Forensics Quickie: Pinpointing Recent File Activity - RecentDocs

Let's revisit the basics...

Scenario
A suspicious employee left your company on January 28, 2014. You'd like to know which files were most recently used (opened, saved) on the employee's system right before he/she left.

The Solution
Pull the user's NTUSER.DAT. Run RegRipper to easily output the the values within the Software\Microsoft\Windows\CurrentVersion\Explorer\RecentDocs subkey.

To give some background, the RecentDocs subkey will contain a few things:

• Subkeys named after every file extension used on the system (e.g. a subkey for .zip, .doc, .mp4, etc.). 
• Each of these subkeys will contain its own MRUListEx value that keeps track of the order in which files were opened. (Sound familiar? We also saw use of the MRUListEx value upon analyzing shellbags artifacts). 
• Each subkey will store up to 10 numbered values; each numbered value represents a recently opened file with the extension found in the subkey's name.
  
 

The .gif subkey showing the 10 most recently opened .gif files for a specific user. The order is stored in MRUListEx.


• A subkey for folders most recently opened.
• Can store up to 30 numbered values (i.e. the 30 most recently opened folders).
• A root MRUListEx value
• Stores the 149 most recently opened documents and folders across all file extensions.

With that, take note that MRU times are powerful; here's why:

Since every extension has its own subkey, we will have quite a few registry key LastWrite times with which to work. But so what? When dealing with RecentDocs, the LastWrite time only applies to the most recently opened file within that subkey. While this is certainly the case, we mustn't forget that we also have an all-encompassing MRUListEx in the root of the RecentDocs subkey.

So what does this mean? By using the LastWrite times of each subkey and the root MRUListEx list, we can more accurately determine when certain files were last opened. While we won't be able to get an EXACT time for non-MRU (i.e. files that are not the first entry in the MRUListEx value), we may be able to determine a time range in which these files were opened. Consider the following example:

---

Our root MRUListEx shows:

RecentDocs
**All values printed in MRUList\MRUListEx order.
Software\Microsoft\Windows\CurrentVersion\Explorer\RecentDocs
LastWrite Time Thu Feb  6 14:08:49 2014 (UTC)

  26 = smiley.gif
  99 = strange_bird.jpg
  42 = books_to_read.csv
  130 = secret_clients.csv
  55 = zip_passwords.rtf
  87 = my_diet.csv
  3 = contacts.csv
  74 = phillip.csv
  72 = notes.txt
  ...

At this point, if we look at the first few entries of the root MRUListEx, we can really only tell that smiley.gif was last opened on Feb 6, 2014. So let's mark it:

  26 = smiley.gif ............................Feb  6 14:08:49 2014
  99 = strange_bird.jpg
  42 = books_to_read.csv
  130 = secret_clients.csv
  55 = zip_passwords.rtf
  87 = my_diet.csv
  3 = contacts.csv
  74 = phillip.csv
  72 = notes.txt
  ...

That secret_clients.csv looks pretty interesting. Let's look at the .csv subkey to see if we can find out when it was opened.

The .csv subkey's MRUListEx shows:

Software\Microsoft\Windows\CurrentVersion\Explorer\RecentDocs\.csv
LastWrite Time Sat Jan 18 19:19:22 2014 (UTC)
MRUListEx = 6,2,1,7,3,5,4
  
  6 = books_to_read.csv  
  2 = secret_clients.csv
  1 = my_diet.csv
  7 = contacts.csv
  3 = phillip.csv
  ...

Dang. Looks like the books_to_read.csv file is the most recently opened .csv, which means that the Jan 18 19:19:22 2014 LastWrite time of the .csv subkey is going to represent the last time that that file was opened -- not secret_clients.csv. I guess we can give up on this one...

...or...we could look at the surrounding files to glean some more information. First, let's mark the known LastWrite time in our root MRUListEx list.

  26 = smiley.gif ............................Feb  6 14:08:49 2014
  99 = strange_bird.jpg
  42 = books_to_read.csv...............Jan 18 19:19:22 2014
  130 = secret_clients.csv
  55 = zip_passwords.rtf
  87 = my_diet.csv
  3 = contacts.csv
  74 = phillip.csv
  72 = notes.txt
  ...

Looking at our list, we can see that there are two open times that we know for sure. We can also see that there is another file that was opened between these times. It looks like strange_bird.jpg was opened after smiley.gif, but before books_to_read.csv. Since this list is an all-encompassing list, it would be safe to say that between January 18 and Feb 6, strange_bird.jpg was opened. But let's go ahead and confirm that.

The .jpg subkey's MRUListEx shows:

Software\Microsoft\Windows\CurrentVersion\Explorer\RecentDocs\.jpg
LastWrite Time Sat Jan 18 21:54:57 2014 (UTC)
MRUListEx = 4,2,3,1
  
  4 = strange_bird.jpg 
  2 = stone_tanooki.jpg
  3 = camel.jpg
  1 = fighting_frogs.jpg

It looks like strange_bird.jpg is the most recently opened .jpg on the system. That means that the Jan 18 21:54:57 2014 LastWrite time of the .jpg subkey represents the the time at which strange_bird.jpg was opened. Again, let's mark it in our root MRUListEx list:

  26 = smiley.gif ............................Feb  6 14:08:49 2014
  99 = strange_bird.jpg...................Jan 18 21:54:57 2014
  42 = books_to_read.csv...............Jan 18 19:19:22 2014
  130 = secret_clients.csv
  55 = zip_passwords.rtf
  87 = my_diet.csv
  3 = contacts.csv
  74 = phillip.csv
  72 = notes.txt
  ...

At this point, we still don't know when secret_clients.csv was last opened. Upon finding its LNK file, we note that it had been first opened in late 2013 (LNK creation time). If this machine was running XP, we would be able to grab the access time of this LNK file to see when it *might* have been last opened. Alas, we are using Windows 7 (disclaimer: there are tons of other things you should be looking at anyway; this is merely one example of why we would want to look at RecentDocs). Let's keep moving.

Taking a second look at our root MRUListEx list, we notice that the next item after secret_clients.csv is zip_password.rtf. That also looks interesting, so let's see when that was opened.

The .rtf subkey's MRUListEx shows:

Software\Microsoft\Windows\CurrentVersion\Explorer\RecentDocs\.rtf
LastWrite Time Sat Jan 17 14:15:31 2014 (UTC)
MRUListEx = 2,1
  
  2 = zip_passwords.rtf 
  1 = fiction_novel.rtf

It looks to be the first one, so we can use the LastWrite time to mark it in our list.

  26 = smiley.gif ............................Feb  6 14:08:49 2014
  99 = strange_bird.jpg...................Jan 18 21:54:57 2014
  42 = books_to_read.csv...............Jan 18 19:19:22 2014
  130 = secret_clients.csv
  55 = zip_passwords.rtf.................Jan 17 14:15:31 2014
  87 = my_diet.csv
  3 = contacts.csv
  74 = phillip.csv
  72 = notes.txt
  ...

With that, we have found a pretty tight time frame as to when secret_clients.csv was last opened. We don't have the exact date and time, but we can now say that is was last opened somewhere between Jan 17 and Jan 18.

It looks as though there are some more .csv files that were recently opened, and by the looks of it, that .txt file might be able to help us hone in on a time frame.

The .txt subkey's MRUListEx shows:

Software\Microsoft\Windows\CurrentVersion\Explorer\RecentDocs\.txt
LastWrite Time Sat Jan 10 18:23:08 2014 (UTC)
MRUListEx = 4,5,2,1,3
  
  4 = notes.txt 
  5 = groceries.txt
  2 = how_to_make_toast.txt
  1 = my_novel.txt
  3 = address.txt
  ...

Just as before, let's mark the LastWrite time in our root MRUListEx list.

  26 = smiley.gif ............................Feb  6 14:08:49 2014
  99 = strange_bird.jpg...................Jan 18 21:54:57 2014
  42 = books_to_read.csv...............Jan 18 19:19:22 2014
  130 = secret_clients.csv
  55 = zip_passwords.rtf.................Jan 17 14:15:31 2014
  87 = my_diet.csv
  3 = contacts.csv
  74 = phillip.csv
  72 = notes.txt...............................Jan 10 18:23:08 2014
  ...

Again, we don't have an exact date and time as to when the my_diet.csv, contacts.csv, and phillip.csv files were last opened. But, based on the LastWrite time of the .txt subkey, we do know the time frame in which these files were opened.

Now imagine that a USB device was also plugged in at some point within this time frame. It is possible that the suspicious employee may have done a "Save As..." on contacts.csv and saved it to the external device. It is also possible that the employee opened the file and copy/pasted the information into an email. Of course, the possibilities are endless, but you get the point.

And it goes without saying that other artifacts, such as LNK files, jump lists, ComDlg32, OfficeDocs, and more should also be used to complement RecentDocs analysis. In particular, ComDlg32 would be of interest in this scenario, as it will show you files recently opened/saved. It offers more information (e.g. the executable used to open the file, the file's path, etc.) and stores saves/opens by extension, as well. As such, we can use this same technique to pinpoint file open/save times (though, it is a bit limited, as the * extension subkey will only show up to 20 entries).

This post quickly became longer than expected, but it's still a quickie. I'll end with this:

This is nothing new. All of this data is available to you in the Windows registry. This is a fairly basic technique, but it is also one that shouldn't be overlooked. And as always, use more than one artifact to make your case.

-Dan Pullega (@4n6k)

Fuente: http://www.4n6k.com/2014/02/forensics-quickie-pinpointing-recent.html