Monday, June 28, 2010

Skillz

Remember that scene from Napoleon Dynamite where he talks about having "skillz"? Well, analysts have to have skillz, right?

I was working on a malware exam recently...samples had already been provided to another analyst for reverse engineering, and it was my job to analyze acquired images and answer a couple of questions. We knew the name of the malware, and when I was reviewing information about it at various sites (to prepare my analysis plan), I found that when the malware files are installed, their MAC times are copied from kernel32.dll. Okay, no problem, right? I'll just parse the MFT and get the time stamps from the $FILE_NAME attribute.

So I received the images and began my case in-processing. I then got to the point where I extracted the MFT from the image, and the first thing I did was run David Kovar's analyzemft.py against it. I got concerned after I ran it for over an hour, and all I got was a 9Kb file. I hit Ctrl-C in the command prompt and killed the process. I then ran Mark Menz's MFTRipperBE against the file and when I opened the output .csv file and ran a search for the file name, Excel told me that it couldn't find the string. I even tried opening the .csv file in an editor and ran the same search, with the same results. Nada.

Fortunately, as part of my in-processing, I had verified the file structure with FTK Imager, and then created a ProDiscover v6.5 project and navigated to the appropriate directory. From there, I could select the file within the Content View of the project and see the $FILE_NAME attribute times in the viewer.

I was a bit curious about the issue I'd had with the first two tools, so I ran my Perl code for parsing the MFT and found an issue with part of the processing. I don't know if this is the same issue that analyzemft.py encountered, but I made a couple of quick adjustments to my Perl script, and I was able to fairly quickly get the information I needed. I can see that the file has $STANDARD_INFORMATION and $FILE_NAME attributes, as well as as data attribute, that the file is allocated (from the flags), and that the MFT sequence number is 2. Pretty cool.

The points of this post are:

1. If you run a tool and do not find the output that you expect, there's likely a reason for it. Validate your findings with other tools or processes, and document what you do. I've said (and written in my books) that the absence of an artifact where you would expect to find one is itself an artifact.

2. Analysts need to have an understanding of what they're looking at and for, as well as some troubleshooting skills, particularly when it comes to running tools. Note that I did not say "programming" skills. Not everyone can, or wants to, program. However, if you don't have the skills, develop relationships with folks who do. But if you're going to ask someone for help, you need to be able to provide enough information that they can help you.

3. Have multiple tools available to validate your findings, should you need to do so. I ran three tools to get the same piece of information, of which I had documented the need in my analysis plan prior to receiving the data. One tool hung, another completed without providing the information, and I was able to get what I needed from the third, and then validate it with a fourth. And to be honest, it didn't take me days to accomplish that.

4. The GUI tool that provided the information doesn't differentiate between "MFT Entry Modified" and "File Modified"...I just have two time stamps from the $FILE_NAME attribute called "Modified". So I tweaked my own code to print out the time stamps in MACB format, along with the offset of the MFT entry within the MFT itself. Now, everything I need is documented, so if need be, it can be validated by others.

3 comments:

Anonymous said...

Great article. I just came from Chad Tilbury's SANS 408 class in Balt & he stressed using multiple tools and validating your findings.
As far as "skillz", what in your opinion would be the most important technical skills a forensic examiner would need to be successful.
thanks

Keydet89 said...

The most important skills an analyst should possess are not technical...the desire to learn, and documenting what you do are probably the most valuable "skills" you can have.

David Kovar said...

Greetings,

The bug in analyzeMFT has since been fixed. Someone ran into the same problem and sent me the $MFT that drove the bug.

-David