4n6k: May 2013

Everything I've learned on the subject of digital forensics has been a direct result of both experience and reading forensics books, blogs, and list-serv responses written by people like Ken Pryor, Harlan Carvey, Eoghan Casey, Chad Gough, Rob Lee, Lee Whitfield, Brad Garnett, David Kovar, Andrew Case, Dave Hull, Dan O'Day, Shafik Punja, Frank McClain, Cory Altheide, Joe Garcia, Hal Pomeranz, *deep breath* Troy Larson, Chad Tilbury, Andrew Hoog, Andrew Hay, cci, pr0neer, Eric Huber, Luby Novitovic, Michael Hale Ligh, Alex Bond, Jimmy Weg, Corey Harrell, Dan Farmer, Larry E. Daniel, Jamie Levy, Tom Yarrish, Andreas Schuster, Loc Nguyen, David Cowen, Didier Stevens, Joachim Metz, Ovie Carroll, David Nides, Brian Carrier, Jesse Kornblum, Patrick Olsen, and many more (I know I've forgotten some; I apologize to those I've missed). So I just wanted to start off with a quick 'thank you,' as you've all driven me to this point. Your work is very much appreciated.
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Before I get into the bulk of it all, let me note that UserAssist artifacts are nothing new. Didier Stevens and Harlan Carvey have written some great posts about what the UserAssist keys are and how they are laid out. Richard Drinkwater and Sploited have also posted about them. Harlan's Windows Registry Forensics also serves as a fantastic reference for not only this topic, but other registry artifacts as well; highly recommended. I will be covering the practical interpretation of the existence of these artifacts -- not their structures. So, as always, I would highly recommend that you take a look at all of the articles referenced at the bottom of this post for more in-depth information on the specifics. With that said, let's sum up what we already know about UserAssist artifacts.

UserAssist Summary

It is a registry key. It has values in subkeys that relate to each item executed on the system.

Location: NTUSER.DAT\Software\Microsoft\Windows\CurrentVersion\Explorer\UserAssist

Contains standard subkeys related to program/LNK file execution and toolbar interaction. Typical layout under UserAssist key is as follows:

Windows XP

{5E6AB780-7743-11CF-A12B-00AA004AE837}

Count

{75048700-EF1F-11D0-9888-006097DEACF9}

Count

Windows 7

{CEBFF5CD-ACE2-4F4F-9178-9926F41749EA}

Count

{F4E57C4B-2036-45F0-A9AB-443BCFE33D9F}

Count

Each count subkey contains ROT-13 encoded values; each value is a separate UserAssist entry.

Windows XP and Windows 7 UserAssist registry values have different binary structures.

Win7 adds time of app interaction/focus; minor changes to time offsets

There are many "UEME_" prefixes in Windows XP; they are nerfed in Windows Vista and further so (almost non-existent) in Windows 7.
The raw "run count" starts with the number "5" instead of "0." Most UserAssist tools will convert this number to alleviate the extra run counts, though (this is fixed in Win7).

It keeps track of the number of times applications/applets are launched (via shortcuts, Explorer shell, control panel, etc); this influences which items are automatically listed in the Start menu (i.e. programs used most often) for each user.

This differs from Prefetch execution count; prefetch count is not user-specific like UserAssist count.

It can keep track of how a program was launched (easier to determine on WinXP).

Forensic Value (or Why It's Important)

We now know the basics of what UserAssist artifacts are...so how can we use them to our advantage? Through them, we can determine the following:

Frequency of program execution -- per user.
Last time a program was launched.
From whence items were being launched most often.
System date/time changes.
Evidence of programs after deletion/uninstall.
How long a user has interacted with a given program (Win7).
Evidence of absence. (i.e. "items were in a specific location at one time." e.g. "My Documents" folder is empty...but was launched 224 times)

That last bullet is oddly specific because, well, I ran into that on a case recently. I was able to use information from the UserAssist key to determine that a [now empty] My Documents folder was opened 200+ times and was launched immediately before file copy activity and system cleaning/wiping. Couple this with shellbag artifacts showing what used to be there and the fact that the My Documents folder was not being redirected and...well, you get the point. These artifacts are valuable -- especially those found within the UserAssist key.

Pre-Testing (or What To Expect And Why)

Much of my interest and curiosity in regard to UserAssist artifacts was spawned by reviewing timelines. I had always seen red UserAssist entries in my colored timelines, but I never completely understood the forensic potential that they held. In the case above, I saw one entry in particular that confused me at the time:

This timeline entry shows the usual elements: time, artifact + type of source, type of artifact, and a [short] description of the artifact value. Since red entries suggest Program Execution and we see that the sourcetype is a UserAssist key, we shift our focus to the artifact value. To offset any confusion right of the bat, let's take a look at a more standard UserAssist timeline entry first:

We see the "UEME_" prefix followed by "RUNPATH" and then the full path to the executed program. This essentially tells us that the program, CCleaner, which is located at the given path, was run in some way at 8:13:53.

If you'd like, you could stop there...or...you could dig a little deeper to understand exactly how and why you are seeing what you're seeing. I suggest the latter. So, now that we have a little better understanding of what that value means, let's go back to our initial, confusing entry:

We see the familiar "UEME_" prefix and "RUNPATH." But then we see a triple colon ":::" followed by "[My Documents] VIRTUAL." Based on our CCleaner entry, we might expect to see a full path to an executable. So what gives?

What we see above is an artifact left as a result of a user opening a Windows virtual folder (in this case, My Documents). These folders are also known as special folders. Other special folders include the Recycle Bin, My Network Places, and the like. Log2timeline is nice enough to let us know that this is a virtual folder by ending the value with "VIRTUAL."

On that note, it's important to understand what log2timeline is doing to allow you to see that "[My Documents] VIRTUAL" portion of the value. The log2timeline source tells us that when log2timeline comes across a "CSIDL" or "known folder" GUID, it will translate the GUID to its human-readable counterpart (there are many officially recognized GUIDs, and even more unofficially recognized GUIDs). To illustrate this, consider the following output from RegRipper's userassist module:

Mon Jan 7 13:48:31 2013 Z
UEME_RUNPATH:::{450D8FBA-AD25-11D0-98A8-0800361B1103} (224)

This is the same exact data we are seeing in the timeline entry (with the added run count in parentheses); the only difference being that the {450D8FBA-AD25-11D0-98A8-0800361B1103} GUID is translated to "[My Documents] VIRTUAL" in the timeline. Of course, you will have to look up any GUIDs that do not have a name associated with them in log2timeline.

As the above example suggests, UserAssist artifacts will show up with more than just paths to executables. You will also see entries that point to certain folders and LNK files (e.g. a user double-clicking the Recycle Bin folder on the desktop; a user double-clicking a program shortcut; etc).

UserAssist Entry Variations

The above gives an example of what we'll see when we encounter a RUNPATH entry. I would argue that this kind of entry will be the most common that you will see. There are, however, some different UserAssist entries, as well. The absolute best explanation of these entry types that I have found is from Didier Stevens's UserAssist article in the first issue of Into The Boxes:

UEME_CTLSESSION: This entry is for the session ID, it doesn't hold data about executed programs

UEME_UIQCUT: Counts the programs launched via a Quick Launch menu shortcut

UEME_UISCUT: Counts the programs launched via a Desktop shortcut

UEME_RUNCPL: This entry keeps data about executed control applets (.cpl)

UEME_RUNPATH: This entry keeps data about executed programs

UEME_RUNPIDL: This entry keeps data about executed PIDLs

UEME_UITOOLBAR: This entry keeps data about clicks on the Windows Explorer Toolbar buttons

In my experience, the RUNPATH, RUNCPL, and RUNPIDL entries serve as the most useful. We'll see some examples of what causes each of these entries in the following section.

Testing (or How It Really Looks In Practice)

Tests were run on some of the more common user activities (and POC actions) to see exactly how UserAssist entries are triggered. The 13 scenarios below illustrate how UserAssist entries look both in the registry and in timelines after a given user action is performed. Note that the applications used are merely examples and can be substituted with countless others.

Action #1: Click Internet Explorer via Start Menu

We see a virtual folder for the default internet browser along with the Internet Explorer executable