. Most of these viruses did not have the ability to send infected e-mail. Those viruses which did spread through e-mail took advantage of the Microsoft Outlook COM interface.[citation needed]
Macro viruses pose unique problems for detection software[citation needed]. For example, some versions of Microsoft Word allowed macros to replicate themselves with additional blank lines. The virus behaved identically but would be misidentified as a new virus. In another example, if two macro viruses simultaneously infect a document, the combination of the two, if also self-replicating, can appear as a "mating" of the two and would likely be detected as a virus unique from the "parents".[5]
A virus may also send a web address link as an instant message to all the contacts on an infected machine. If the recipient, thinking the link is from a friend (a trusted source) follows the link to the website, the virus hosted at the site may be able to infect this new computer and continue propagating.
The newest species of the virus family is the cross-site scripting virus.[citation needed] The virus emerged from research and was academically demonstrated in 2005.[6] This virus utilizes cross-site scripting vulnerabilities to propagate. Since 2005 there have been multiple instances of the cross-site scripting viruses in the wild, most notable sites affected have been MySpace and Yahoo.
A common misconception is that a program called "Rother J" was the first computer virus to appear "in the wild" — that is, outside the single computer or lab where it was created, but that claim is false. See the Timeline of notable computer viruses and worms for other earlier viruses. It was however the first virus to infect computers "in the home". Written in 1982 by Richard Skrenta, it attached itself to the Apple DOS 3.3 operating system and spread by floppy disk.[2] This virus was originally a joke, created by a high school student and put onto a game on floppy disk. On its 50th use the Elk Cloner virus would be activated, infecting the computer and displaying a short poem beginning "Elk Cloner: The program with a personality".
The first PC virus in the wild was a boot sector virus called (c)Brain[3], created in 1986 by the Farooq Alvi Brothers, operating out of Lahore, Pakistan. The brothers reportedly created the virus to deter pirated copies of software they had written. However, analysts have claimed that the Ashar virus, a variant of Brain, possibly predated it based on code within the virus.[original research?]
Before computer networks became widespread, most viruses spread on removable media, particularly floppy disks. In the early days of the personal computer, many users regularly exchanged information and programs on floppies. Some viruses spread by infecting programs stored on these disks, while others installed themselves into the disk boot sector, ensuring that they would be run when the user booted the computer from the disk, usually inadvertently. PCs of the era would attempt to boot first from a floppy if one had been left in the drive. This was the most successful infection strategy until floppy disks fell from favour, making boot sector viruses the most common in the wild[4].
Traditional computer viruses emerged in the 1980s, driven by the spread of personal computers and the resultant increase in BBS and modem use, and software sharing. Bulletin board driven software sharing contributed directly to the spread of Trojan horse programs, and viruses were written to infect popularly traded software. Shareware and bootleg software were equally common vectors for viruses on BBS's.[citation needed] Within the "pirate scene" of hobbyists trading illicit copies of retail software, traders in a hurry to obtain the latest applications and games were easy targets for viruses.[original research?]
Since the mid-1990s, macro viruses have become common. Most of these viruses are written in the scripting languages for Microsoft programs such as Word and Excel. These viruses spread in Microsoft Office by infecting documents and spreadsheets. Since Word and Excel were also available for Mac OS, most of these viruses were able to spread on Macintosh computers as well. Most of these viruses did not have the ability to send infected e-mail. Those viruses which did spread through e-mail took advantage of the Microsoft Outlook COM interface.[citation needed]
Macro viruses pose unique problems for detection software[citation needed]. For example, some versions of Microsoft Word allowed macros to replicate themselves with additional blank lines. The virus behaved identically but would be misidentified as a new virus. In another example, if two macro viruses simultaneously infect a document, the combination of the two, if also self-replicating, can appear as a "mating" of the two and would likely be detected as a virus unique from the "parents".[5]
A virus may also send a web address link as an instant message to all the contacts on an infected machine. If the recipient, thinking the link is from a friend (a trusted source) follows the link to the website, the virus hosted at the site may be able to infect this new computer and continue propagating.
The newest species of the virus family is the cross-site scripting virus.[citation needed] The virus emerged from research and was academically demonstrated in 2005.[6] This virus utilizes cross-site scripting vulnerabilities to propagate. Since 2005 there have been multiple instances of the cross-site scripting viruses in the wild, most notable sites affected have been MySpace and Yahoo.
Computer virus
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A computer virus is a computer program that can copy itself and infect a computer without permission or knowledge of the user. The term "virus" is also commonly used, albeit erroneously, to refer to many different types of malware and adware programs. The original virus may modify the copies, or the copies may modify themselves, as occurs in a metamorphic virus. A virus can only spread from one computer to another when its host is taken to the uninfected computer, for instance by a user sending it over a network or the Internet, or by carrying it on a removable medium such as a floppy disk, CD, or USB drive. Meanwhile viruses can spread to other computers by infecting files on a network file system or a file system that is accessed by another computer. Viruses are sometimes confused with computer worms and Trojan horses. A worm can spread itself to other computers without needing to be transferred as part of a host, and a Trojan horse is a file that appears harmless. Worms and Trojans may cause harm to either a computer system's hosted data, functional performance, or networking throughput, when executed. In general, a worm does not actually harm either the system's hardware or software, while at least in theory, a Trojan's payload may be capable of almost any type of harm if executed. Some can't be seen when the program is not running, but as soon as the infected code is run, the Trojan horse kicks in. That is why it is so hard for people to find viruses and other malware themselves and why they have to use spyware programs and registry processors.
Most personal computers are now connected to the Internet and to local area networks, facilitating the spread of malicious code. Today's viruses may also take advantage of network services such as the World Wide Web, e-mail, Instant Messaging and file sharing systems to spread, blurring the line between viruses and worms. Furthermore, some sources use an alternative terminology in which a virus is any form of self-replicating malware.
Some malware is programmed to damage the computer by damaging programs, deleting files, or reformatting the hard disk. Other malware programs are not designed to do any damage, but simply replicate themselves and perhaps make their presence known by presenting text, video, or audio messages. Even these less sinister malware programs can create problems for the computer user. They typically take up computer memory used by legitimate programs. As a result, they often cause erratic behavior and can result in system crashes. In addition, much malware is bug-ridden, and these bugs may lead to system crashes and data loss. Many CiD programs are programs that have been downloaded by the user and pop up every so often. This results in slowing down of the computer, but it is also very difficult to find and stop the problem.
Jump to: navigation, search
This article or section may contain original research or unverified claims.
Please improve the article by adding references. See the talk page for details. (September 2008)
This article needs additional citations for verification.
Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (September 2008)
A computer virus is a computer program that can copy itself and infect a computer without permission or knowledge of the user. The term "virus" is also commonly used, albeit erroneously, to refer to many different types of malware and adware programs. The original virus may modify the copies, or the copies may modify themselves, as occurs in a metamorphic virus. A virus can only spread from one computer to another when its host is taken to the uninfected computer, for instance by a user sending it over a network or the Internet, or by carrying it on a removable medium such as a floppy disk, CD, or USB drive. Meanwhile viruses can spread to other computers by infecting files on a network file system or a file system that is accessed by another computer. Viruses are sometimes confused with computer worms and Trojan horses. A worm can spread itself to other computers without needing to be transferred as part of a host, and a Trojan horse is a file that appears harmless. Worms and Trojans may cause harm to either a computer system's hosted data, functional performance, or networking throughput, when executed. In general, a worm does not actually harm either the system's hardware or software, while at least in theory, a Trojan's payload may be capable of almost any type of harm if executed. Some can't be seen when the program is not running, but as soon as the infected code is run, the Trojan horse kicks in. That is why it is so hard for people to find viruses and other malware themselves and why they have to use spyware programs and registry processors.
Most personal computers are now connected to the Internet and to local area networks, facilitating the spread of malicious code. Today's viruses may also take advantage of network services such as the World Wide Web, e-mail, Instant Messaging and file sharing systems to spread, blurring the line between viruses and worms. Furthermore, some sources use an alternative terminology in which a virus is any form of self-replicating malware.
Some malware is programmed to damage the computer by damaging programs, deleting files, or reformatting the hard disk. Other malware programs are not designed to do any damage, but simply replicate themselves and perhaps make their presence known by presenting text, video, or audio messages. Even these less sinister malware programs can create problems for the computer user. They typically take up computer memory used by legitimate programs. As a result, they often cause erratic behavior and can result in system crashes. In addition, much malware is bug-ridden, and these bugs may lead to system crashes and data loss. Many CiD programs are programs that have been downloaded by the user and pop up every so often. This results in slowing down of the computer, but it is also very difficult to find and stop the problem.
History of Worm
Naming and history
The name worm comes from The Shockwave Rider, a science fiction novel published in 1975 by John Brunner[1][2].
Payloads
Many worms have been created which are only designed to spread, and don't attempt to alter the systems they pass through. However, as the Morris worm and Mydoom showed, the network traffic and other unintended effects can often cause major disruption. A "payload" is code designed to do more than spread the worm - it might delete files on a host system (e.g., the ExploreZip worm), encrypt files in a cryptoviral extortion attack, or send documents via e-mail. A very common payload for worms is to install a backdoor in the infected computer to allow the creation of a "zombie" under control of the worm author - Sobig and Mydoom are examples which created zombies. Networks of such machines are often referred to as botnets and are very commonly used by spam senders for sending junk email or to cloak their website's address.[3] Spammers are therefore thought to be a source of funding for the creation of such worms,[4][5] and worm writers have been caught selling lists of IP addresses of infected machines.[6] Others try to blackmail companies with threatened DoS attacks.[7]
Backdoors can be exploited by other malware, including worms. Examples include Doomjuice, which spreads better using the backdoor opened by Mydoom, and at least one instance of malware taking advantage of the rootkit and backdoor installed by the Sony/BMG DRM software utilized by millions of music CDs prior to late 2005.
Worms with good intent
Beginning with the very first research into worms at Xerox PARC there have been attempts to create useful worms. The Nachi family of worms, for example, tried to download and install patches from Microsoft's website to fix vulnerabilities in the host system — by exploiting those same vulnerabilities. In practice, although this may have made these systems more secure, it generated considerable network traffic, rebooted the machine in the course of patching it, and did its work without the consent of the computer's owner or user.
Other worms, such as XSS worms have been written for research to determine the factors of how worms spread, such as social activity and change in user behavior.
Still more worms do very little, or are pranks, such as one that sends the popular picture of the lolowl with the phrase "O RLY?" to a print queue in the infected computer.
Most security experts regard all worms as malware, whatever their payload or their writers' intentions.
Protecting against dangerous computer worms
Worms spread by exploiting vulnerabilities in operating systems. All vendors supply regular security updates[8] (see "Patch Tuesday"), and if these are installed to a machine then the majority of worms are unable to spread to it. If a vendor acknowledges a vulnerability but has yet to release a security update to patch it, a zero day exploit is possible. However, these are relatively rare.
Users need to be wary of opening unexpected email[9], and should not run attached files or programs, or visit web sites that are linked to such emails. However, as with the ILOVEYOU worm, and with the increased growth and efficiency of phishing attacks, it remains possible to trick the end-user into running a malicious code.
Anti-virus and anti-spyware software are helpful, but must be kept up-to-date with new pattern files at least every few days. The use of a firewall is also recommended.
In the April-June, 2008, issue of IEEE Transactions on Dependable and Secure Computing, computer scientists describe a potential new way to combat internet worms. The researchers discovered how to contain the kind of worm that scans the Internet randomly, looking for vulnerable hosts to infect. They found that the key is for software to monitor the number of scans that machines on a network send out. When a machine starts sending out too many scans, it is a sign that it has been infected, allowing administrators to take it off line and check it for viruses.
The name worm comes from The Shockwave Rider, a science fiction novel published in 1975 by John Brunner[1][2].
Payloads
Many worms have been created which are only designed to spread, and don't attempt to alter the systems they pass through. However, as the Morris worm and Mydoom showed, the network traffic and other unintended effects can often cause major disruption. A "payload" is code designed to do more than spread the worm - it might delete files on a host system (e.g., the ExploreZip worm), encrypt files in a cryptoviral extortion attack, or send documents via e-mail. A very common payload for worms is to install a backdoor in the infected computer to allow the creation of a "zombie" under control of the worm author - Sobig and Mydoom are examples which created zombies. Networks of such machines are often referred to as botnets and are very commonly used by spam senders for sending junk email or to cloak their website's address.[3] Spammers are therefore thought to be a source of funding for the creation of such worms,[4][5] and worm writers have been caught selling lists of IP addresses of infected machines.[6] Others try to blackmail companies with threatened DoS attacks.[7]
Backdoors can be exploited by other malware, including worms. Examples include Doomjuice, which spreads better using the backdoor opened by Mydoom, and at least one instance of malware taking advantage of the rootkit and backdoor installed by the Sony/BMG DRM software utilized by millions of music CDs prior to late 2005.
Worms with good intent
Beginning with the very first research into worms at Xerox PARC there have been attempts to create useful worms. The Nachi family of worms, for example, tried to download and install patches from Microsoft's website to fix vulnerabilities in the host system — by exploiting those same vulnerabilities. In practice, although this may have made these systems more secure, it generated considerable network traffic, rebooted the machine in the course of patching it, and did its work without the consent of the computer's owner or user.
Other worms, such as XSS worms have been written for research to determine the factors of how worms spread, such as social activity and change in user behavior.
Still more worms do very little, or are pranks, such as one that sends the popular picture of the lolowl with the phrase "O RLY?" to a print queue in the infected computer.
Most security experts regard all worms as malware, whatever their payload or their writers' intentions.
Protecting against dangerous computer worms
Worms spread by exploiting vulnerabilities in operating systems. All vendors supply regular security updates[8] (see "Patch Tuesday"), and if these are installed to a machine then the majority of worms are unable to spread to it. If a vendor acknowledges a vulnerability but has yet to release a security update to patch it, a zero day exploit is possible. However, these are relatively rare.
Users need to be wary of opening unexpected email[9], and should not run attached files or programs, or visit web sites that are linked to such emails. However, as with the ILOVEYOU worm, and with the increased growth and efficiency of phishing attacks, it remains possible to trick the end-user into running a malicious code.
Anti-virus and anti-spyware software are helpful, but must be kept up-to-date with new pattern files at least every few days. The use of a firewall is also recommended.
In the April-June, 2008, issue of IEEE Transactions on Dependable and Secure Computing, computer scientists describe a potential new way to combat internet worms. The researchers discovered how to contain the kind of worm that scans the Internet randomly, looking for vulnerable hosts to infect. They found that the key is for software to monitor the number of scans that machines on a network send out. When a machine starts sending out too many scans, it is a sign that it has been infected, allowing administrators to take it off line and check it for viruses.
Computer worm
From Wikipedia, the free encyclopedia
Jump to: navigation, search
A computer worm is a self-replicating computer program. It uses a network to send copies of itself to other nodes (computer terminals on the network) and it may do so without any user intervention. Unlike a virus, it does not need to attach itself to an existing program. Worms almost always cause harm to the network, if only by consuming bandwidth, whereas viruses almost always corrupt or modify files on a targeted computer.
Jump to: navigation, search
A computer worm is a self-replicating computer program. It uses a network to send copies of itself to other nodes (computer terminals on the network) and it may do so without any user intervention. Unlike a virus, it does not need to attach itself to an existing program. Worms almost always cause harm to the network, if only by consuming bandwidth, whereas viruses almost always corrupt or modify files on a targeted computer.
Virus removal tools
Virus removal tools
A virus removal tool is software for removing specific viruses from infected computers. Unlike general-purpose virus scanners, it is not intended to detect and remove, ideally, all known viruses; rather it is designed to remove specific viruses more effectively and completely than a general-purpose program. Many single-virus tools will be found searching the Worldwide-Web for "virus removal tool"; others, such as McAfee Stinger and the Microsoft Malicious Software Removal Tool run automatically by Windows update, are designed to remove a limited numbers of viruses. Many of these tools are available for free download.
If a virus is identified by a general-purpose scanner it may not be entirely removed; once the virus has been identified, running a tool designed specifically for it can do a better job of cleaning.
Issues of concern
* The regular appearance of new malware is certainly in the financial interest of vendors of commercial antivirus software, though there is no evidence of collusion. [2]
* Some antivirus software can considerably reduce performance. Users may disable the antivirus protection to overcome the performance loss, thus increasing the risk of infection. For maximum protection, the antivirus software needs to be enabled all the time — often at the cost of slower performance (see also software bloat).
* It is important to note that one should not have more than one memory-resident antivirus software solution installed on a single computer at any given time. Otherwise, the computer may be crippled.[3]
* It is sometimes necessary to temporarily disable virus protection when installing major updates such as Windows Service Packs or updating graphics card drivers.[4] Active antivirus protection may partially or completely prevent the installation of a major update.
* When purchasing antivirus software, the agreement may include a clause that the subscription will be automatically renewed, and the purchaser's credit card automatically billed, at the renewal time without explicit approval. For example, McAfee requires one to unsubscribe at least 60 days before the expiration of the present subscription.[5] Norton Antivirus also renews subscriptions automatically by default. [6]
* Some antivirus programs are actually spyware masquerading as antivirus software. It is best to double-check that the antivirus software which is being downloaded is actually a real antivirus program.[7]
* Some commercial antivirus software programs contain adware.[citation needed]
* Most widely-accepted antivirus programs often do not detect newly-created viruses.
* Anti-virus manufacturers have been criticised for fear mongering by exaggerating the risk that virus pose to consumers.[8]
A virus removal tool is software for removing specific viruses from infected computers. Unlike general-purpose virus scanners, it is not intended to detect and remove, ideally, all known viruses; rather it is designed to remove specific viruses more effectively and completely than a general-purpose program. Many single-virus tools will be found searching the Worldwide-Web for "virus removal tool"; others, such as McAfee Stinger and the Microsoft Malicious Software Removal Tool run automatically by Windows update, are designed to remove a limited numbers of viruses. Many of these tools are available for free download.
If a virus is identified by a general-purpose scanner it may not be entirely removed; once the virus has been identified, running a tool designed specifically for it can do a better job of cleaning.
Issues of concern
* The regular appearance of new malware is certainly in the financial interest of vendors of commercial antivirus software, though there is no evidence of collusion. [2]
* Some antivirus software can considerably reduce performance. Users may disable the antivirus protection to overcome the performance loss, thus increasing the risk of infection. For maximum protection, the antivirus software needs to be enabled all the time — often at the cost of slower performance (see also software bloat).
* It is important to note that one should not have more than one memory-resident antivirus software solution installed on a single computer at any given time. Otherwise, the computer may be crippled.[3]
* It is sometimes necessary to temporarily disable virus protection when installing major updates such as Windows Service Packs or updating graphics card drivers.[4] Active antivirus protection may partially or completely prevent the installation of a major update.
* When purchasing antivirus software, the agreement may include a clause that the subscription will be automatically renewed, and the purchaser's credit card automatically billed, at the renewal time without explicit approval. For example, McAfee requires one to unsubscribe at least 60 days before the expiration of the present subscription.[5] Norton Antivirus also renews subscriptions automatically by default. [6]
* Some antivirus programs are actually spyware masquerading as antivirus software. It is best to double-check that the antivirus software which is being downloaded is actually a real antivirus program.[7]
* Some commercial antivirus software programs contain adware.[citation needed]
* Most widely-accepted antivirus programs often do not detect newly-created viruses.
* Anti-virus manufacturers have been criticised for fear mongering by exaggerating the risk that virus pose to consumers.[8]
Virus scanners
Antivirus scanning software, or a virus scanner, is a program which examines all files in specified locations, the contents of memory, the operating system, the registry, unexpected program behavior, and anywhere else relevant with the intention of identifying and removing any malware.
Typically two different approaches are used to identify malware, often in combination, although with an emphasis on the virus dictionary approach.
* examining (scanning) files, etc., for known viruses matching signatures in a virus dictionary, and
* identifying suspicious behavior from any computer program which might indicate infection. This approach is called heuristic analysis, and may include data captures, port monitoring and other methods.
Network firewalls prevent unknown programs and Internet processes from having access to the system protected; they are not antivirus systems as such, and make no attempt to identify or remove anything, but protect against infection, and limit the activity of any malicious software which is present by blocking incoming requests on certain TCP/IP ports.
Dictionary
In the virus dictionary approach, when the antivirus software looks at a file, it refers to a dictionary of known viruses that the authors of the antivirus software have identified. If a piece of code in the file matches any virus identified in the dictionary, then the antivirus software can take one of the following actions:
1. attempt to repair the file by removing the virus itself from the file,
2. quarantine the file (such that the file remains inaccessible to other programs and its virus can no longer spread), or
3. delete the infected file.
To achieve consistent success in the medium and long term, the virus dictionary approach requires frequent (generally online) downloads of updated virus dictionary entries. Civically-minded and technically-inclined users, and those who want help find viruses not detected by the software, can send their infected files to the authors of antivirus software, who analyze them and include identifying features and removal information in their dictionaries.
Dictionary-based antivirus software typically examines files when the computer's operating system creates, opens, closes, or e-mails them. In this way it can detect a known virus immediately upon receipt. System administrators can schedule antivirus software to examine (scan) all files on the computer's hard disk on a regular basis.
Although the dictionary approach can effectively contain virus outbreaks in the right circumstances, virus authors have tried to stay a step ahead of such software by writing "oligomorphic", "polymorphic" and more recently "metamorphic" viruses, which encrypt parts of themselves or otherwise modify themselves as a method of disguise, so as to not match virus signatures in the dictionary.
An emerging technique to deal with malware in general is whitelisting. Rather than looking for only known bad software, this technique prevents execution of all computer code except that which has been previously identified as trustworthy by the system administrator. By following this "default deny" approach, the limitations inherent in keeping virus signatures up to date are avoided. Additionally, computer applications that are unwanted by the system administrator are prevented from executing since they are not on the whitelist. Since modern enterprise organizations have large quantities of trusted applications, the limitations of adopting this technique rest with the system administrators' ability to properly inventory and maintain the whitelist of trusted applications. Viable implementations of this technique include tools for automating the inventory and whitelist maintenance processes.
Suspicious behavior - heuristics
The suspicious behavior approach, by contrast, doesn't attempt to identify known viruses, but instead monitors the behavior of all programs. If one program tries to write data to an executable program, for example, the antivirus software can flag this suspicious behavior, alert a user, and ask what to do.
Unlike the dictionary approach, the suspicious behavior approach therefore provides protection against brand-new viruses that do not yet exist in any virus dictionaries. However, it can also sound a large number of false positives, and users probably become desensitized to all the warnings. If the user clicks "Accept" on every such warning, then the antivirus software obviously gives no benefit to that user. This problem has worsened since 1997[citation needed], since many more non-malicious program designs came to modify other .exe files without regard to this false positive issue. Therefore, most modern antivirus software uses this technique less and less.
File Emulation - heuristics
Some antivirus software use other types of heuristic analysis. For example, it could try to emulate the beginning of the code of each new executable that the system invokes before transferring control to that executable. If the program seems to use self-modifying code or otherwise appears as a virus (if it immediately tries to find other executables, for example), one could assume that a virus has infected the executable. However, this method could result in a lot of false positives.
Sandbox
Yet another detection method involves using a sandbox. A sandbox emulates the operating system and runs the executable in this simulation. After the program has terminated, software analyzes the sandbox for any changes which might indicate a virus. Because of performance issues, this type of detection normally only takes place during on-demand scans. Also this method may fail as a virus can be nondeterministic and do different things, including doing nothing at all, each time it is executed — so it will be impossible to detect it from one run. [1]
Some virus scanners can warn a user if a file is likely to contain a virus based on the file type.
Typically two different approaches are used to identify malware, often in combination, although with an emphasis on the virus dictionary approach.
* examining (scanning) files, etc., for known viruses matching signatures in a virus dictionary, and
* identifying suspicious behavior from any computer program which might indicate infection. This approach is called heuristic analysis, and may include data captures, port monitoring and other methods.
Network firewalls prevent unknown programs and Internet processes from having access to the system protected; they are not antivirus systems as such, and make no attempt to identify or remove anything, but protect against infection, and limit the activity of any malicious software which is present by blocking incoming requests on certain TCP/IP ports.
Dictionary
In the virus dictionary approach, when the antivirus software looks at a file, it refers to a dictionary of known viruses that the authors of the antivirus software have identified. If a piece of code in the file matches any virus identified in the dictionary, then the antivirus software can take one of the following actions:
1. attempt to repair the file by removing the virus itself from the file,
2. quarantine the file (such that the file remains inaccessible to other programs and its virus can no longer spread), or
3. delete the infected file.
To achieve consistent success in the medium and long term, the virus dictionary approach requires frequent (generally online) downloads of updated virus dictionary entries. Civically-minded and technically-inclined users, and those who want help find viruses not detected by the software, can send their infected files to the authors of antivirus software, who analyze them and include identifying features and removal information in their dictionaries.
Dictionary-based antivirus software typically examines files when the computer's operating system creates, opens, closes, or e-mails them. In this way it can detect a known virus immediately upon receipt. System administrators can schedule antivirus software to examine (scan) all files on the computer's hard disk on a regular basis.
Although the dictionary approach can effectively contain virus outbreaks in the right circumstances, virus authors have tried to stay a step ahead of such software by writing "oligomorphic", "polymorphic" and more recently "metamorphic" viruses, which encrypt parts of themselves or otherwise modify themselves as a method of disguise, so as to not match virus signatures in the dictionary.
An emerging technique to deal with malware in general is whitelisting. Rather than looking for only known bad software, this technique prevents execution of all computer code except that which has been previously identified as trustworthy by the system administrator. By following this "default deny" approach, the limitations inherent in keeping virus signatures up to date are avoided. Additionally, computer applications that are unwanted by the system administrator are prevented from executing since they are not on the whitelist. Since modern enterprise organizations have large quantities of trusted applications, the limitations of adopting this technique rest with the system administrators' ability to properly inventory and maintain the whitelist of trusted applications. Viable implementations of this technique include tools for automating the inventory and whitelist maintenance processes.
Suspicious behavior - heuristics
The suspicious behavior approach, by contrast, doesn't attempt to identify known viruses, but instead monitors the behavior of all programs. If one program tries to write data to an executable program, for example, the antivirus software can flag this suspicious behavior, alert a user, and ask what to do.
Unlike the dictionary approach, the suspicious behavior approach therefore provides protection against brand-new viruses that do not yet exist in any virus dictionaries. However, it can also sound a large number of false positives, and users probably become desensitized to all the warnings. If the user clicks "Accept" on every such warning, then the antivirus software obviously gives no benefit to that user. This problem has worsened since 1997[citation needed], since many more non-malicious program designs came to modify other .exe files without regard to this false positive issue. Therefore, most modern antivirus software uses this technique less and less.
File Emulation - heuristics
Some antivirus software use other types of heuristic analysis. For example, it could try to emulate the beginning of the code of each new executable that the system invokes before transferring control to that executable. If the program seems to use self-modifying code or otherwise appears as a virus (if it immediately tries to find other executables, for example), one could assume that a virus has infected the executable. However, this method could result in a lot of false positives.
Sandbox
Yet another detection method involves using a sandbox. A sandbox emulates the operating system and runs the executable in this simulation. After the program has terminated, software analyzes the sandbox for any changes which might indicate a virus. Because of performance issues, this type of detection normally only takes place during on-demand scans. Also this method may fail as a virus can be nondeterministic and do different things, including doing nothing at all, each time it is executed — so it will be impossible to detect it from one run. [1]
Some virus scanners can warn a user if a file is likely to contain a virus based on the file type.
Antivirus software
From Wikipedia, the free encyclopedia
(Redirected from Anti-virus software)
Jump to: navigation, search
"Antivirus" redirects here. For antiviral medication, see antiviral drug.
Antivirus software (sometimes spelled Anti-Virus or anti-virus with the hyphen) are computer programs that attempt to identify, neutralize or eliminate malicious software. The term "antivirus" is used because the earliest examples were designed exclusively to combat computer viruses; however most modern antivirus software is now designed to combat a wide range of threats, including worms, phishing attacks, rootkits, Trojans, often described collectively as malware.
(Redirected from Anti-virus software)
Jump to: navigation, search
"Antivirus" redirects here. For antiviral medication, see antiviral drug.
Antivirus software (sometimes spelled Anti-Virus or anti-virus with the hyphen) are computer programs that attempt to identify, neutralize or eliminate malicious software. The term "antivirus" is used because the earliest examples were designed exclusively to combat computer viruses; however most modern antivirus software is now designed to combat a wide range of threats, including worms, phishing attacks, rootkits, Trojans, often described collectively as malware.
antivirus program
A utility that searches a hard disk for viruses and removes any that are found. Most antivirus programs include an auto-update feature that enables the program to download profiles of new viruses so that it can check for the new viruses as soon as they are discovered.
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