Filtering inbound and outbound data is most certainly a part of your information security infrastructure. A prominent component are anti-virus content filters. Your desktop clients probably have one. Your emails will be first read by these filters. While techniques like this have been around for a long time, they regularly draw criticism. According to some opinions the concept of anti-virus is dead. Nevertheless it’s still a major building block of security architecture. The choice can be hard, though. DeepSec 2014 features a talk by Daniel Sauder, giving you an idea why anti-virus software can fail.
Someone who is starting to think about anti-virus evasion will see, that this can be reached easy (see for example last year’s DeepSec talk by Attila Marosi). If an attacker wants to hide a binary executable file with a Metasploit payload, the main points for accomplish this goal is mainly
- encrypting/encoding the payload and have an own shellcode binder for escaping signature scanning, and
- using a technique for evading the sandbox.
By developing further evasion techniques it is possible to research the internal functionality of anti-virus products. For example it can be determined whether a product is using x86 emulation or not, and what the emulation is capable of, and which Microsoft Windows API calls can disturb the anti-virus engine itself. Other tests include building an .exe file without a payload generated with msfpayload and well known attacking tools as well as 64-bit payloads and escaping techniques.
At the time of this writing Daniel Sauder developed 36 different techniques as proof of concept code and tested them against 8 different anti-virus products. More techniques and engines are pending. Together with documentation, papers, and talks from other researchers, this gives a deeper understanding for the functionality of anti-virus software and shows, where it is failing generally and in particular.
Anti-virus software is no magic solution that will always perfectly work. If you run filters of this kind, we recommend attending Daniel’s talk. Once you know how your defence mechanisms fail, you can work to improve them.