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6.3. Aggressive Retroviruses

A retrovirus is a computer virus that specifically tries to bypass or hinder the operation of an antivirus, personal firewall, or other security programs17.

There are many possible ways for an attacker to achieve this because most Windows users work with their computers as a user with administrative privileges. This gives computer viruses the potential to kill the processes and files that belong to antivirus software or to disable the antivirus programs. When Microsoft DOS was first released with the MSAV antivirus product, many viruses deployed techniques to kill the integrity checksum files of the antivirus scanner and the antivirus signatures; they also disabled the on-access virus protection18. At one point, the MSAV/VSAFE disabling routine (a single interrupt call with special parameters) was so popular in computer viruses that it became one of the best heuristic scanning methods to generically pinpoint possible retroviruses!

Retroviruses have the potential to make way for other computer viruses that are otherwise known and easy for the antivirus software to handle. Therefore, virus writers routinely reverse engineer antivirus products to learn tricks that can be used in retro attacks. For example, aggressive retroviruses often do the following actions upon execution:

  • Disable or kill the antivirus programs in memory and/or on the disk.

  • Disable or bypass behavior-blocking products.

  • Bypass or kill personal firewall software.

  • Delete the integrity-checking database files.

  • Modify the integrity-checking database files. (For example, the IDEA.6155 virus of Spanska attacks TBSCAN's ANTI-VIR.DAT file by patching it. The virus modifies the first character of the non-encrypted host filename stored in ANTI-VIR.DAT. Thus the integrity checker component will not find out that the virus infected the host and simply takes a new checksum of the infected file the next time the integrity checker is used19.)

  • Execute virus code via the antivirus program. (For example, the Varicella virus escapes when Frans Veldman's TBCLEAN program attempts to clean it generically.)

  • Trojanize the antivirus database files. Such viruses activate when the scanner searches for viruses. (For instance, the AVP antivirus was challenged by a few viruses, such as a Mr. Sandman creation called Anti-AVP, intended to be called AntiCARO. This virus inserts a new, hacked AVP database into AVP's set to force the Boza virus being detected as Bizatch. Another variation of this attack forces AVP to not detect any viruses and to delete other antivirus programs from the disk during scanning. This virus was authored by the virus writer nicknamed TCP.)

  • Detect the execution of antivirus software and perform damage.

  • Take action against disinfectors by making a clean boot more difficult. (An example of this is the Ginger virus that causes circular partition problems.)

  • Remove the antivirus program's integrity mark from files. (An older version of McAfee SCAN appended an integrity check record to the end of the executable files, which the Tequila virus removed before infection.)

  • Attack common CRC checksum algorithms. (For example, the HybrisF virus of the virus writer, Vecna, attacks CRC checksums. This technique was previously developed by the virus writer, Zhengxi. CRC checksums are not cryptographically strong. To prove this, the HybrisF virus infected files in such a way that their CRC checksum remained unchanged after infectioneven the file size was the same. Many integrity-checker products did not manage to detect changes in the files.)

  • Prevent infected systems from connecting and downloading updates from antivirus Web sites. (W95/MTX and W32/Mydoom are examples of this. Other retroviruses can easily block access to security updates as well.)

  • Require extra password protection. (For example, several members of the W32/Beagle@mm family send themselves in password-encrypted ZIP files. The password is randomly set and sent in the e-mail message with the worm. Although brute-force encryption of ZIP files is feasible, antivirus products cannot always brute-force attack such files because it can take minutes to decrypt even a simple file. An obvious solution is to use the words in the e-mail message to decrypt the content of the e-mail message. However, the attacker could send a second e-mail message saying, "Here is the password that I forgot to send you," or send an image file such as a JPEG or BMP file that contains the password in handwritten form. (I only wrote this paragraph a few weeks ago, and several variants of Beagle appeared in the meantime that use Microsoft GDIPLUS APIs to convert randomly generated passwords to picture files. Thus antivirus solutions that attempt to pick the password from the e-mail quickly failed to decrypt these variants.)

  • A few retroviruses not only target antivirus software, but virus analysis tools as well. For example, viruses might interfere with tools such as Filemon and Regmon that are commonly used to perform dynamical code analysis, as I illustrate in Chapter 15.

Similar attacks are possible using other file formats, such as self-extracting archives and Microsoft document formats. When documents are protected with a password, the macros in the document are also protected. In early editions of Microsoft Office products, password protection was weak, and therefore antivirus products could decrypt password-protected macros to find the virus in a matter of seconds. Newer Microsoft Office releases have a stronger password protection for documents that can withstand a known plain-text attack and thus cannot be scanned anymore. Although the PKZIP password protection is breakable, it cannot be done in seconds, but minutes only, and so antivirus programs do not have the luxury to execute a brute-force attack to scan them.

Retroviruses are particularly challenging for antivirus software. Modern antivirus solutions require extra protection to prevent attacks such as process termination to protect themselves better from unknown computer viruses.

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