Friday, February 17, 2006

Determining group membership from an image

A while back, I received this question..."how do I figure out a user's group membership from an image?" Well, I didn't know off the top of my head, and decided that knowing would be useful, so I started digging around. After receiving some much-needed insight from a friend on one of the online forums I frequent, I was able to create a ProScript (Perl script) for use with ProDiscover. I posted the script to their online forum, along with one to enumerate users from an image...and that one is just the beginning, as it only retrieves the user's name from the V structure. There is still quite a bit of information embedded in the F and V structures.

In a nutshell, the way it works is this...within the HKLM\SAM\SAM\Domains\Builtin\Aliases key, there are several subkeys...00000220, etc. These keys have a value named "C", which is binary, and contains the name of the group, the comment describing the group, etc. The first 52 (13 DWORDS) bytes of this value is the header, and the last three DWORDs describe the offset to the listing of user SIDs, the length of the data, and how many users there are. One of the SIDs for the Administrators group will end with the RID of "1F4", which is 500 in decimal...the Administrator account.

Now, to map the user RID to a username, go to HKLM\SAM\SAM\Domains\Account\Users key, where you will find subkeys that look like 000001F4 and 00000E3B, etc. These are the user RIDs, and user info is maintained in the binary V and F values within the key. The offset to the username (ofsName) is stored in the DWORD located 0xC bytes into the binary V structure (the length of the username is maintained in the next DWORD, which starts at offset 0x10). The username itself is in Unicode format and is found at offset 0xCC + ofsName from the beginning of the structure.

So...pretty cool, eh? I got started down the path of looking to the Builtin\Aliases key by running one of the 'net' commands (to enumerate group membership) while also running RegMon, and then filtering on the process I wanted. From there, a little work with a hexeditor and a little help from a friend went a long way.

It's really no surprise that there's nothing in the above post that points to Microsoft as providing assistance of documentation...that information simply is not available. It's not the fault of the folks who have done their best to assist me (and others) over the years...they can't provide what doesn't exist. What I've had to do is go to Linux-based documentation, talk to others, experiment on my own, etc.

On a side note, sometimes when I'm working with or discussing something with someone, particularly things related to the forensic analysis of Windows systems, I'll ask them if they have any sort of documentation or reference. This isn't meant as an indictment of them...I'm not asking them to prove anything, though most of the times, the response I get seems to indicate that they were seriously offended by the question. No, I'm asking for that simply because sometimes I may be able to correlate information from some sort of documentation with something else I've been looking at, or some other little tidbit of information I mayhave. Whenever possible, I try to provide references and/or documentation for what I do as it not only gives everyone a common base to work from, but it also lets others see what I've been looking at, so that they don't have to relearn all that stuff on their own.

Thoughts?

3 comments:

Andreas Schuster said...

After that certain post to the windowsforensics group I finally motivated myself to take some screenshots. So if you're looking for some eyecandy, look here:
http://computer.forensikblog.de/en/2006/02/list_members_of_a_windows_group.html

Kind regards, Andreas

Keydet89 said...

Andreas,

VERY Cool! Thanks for the link, and the comment.

I'm definitely going to be scouring...I mean, checking out your blog next week.

Andreas Schuster said...

Harlan,

thanks for the heads-up, I've added some extra bandwidth for you ;) Most content is in the german section though. Sorry, I can't cope with the translations. Please tell me if you'd like to see a certain article in the english section and I'll try my very best to put it there ASAP.