DeXRAY – Decrypting VBN files, Part 2

September 21, 2012 in Batch Analysis, Compromise Detection, DeXRAY, File Formats ZOO, Forensic Analysis, Incident Response, Malware Analysis, Software Releases

A few months back I posted about DeXRAY –  a generic script/file carver that tries to decrypt various Quarantine files + PE files hidden inside other files under a simple layer of a single-byte xor encryption.

Some time later, in one of my other posts I mentioned that newer VBN files used by Symantec Quarantine use a different encryption scheme; instead of using well-known xor with a 0x5A key, they now use 0xA5 key.

Turns out that I was right about it, but only partially as there is a twist to it – not only these files are encrypted, but they are also divided into chunks separated by a 5 byte ‘chunk divider’ in a form of 0xF6 0x?? 0x?? 0xFF 0xFF. So, to reconstruct the encrypted Quarantine files, one needs to decrypt them with 0xA5 first and then remove the chunk dividers.

Simple, isn’t?

I updated DeXRAY.pl to handle this (quick & dirty patch, but it should work). If you find some VBN files that don’t get decrypted at all or get corrupted after decryption, please let me know.

I bet Symantec guys added this to prevent accidental detection of Quarantine files by theirs and other AV companies’ scanners; the way I think it goes is that many AV companies use X-rays technique during scans (that is, they may find malware even if it is encrypted with a single byte xor) and they could potentially/accidentally decrypt and detect the Quarantine file during scans. That could potentially lead to some ‘funny’ results (recursive detection, etc.); introducing ‘chunk divider’ breaks the file format of the encrypted file and AV scans no longer can ‘understand’ the corrupted file structure (especially if it is an encrypted PE file).

Download a new version from here:  DeXRAY.pl

 

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