Thursday, August 4, 2011

Carving Symantec VBN Files

Those of you who perform IT support or incident response are most likely intimate with corporate antivirus products. While the usefulness of antivirus can be debated, the purpose of this post is to provide some insight into the file structure of Symantec's quarantine files. It is not uncommon for an IT practitioner or an incident responder to restore and perform further analysis on a malicious file to verify the attackers intent. Someone recently posted to the Windows Forensics email group about having issues restoring quarantined files from Symantec Endpoint Protection (SEP) 11 which prompted me to put together this quick post.

Symantec does provide a utility called QExtract that allows for the extraction of quarantined files. Documentation on the syntax of the command line utility can be found in Symantec's online knowledge base. As an example, the following is the output obtained from using the /DETAILED switch with qextract.exe on a system that the Mebroot rootkit payload was detected on.

QExtract can restore the malicious file by using the session ID, file name, or risk name obtained from this output (see the aforementioned documentation for syntax). The utility works, but is limited. It only runs on Windows. Additionally, you cannot point QExtract to an alternate source location. If SEP is not installed, then the default path to the quarantine files must be manually created. Moreover, when restoring something from a quarantine file, the original path of the file must exist or restoration will fail.

The file structure of the quarantine files in Symantec's AV products has been known for some time, however. Since 2007 there has been an Encase script available that will extract these files. SEP Quarantine files, also known as Virus Bin (VBN) files, are located in the C:\ProgramData\Symantec\Symantec Endpoint Protection\Quarantine folder. For the purpose of this post, I am looking at the detection of the aforementioned Mebroot rootkit. Some details including hashes and statistics from Virus Total are as follows.
Symantec: Trojan.Mebroot
MD5: fd543137a51fc24e07e00f9bc7c3c06e
SHA1: 357ac149ba2c864a5f0fc2276c2fa437b5c5533b
Looking at a VBN file using X-Ways WinHex Editor we see the file begins with the original location of the detected malware. At offset 0x00000184 (byte 388) SEP stores additional information on detection of the malicious file including the system name, original location/name of file, time of detection, and Symantec unique record ID.

At offset 0x00000E68 (byte 3688) we see something else. It appears that the data has been obfuscated or encrypted. Note the that the value 0x5A is common throughout the file. What are the chances that these are actually spaces (0x20) and the data was XOR'd with the value of 0X5A?

Using Winhex to inverse XOR with the value of 0x5A gives us the malicious file. Note: the file signature of 0x4D5A (MZ) which is for a Windows/DOS executable file.

To carve out the Mebroot payload, simply copy the selected block to a new file and save it.

I would imagine this will work with previous versions of Symantec Corporate Edition but the offsets may be different. If anyone has any experience in that regard let me know.

Happy Hunting!

1 comment:

  1. This is super helpful info, thanks for this post!