Journey Into Incident Response

Data destruction is an anti-forensic technique where data is deleted to limit the amount of forensic evidence left on a system. One data destruction anti-forensic technique leveraged by malware are self-deleting droppers and downloaders. The use of this technique can be seen in the Damballa paper Behind Today's Crimeware Installation Lifecycle: How Advanced Malware Morphs to Remain Stealthy and Persistent which outlines the malware installation lifecycle. In Windows, a program cannot delete itself so malware has to use other avenues to accomplish this. The article Self-deleting Executables: Techniques which allow an executable file to delete itself whilst running goes into detail about these avenues while the article ZeroAccess's Way of Self-Deletion discusses another one. A common avenue used by malware to limit the available forensic evidence is the self-deleting batch file method. If this method is used then there may be evidence of this activity in the Windows prefetch files and NTFS Change Journal.

Self-Deleting Batch File Method

The Catch22 Self-deleting Executables article discusses the self-deleting batch file method. The article states the method "works because MS-DOS batch files are able to delete themselves." All an executable has to do if it wants to delete itself is to create a batch file with code to delete its executable." Next the executable needs to spawn off the batch file using CreateProcess, and then should exit immediately". The batch script will proceed to delete the executable then itself. On a system this action removes valuable information about the infection vector.

To see how this method works I'll use the following Zbot sample MD5 0dc43fa765b042c13b51667e9c05d6e7. The image below shows the malware (bot.exe) creating a batch script named tmp1d2cc71c.bat.


The purpose of the tmp1d2cc71c.bat batch script is to delete the bot.exe executable and then to delete itself as shown in the image below. Notice the "if exist" statement creates a loop to force the bot.exe deletion.


Shortly after the tmp1d2cc71c.bat batch script creation the malware proceeds to launch a command prompt to execute the script.


Looking at the "Process Create" event details shows the command-line the malware used to start the command prompt.


The command prompt proceeds to load; one activity that occurs during process creation is the creation and/or reading of the command prompt prefetch file. 


The command prompt uses the SetDispositionInformationFile function to delete the bot.exe file. The ZwSetInformationFile routine with the FileDispositionInformation parameter "sets the DeleteFile member of a FILE_DISPOSITION_INFORMATION to TRUE, so the file can be deleted when ZwClose is called to release the last open handle to the file object." In other words, the bot.exe file gets deleted when it is closed. The images below show this event in process monitor and the properties of this event where the DeleteFile member is set to True.




Remember the "if exist" statement in the batch script? The image below shows cmd.exe checking to see if the bot.exe exists. The result is "no such file" so the script exits the loop and continues its execution.

Again, the command prompt uses the SetDispositionInformationFile function to delete the tmp1d2cc71c.bat batch script before it exits.

Traces of the Self-Deleting Batch File Method

The self-deleting batch file method used by downloaders and droppers limits the amount of forensic evidence left on a system. Specifically, it removes the downloaders and droppers themselves so they can't be analyzed in a method similar to the process above. However, this method leaves traces on a system to help determine that this method was used. I've seen these traces numerous times left in the Windows prefetch files and the NTFS Change Journal.

 

Prefetch Files

Windows prefetch files contain file paths to the files accessed when the application was starting. Sometime ago Harlan touched on this in his post Prefetch Analysis, Revisited...Again... and I did as well in my post Second Look at Prefetch Files. Traces of the self-deleting batch file method may be left in the command prompt (cmd.exe) prefetch file; this is usually my first indication that this method was used.

Again, let's illustrate this with the Zbot sample MD5 0dc43fa765b042c13b51667e9c05d6e7. The image below shows some suspicious executables (bot.exe and anku.exe) prefetch files followed by a modification to the cmd.exe prefetch file. Note: the modification to the cmd.exe prefetch file may not always align with the malware prefetch files if the command prompt has been used since the initial infection.


The indication that the self-deleting batch file method was used are references to batch scripts located in temporary folders. Looking closer at the cmd.exe prefetch file - as shown below - contains a reference to the tmpccc11cdb.bat  batch script which is located in the temp folder.

NTFS Change Journal

The NTFS Change Journal ($UsnJrnl) contains a wealth of information about file system activity which can provide more context about what occurred on a system. I discussed the importance of this artifact in the post Re-Introducing $UsnJrnl and I even showed how it can help identity the self-deleting batch file method. The $UsnJrnl can help provide more context about the tmpccc11cdb.bat  batch script found in the cmd.exe prefetch file.

The image below shows how the $UsnJrnl recorded the creation of the malware with the file name bot.exe.


The $UsnJrnl even records activity for other malware that gets dropped onto the system such as for the executable anku.exe as shown below.


The image below shows the traces of the self-deleting batch file method in the $UsnJrnl. First the batch script (tmpccc11cdb.bat) is created which is followed by the deletion of an executable (bot.exe) and the batch script itself (tmpccc11cdb.bat).

Wrapping Things Up

When we are confronted with systems impacted by malware it is a given we will encounter numerous anti-forensics techniques. Most likely we will see less forensic evidence left on a system due to data destruction. As the Locard's exchange principle states when two objects come into contact, something from one is exchanged to the other. This principle applies to anti-forensics and traces of this activity may be left on the system. Together, prefetch files and the NTFS Change Journal can shed light on the activity for the self-deleting batch file method.

The NTFS change journal ($UsnJrnl) is not a new artifact and has been discussed before by others. The file's importance may have been overlooked since it wasn’t available in Windows XP by default. As more and more systems running newer Windows operating systems cross our desks it is crucial to know about the $UsnJrnl file. This is because starting with Windows Vista and continuing into Windows 7 the operating systems use the change journal. The $UsnJrnl file contains a wealth of information about file system activity which can provide more context about what occurred on a system. The information can even counter anti-forensics techniques leveraged by malware to subvert post-mortem examinations. Read on to get reacquainted with this NTFS artifact.

What is the $UsnJrnl File?

According to the book File System Forensic Analysis, the change journal “is a file that records when changes are made to files and directories”. The change journal is located at $Extend\$UsnJrnl and the actual journal entries are stored in the $UsnJrnl:$J alternate data stream. The Windows Internals Part 2: Covering Windows Server 2008 R2 and Windows 7 book details the information contained in each entry:

     - Time of change
     - Reason for the change
     - File/directory’s name
     - File/directory’s attributes
     - File/directory’s MFT record number
     - File record number of the file’s parent directory
     - Security ID
     - Update Sequence Number of the record
     - Information about the source of the change

Others have already highlighted the $UsnJrnl file’s forensic significance. One of which was Bugbear over at the Security Braindump. In his post Dear Diary: AntiMalwareLab.exe File_Created he not only goes into detail about the $UsnJrnl file structure but he even shared how the artifact was beneficial on a case. The change journal’s significance is not limited to malware type cases; it’s relevant to any type of case where it is important to know what changes occurred to the NTFS file system. Case in point, on a fraud case the $UsnJrnl file showed how a user performed a mass deletion of financial files.

Malware Anti-Forensic Techniques

There are various definitions explaining what anti-forensics is. The one I tend to prefer is Marcus Rogers where he stated anti-forensics are “attempts to negatively effect the existence, amount and/or quality of evidence from a crime scene, or make the analysis and examination of evidence difficult or impossible to conduct”. The Rootkit Arsenal 2nd Edition goes on to explain that the goal of anti-forensics is to “minimize the quantity of useful trace evidence that’s generated in addition to ensuring that the quality of this information is also limited (as far as a forensic investigation is concerned).” Malware can leverage anti-forensics to undermine examinations conducted post mortem and live as well as against the malware itself. As it relates to post mortem examinations, two anti-forensic strategies are data hiding and data destruction; both of which can be countered using the information in the $UsnJrnl file.

Data Hiding in Obscure Location

As the name implies, data hiding is a technique to store data where it is unlikely to be found. Remember the Rootkit Paradox and the first part of the predicament; they want to remain hidden. One area malware tries to hide is in obscure locations on the system with hopes that people won’t notice it.

Some ZeroAccess Trojan variants (i.e. MD5 3dbb09aa9b6eff5a180b3412b54e9862) use the recycle bin as a hiding location. The best way to illustrate this is to use a hypothetical scenario where an IDS alert provided the lead to use when examining the $UsnJrnl:$J file. Hypothetically, let’s say an IDS alert went off around 12/14/2012 15:38 UTC for Zeroaccess Outbound UDP traffic. The $J file was parsed with TZwork’s Windows Journal Parser (command used was: jp64.exe –file $J) and the output showed the following at the time of interest:


The output shows at 15:38:30 the following items were created: $5da39e9580074308c6cfbcce61795d0d, L, U, @, and n. The $J file entries only contain file/directory names but the MFT entry can be used to obtain the full file path. The first item of interest is $5da39e9580074308c6cfbcce61795d0d and its MFT entry is 0x00000000a477 (converted to decimal the value is 42103). Locating the MFT record 42103 in the parsed $MFT showed the following:


The item’s full path was /$Recycle.Bin/S-1-5-21-2793522790-2301028668-542554750-1000/$5da39e9580074308c6cfbcce61795d0d. Correlating the MFT entries listed in the $J file for the L, U, and @ files shows they were located inside the $5da39e9580074308c6cfbcce61795d0d directory. The information in the $J file was able to reveal the hiding spot used by this ZeroAccess variant.

Data Destruction

Data destruction is a technique where data is deleted to limit the amount of forensic evidence left on a system. The technique makes it more challenging to determine what happened. Two data destruction techniques to thwart post-mortem forensics are self deleting droppers/downloaders and overwriting file system metadata.

Self Deleting Droppers/Downloaders

A dropper is a type of program when executed will install malicious components. Typically, the malicious components are embedded within the dropper itself. A downloader on the other hand is a program when executed will download and install malicious components. One anti-forensic technique leveraged by droppers and downloaders is to delete the initial executable after its done its job. By deleting the executable it removes evidence about the initial infection vector. One issue faced with self deletion is that Windows does not allow for a process to delete itself. To get around this issue malware can use a self dissolving batch script or another type of shell script. Basically, the malicious process will spawn a new shell process (such as the command prompt) and the shell process will perform the deletion. The ZeroAccess Trojan MD5 3dbb09aa9b6eff5a180b3412b54e9862 is a self deleting dropper and the image below is the portion of the $J file where the Trojan (9862.exe) was deleted.


The change journal logged the 9862.exe file being deleted followed by the cmd.exe prefetch file being modified. The most interesting thing is over time the MFT record for the 9862.exe file will be reused and the cmd.exe prefetch file will change; both of which may limit the ability to determine the 9862.exe file was self deleting. However, the change journal may still maintain entries showing what occurred on the system when other artifacts no longer do.

Overwriting File System Metadata

The timestamps stored in the NTFS metadata are crucial when trying to determine what happened on a system. The metadata includes timestamps which are used to construct a timeline of activity showing how the events unfolded that resulted in a compromise. One anti-forensic technique leveraged by malware is to overwrite the timestamps making it harder to build a timeline. The ZeroAccess Trojan MD5 b437274492e215c25f96711d04b44050 uses two different methods to over write timestamps.

The $MFT maintains two sets of timestamps for files and directories; the Standard Information (SIA) and Filename Attribute (FNA). Most times when timestamps are overwritten it only impacts the SIA timestamps leaving the FNA timestamps intact. Comparing the FNA timestamps to the SIA timestamps is an effective way to determine if timestamps have been altered. To illustrate the following are the timestamps from the file /Users/lab/AppData/Local/{5da39e95-8007-4308-c6cf-bcce61795d0d}/n

Standard Information Attribute

Creation:       7/13/2009 23:11:59 UTC
Access:         7/13/2009 23:11:59 UTC
Modification:  7/14/2009 1:17:52 UTC


Filename Attribute

Creation:       12/6/2012 22:18:00 UTC
Access:         12/6/2012 22:18:00 UTC
Modification:  12/6/2012 22:18:00 UTC

The n file’s timestamps have been altered making it appear as if the file was created and last modified back in 2009. The $J file provides another method to determine if a file’s timestamps were altered since it records when items are created. The image below shows the $J file entry when the n file was created.


As can be seen, the entry reveals the n file was created on 12/6/2012 at 22:18:00 UTC as opposed to 7/13/2009 23:11:59 UTC which was reflected in the SIA timestamp . To detect timestamp manipulation the file/directory’s creation date can be compared to its record in the $J file.

     File System Tunneling

When I was looking at the artifacts associated with the ZeroAccess Trojan levering the NTFS Extended Attributes I quickly noticed how the malware altered its files’ timestamps. I even found the activity in the $J file showing the services.exe file being modified. However, the activity didn’t become clear until I shared the $MFT file with Harlan. Harlan was parsing the $MFT file when he noticed the Trojan used file system tunneling to alter the services.exe timestamps. Jason Hale over at The Digital Forensics Stream post File System Tunneling in Windows does an outstanding job demonstrating how file system tunneling works. Microsoft’s explanation about file system tunneling in Windows is to:

“enable compatibility with programs that rely on file systems being able to hold onto file meta-info for a short period of time. This occurs after deletion or renaming and re-introducing a new directory entry with that meta-info (if a create or rename occurs to cause a file of that name to appear again in a short period of time).”

File system tunneling means if a file is renamed or deleted and then if a file with the same name is created in the same directory then the new file will retain the timestamps from the old file. The $UsnJrnl file contains entries showing file system tunneling was used and this is what became clear after Harlan told me what he saw.


The entries show that the original services.exe file was renamed and within a fraction of a second a new services.exe file was created in the same directory. As shown below the new services.exe file retained the timestamps from the original:

Standard Information Attribute

Creation:       7/13/2009 23:11:26 UTC
Access:         7/13/2009 23:11:26 UTC
Modification:  7/14/2009 1:14:36 UTC


Filename Attribute

Creation:       7/13/2009 23:11:26 UTC
Access:         12/6/2012 22:18:06 UTC
Modification:  12/6/2012 22:18:06 UTC

Both the SIA and FNA timestamps indicate the services.exe file was created back in 2009 but the entries in the $J file shows otherwise. The services.exe was actually created on 12/6/2012 and file system tunneling was used to alter its timestamps.

Summary

The $UsnJrnl file contains a wealth of information about file system activity and its worthy for us to re-introduce ourselves to this artifact. This re-introduction shouldn’t be viewed in the same light as when Jay Z reintroduced himself. Jay Z was saying he was the top MC when he said “allow me to re-introduce myself. My name is Hov’, OH, H to the OV”. The $UsnJrnl is only one of the available NTFS artifacts; it’s not even the only artifact that logs file system activity. The $Logfile is another NTFS artifact with entries recording activity. David Cowen has been releasing information about the $Logfile and the program he wrote to parse it. A few posts include: Updates and DFIR Conferences discussing what the NTFS $Log Parser is and PFIC 2012 Slides & Bsides DFW containing a link to his slide deck where he breaks down the $Logfile. I highly recommend reading what David posted if you haven’t already. The $Logfile is another artifact that contains a wealth of information. The limited testing I did showed it contains information which wasn’t present in the $MFT and $UsnJrnl files. And vice versa about the $UsnJrnl compared to the $Logfile and $MFT. To get an accurate picture about file system activity then all the artifacts ($MFT, $Logfile, and $UsnJrnl) should be examined. Combining the information inside each artifact will counter anti-forensics techniques leveraged by malware to thwart post-mortem examinations.