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Chapter 11. Antivirus Defense Techniques

" But who is to guard the guards themselves?"

Juvenal

This chapter is a collection of techniques that were deployed in antivirus software to protect against computer viruses. In particular, antivirus scanner techniques will be discussed, which have evolved with computer virus attacks during the last 15 years. During the long evolution of antivirus software, these common techniques became fine-tuned and widely used. Although other methods will likely emerge, those collected in this chapter have been in use long enough to remain the core of antivirus software for the foreseeable future.

I will provide examples of computer virus detection in order of increasing complexity:

  • Simple pattern-based virus detection

  • Exact identification

  • Detection of encrypted, polymorphic, and metamorphic viruses1

I will also illustrate the use of generic and heuristic methods2 that can detect classes of computer viruses rather than only specific variants. This chapter also will familiarize you with repair techniques (including generic and heuristic methods) that are used to restore the clean state of infected files. State-of-the-art antivirus software uses sophisticated code emulation (virtual machine) for heuristics3 as well as complex virus detections. It is crucial to understand this critical component of the antivirus software because this is the "secret weapon" that has kept antivirus scanners alive for so long.

There are two basic kinds of scanners: on-demand and on-access scanners. On-demand scanning is executed only at the user's request. On-demand scanning can also be loaded from system startup points and similar locations to achieve better success in virus detection. On the other hand, on-access scanners are memory-resident. They load as a simple application and hook interrupts related to file and disk access, or they are implemented as device drivers that attach themselves to file systems4. For example, on Windows NT/2000/XP/2003 systems, on-access scanners are typically implemented as file-system filter drivers that attach themselves to file systems such as FAT, NTFS, and so on.

Figure 11.1 demonstrates a loaded file system filter driver attached to a set of file systems using a tool from OSR.

Figure 11.1. File system filter drivers attached to file system drivers.


On-access scanners typically scan files when they are opened, created, or closed. In this way, the virus infection can be prevented if a known virus is executed on the system. An interesting problem is caused by network infectors such as W32/Funlove. Funlove infects files across network shares. Thus the infections on the remote system will be detected only if the file is already written to the disk. This means that in some circumstances, even the on-access scanners have trouble stopping viruses effectively.

Note

This risk can be reduced further by scanning the disk cache before the file is written to the disk. Furthermore, other defense methods, such as behavior blocking or network intrusion prevention software, can be used.


This chapter focuses on techniques that prevent, detect, and repair viruses in files or file system areas. Other generic solutions are also subject of this chapter, including the following:

  • On-demand integrity checkers

  • On-access integrity shells

  • Behavior blockers

  • Access controls

  • Inoculation

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