Wednesday, July 18, 2018

Daily Blog #425: How I Use It: Userassist

Hello Reader,
             I'm currently teaching in Abu Dhabi and hanging out with my family at night which means I'm not investing the time to do the next level of MAPI testing I need to do. Instead after the warm reception yesterdays post received I thought I would follow it up with a new series I will add to over time called ' How I Use It'.

Often times we talk about artifacts and evidence sources and how to interpret them, in fact there are so many that most people often forget what they know about them. What we don't often talk about is how we as examiners use that data within their casework to make conclusions or points.

So in this first post in this series of how I use different artifacts I want to talk about the Userassist key. This isn't a new key, it's been around since I first saw it in 2002 and wrote about it in the first Hacking Exposed Computer Forensics book in 2004 but people seem to overlook its usefulness.

Userassist records those programs that a user has executed from the GUI, that I would hope is well known at this point. I've posted about it once in 2013 (http://www.hecfblog.com/2013/08/daily-blog-45-understanding-artifacts.html) and even earlierin 2009 (http://www.hecfblog.com/2009/03/what-did-they-take-when-they-left-part_25.html) both times I didn't really go into detail of what I use it for.

So here are my main analysis points from reviewing this artifacts:


1. What kinds of programs is my suspect executing?


It's difficult to judge the technical proficiency of a suspect from the statements of the people who knew them as their frame of reference in judging their technical abilities is usually focused around how well they use Excel or Outlook. So rather than letting their statements of their abilities to be 'master hackers'  I like to see what kinds of GUI programs they've been executing.

Is it just Office, Outlook and IE?  Or are they loading regedit, going into the command prompt and looking into different system mmc's.  The difference in what they are executing helps me judge the types of artifacts I should expect to find and how closely I need to inspect the dates and artifacts the system is showing me.

In addition I can look for evidence of encryption tools (Veracrypt/Truecrypt), wipers (eraser/bcwipe) and even anti forensics tools (Ccleaner, everyone's favorite). All from one helpful key that will even tell me how many times they've executed it.


2. How far back does the Userassist go? Is it complete?


The Userassist key starts populating data when your first profile is first created and you've logged in for the first time. That means that the history of programs within the key should go back as far as the user profile creation. If there is a gap, especially if it is a large gap, you could be seeing evidence of anti forensics. Start looking for what happened immediately before the gap began to see what could have cleared the data.

Also remember that deleted registry keys, just like deleted files, are not gone just because we delete them. So make sure to use a tool that will show you any potential deleted userassist keys or values. tools like Registry Explorer, YARU and others can expose this data to you while exploring the keys.

3. What email clients is my suspect using?


I'm often looking for my suspects email archives. Rather than guessing what Email client they are using (yes some people don't use Outlook) I can just go to the Userassist key to find out. Unless you have a very interesting suspect using a command line based email reader (pine in windows subsystem for linux?)  the rest are GUI based and should be recorded. If I find no email clients I need to look for what web mail services my suspect is using in their browser history.


4. What web browsers is my suspect using?


Lastly in my normal inspection of the Userassist I'm looking to understand what web browsers by suspect is using. It is not uncommon now for a suspect to be using 2, 3 or even 4 different web browsers on their system during one day. So I use this as a sanity check to make sure:

1. I'm looking for history from all of those browsers, its easy to miss one and focus on the others
2.  I need to make sure my forensic tools support the browser being used, I'm looking at you Maxthon, as its easy to miss a whole browser history cache because you didn't realize the limits of your tool.

So that's the first 4 questions I would answer when reviewing this key and I review this key early in my investigation along with other basic artifacts (execution history, file access and device usage) to start understanding what to expect from this system.


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