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Bypass Antivirus Dynamic Analysis

Limitations of the AV model and how to exploit them

D 24 August 2014     H 18:55     A Emeric Nasi     C 2 messages


Note: This paper requires some knowledge on Windows system programming.

I Introduction.

« Antivirus are easy to bypass », « Antivirus are mandatory in defense in depth », «This Cryptor is FUD» are some of the sentence you hear when doing some researches on antivirus security.
I asked myself, hey is it really that simple to bypass AV? After some research I came (like others) to the conclusion that bypassing Antivirus consists in two big steps:

  • Hide the code which may be recognized as malicious. This is generally done using encryption.
  • Code the decryption stub in such a way it is not detected as a virus nor bypassed by emulation/sandboxing.

In this paper I will mainly focus on the last one, how to fool antivirus emulation/sandboxing systems.
I’ve set myself a challenge to find half a dozen of ways to make a fully undetectable decryption stub (in fact I found way more than that). Here is a collection of methods. Some of those are very complex (and most “FUD cryptor” sellers use one of these). Others are so simple I don’t understand why I’ve never seen these before. I am pretty sure underground and official virus writers are fully aware about these methods so I wanted to share these with the public.

If you wish to read more about this, the document can be downloaded in a PDF format

PDF - 994 kb

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