Monday, September 23, 2013

Shell Item Artifacts

I was watching the 9/20 Forensic Lunch with David Cowen and crew recently, and when Jonathan Tomczak TZWorks was initially speaking, there was a discussion of MFT file reference numbers found in shellbags artifacts.  Jonathan pointed out that these artifacts are also found in Windows shortcut/LNK files and Jump Lists.  From there, Dave posed a question (which I think was based off of the mention of Jump Lists), asking if this was an artifact specifically related to Windows 7.  As it turns out, this isn't so much a function of Windows 7, as how shell items are crafted on different versions of Windows; if you remember this post, shell items are becoming more and more prominent on Windows platforms.  They've existed in shellbags and LNK files since XP, and as of Windows 7, they can be found in Jump Lists (streams in Jump Lists are, with the exception of the DestList stream, LNK format).  Windows 8 has Jump Lists, as well, and thanks to Jason's research, we know that LNK-formatted data can also be found in the Registry.

Shell Items in the Registry
There are a number of RegRipper plugins that parse shell items;, (for Vista+ systems),,, (for Windows 8 systems). This simply illustrates how pervasive shell items are on the different versions of Windows.

Willi mentions the existence of these artifacts here, in his description of the ItemPos* section of the post; look for the section of code that reads:

if (ext_version >= 0x0007) {
        FILEREFERENCE file_ref;

What this says, essentially, is that for certain types of shell items, when a specific ext_version value is found (in this case, greater than 7, which indicates Vista...), there may be file reference available within the shell item.  I say "may be" to reiterate Jonathan's comments; I have only looked at a very limited set of artifacts, and Jonathan made no specific reference to the types of shell items that did or did not contain file reference numbers.

This is also mentioned in Joachim Metz's Windows Shell Item format specification, specifically on pg 25, within the discussion of the extension block.  Joachim has put a lot of effort into documenting a great deal of information regarding the actual structure of a good number of shell items; in his documentation, if the ext_version is 7 or greater, certain types of shell items appear to contain the MFT file reference.

So, again...this is not something that you should expect to see in all types of shell items...many types of shell items simply will not contain this information.  However, those shell items that point to files and folders...type 0x31, 0x32, 0xB1, etc...and those on Vista systems and beyond...may contain MFT file reference numbers.

I had a quick chat with David, and he pointed out that making use of the MFT file reference number from within the shellbags artifacts can show you what existed on the system at some point in the past, as the file reference number is essentially the MFT record number concatenated with the sequence number for the record.  This works in very well with David's TriForce analysis methodology, and can be extremely valuable to an examiner.

The only shortcoming I can see here is that the time stamps embedded within these shell items are not of the same granularity as the time stamps found within the MFT; see this MS API for translating FILETIME time stamps to DOSDate format, which is how the time stamps are stored in the shell items.  As such, the time values will be different from what's found in the MFT.

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