Methods to avoid detection

In order to avoid detection by users, some viruses employ different kinds of obfuscation. 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.

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.

Recent viruses avoid any kind of detection attempt by attempting to forcefully kill the tasks associated with the virus scanner before it can detect them.

As computers and operating systems grow larger and more complex, old hiding techniques need to be updated or replaced.

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 however. 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 hosts 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 behaviour 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 amount 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 fool anti-virus software by intercepting its requests to the operating system. A virus can hide itself by ensuring that a request of anti-virus software to read an infected file is passed to the virus, instead of to the operating system. 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” the infected file. Some viruses employ techniques that make detection by means of signatures difficult or impossible. These viruses modify their code on each infection. That is, each infected file contains a different variant of the virus.

Simple self-modifications

In the past, some viruses modified themselves only in fairly simple ways. For example, they regularly exchanged subroutines in their code. This poses no problems to a somewhat advanced virus scanner however.

Encryption with a variable key

A more advanced method is the use of simple encryption to encode 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. 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.

Mostly, the decryption techniques that these viruses employ are fairly simple and mostly done by just xoring each byte with a randomized key that was saved by the parent virus. The use of XOR-operations has the additional advantage that the encryption and decryption routine are the same (a xor b = c, c xor b = a.)

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 that stay the same on each infection, making it impossible 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.

Some viruses employ polymorphic code in a way which 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, as a result of this, 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 uses this technique are said to be metamorphic. To enable metamorphism, a metamorphic engine is needed. A metamorphic virus is usually very large and complex. W32/Simile consisted of over 14000 lines of assembly code, for example. 90% of it is part of the metamorphic engine.

Add comment September 12th, 2005