Reverse malware

Topics: Cryptographic hash function, File format, Filename extension Pages: 8 (2821 words) Published: October 11, 2013
There are a number of schools of thought on how to approach reversing malware. Some people jump right into dynamic analysis in effort to quickly learn what the specimen is doing so they can put rules in place on their network to stop it's functionality or see who else might be infected. Some of these people, will then perform static analysis to see if they may have missed something. Others leave it at the results found from their dynamic analysis. Other people do static analysis first to fully understand the expected behavior so they know if something is happening with the sample when running it in a dynamic lab other than what is expected. They will then run dynamic analysis on it to see if their findings are correct. Some people just submit the sample to places like Virus Total, Anubis, or CWSanbox, just to name a few . They then take action based on the results they get back from these tools. Finally there are some that just submit it to their Anti Virus vendor and wait for a signature to be released.

There is nothing wrong with any of these methods. Most are done because of either lack of time, skills or understanding of how to reverse malware. Some may think, why reinvent the wheel? This is all OK.

Overall process of static analysis:
The first step in your process should be to start a log or collection of the details you are about to find. Some people do this is a text file, others may just jot things down in a notebook. Others like to use a mind mapping software such as FreeMind. Lenny Zeltser, a SANS instructor and an overall excellent resource for reversing knowledge, has freely released a template for FreeMind specifically for analyzing malicious code. You can download it here.

If you are looking for very good training in this area, Lenny also offers a few courses with SANS that can be taken online or at a conference. He has specifically created, along with others, the Forensics 610: Reverse Engineering Malware , and the Security 569: Combating Malware in the Enterprise courses. I highly encourage you to take these courses if you are interested in malware analysis.

It doesn't matter which method you choose to write your notes down, but it is very important that you do. Documenting this will help you to keep on track and assist you in writing reports of your analysis if you do this professionally. Additionally if you choose a method that allows you to compile all of your findings centrally, you will be able to see trends or recognize similar behaviors of samples that could help in reversing future samples that exhibit similar characteristics.

With that aside, take note of the system you found the malware on. Take notice of the operating system, patch level, applications installed etc. Write down where you found the code (i.e. C:\windows\system32). Add any information that may be relevant on how the code was found (i.e. the system administrator noticed the system was running slowly, or found the system blue screened).

Next take a hash of the file or files found. A hash, in this case, is a mathematical computation on the bits of a specimen. This will help with identification of other copies of the malware even if the name is changed. You can think of this much like doctors and scientists look for specific characteristics of a virus in the human body. That way, other doctors or scientists can identify the same virus in another person. It is generally accepted to perform a MD5 hash on the file. Some people will also do a SHA1 or other computations as well. There is also a newer method called fuzzy hashing or peicewise hashing that can be done. This actually hashes portions of a file, rather than the whole thing. This method allows for identification of portions of the code which may be useful in catching malware that has changed just a little in order to avoid detection by a person or Anti Virus application for example.

There are many tools out there to do this. WinMD5 is an easy to use tool which is freely...
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