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The Antivirus Hacker's Handbook by Elias Bachaalany, Joxean Koret

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Chapter 6Antivirus Software Evasion

Antivirus evasion techniques are used by malware writers, as well as by penetration testers and vulnerability researchers, in order to bypass one or more antivirus software applications. This ensures the payload the attacker wants to execute in the target machine or machines is not blocked by antivirus software and can perform the required actions.

Evasion techniques for bypassing antivirus software can be divided into two categories: dynamic and static. Static means that you simply want to bypass detection based on the antivirus's signature-scanning algorithms, while dynamic means that you want to bypass detection of the sample's behavior when it is executed. That is, statically, you try to bypass signature-based detection using cyclic redundancy check algorithms (CRCs), some other fuzzy hashing techniques, or cryptographic hashes by altering the binary contents of the sample, or you try changing the graph of the program so basic block- and function-based signatures can be tricked into believing the program is different. When trying to dynamically evade detection, the sample in question should change its behavior when it detects that it is running inside a sandbox or an antivirus emulator, or it could execute an instruction that the emulator does not support. It could also try to get out of the sandbox or the “safe execution” environment that is set up by the antivirus software so it can run the malicious programs without being monitored. ...

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