Category Archives: Digital Forensics and Incident Response

Digital Forensics – PlugX and Artifacts left behind

When an attacker conducts an intrusion using A, B or C technique, some of his actions leave artifact X, Y or Z behind. So, based on the scenario from the last article about PlugX, I collected a disk image and memory image from the domain controller. Over the past years I wrote several articles on how to perform acquisition, mounting and processing of such images and analyze them by creating super timelines, look at different artifacts like Event Logs, Prefetch, ShimCache, AMCache, etc., or analyze NTFS metadata or look for artifacts related to interactive sessions. Today, I’m not going to perform analysis but I’m going to list a quick overview about some of the Windows endpoint artifacts that might give us evidence about the actions that were executed in the previous scenario and help us produce a meaningful timeline. In addition, I list some tools that could be used to analyze those artifacts.

Scenario 1: The attacker placed the filename “kas.exe” on the folder “c:\PerfLogs\Admin”. Which artifacts could record evidence about this action?

  • NTFS MFT
    • Description: The Master File Table (MFT) is a special system file that resides on the root of every NTFS partition. The file is named $MFT and is not accessible via user mode API’s but can been seen when you have raw access to the disk e.g, forensic image. This special file is a hierarchical database and inside you have records that contains a series of attributes about a file, directory and indicates where it resides on the physical disk and if is active or inactive. The size of each MFT record is usually 1024-bytes. Each record contains a set of attributes. Some of the most important attributes in a MFT entry are the $STANDART_INFORMATION, $FILENAME and $DATA. The first two are rather important because among other things they contain the file time stamps. Each MFT entry for a given file or directory will contain 8 timestamps. 4 in the $STANDARD_INFORMATION and another 4 in the $FILENAME. These time stamps are known as MACE.
    • Tools: Parse and analyze it with SleuthKit originally written by Brian CarrierMFT2CSV from Joakim Schicht or PLASO/log2timeline originally created by Kristinn Gudjonsson
  • NTFS INDX Attribute
    • Description: The MFT records for directories contain a special attribute called $I30. This attribute contains information about file names and directories that are stored inside a directory. This special attribute is also known as $INDX and consists of three attributes, the $INDEX_ROOT, $INDEX_ALLOCATION and $BITMAP. So, What? Well, this attribute stores information in a B-tree data structure that keeps data sorted so the operating system can perform fast searches in order to determine if a file is present. In addition, this attribute grows to keep track of file names inside the directory. However, when you delete a file from a directory the B-tree re-balances itself but the tree node with metadata about the deleted file remains in a form of slack space until it gets reused. This means we can view the $I30 attribute contents and we might find evidence of files that once existed in a directory but are no longer there.
      Tools: o Parse it and analyze it with INDXParse from William Ballenthin or MFT2CSV from Joakim Schicht.
  • NTFS $LogFile
    • Description: NTFS has been developed over years with many features in mind, one being data recovery. One of the features used by NTFS to perform data recovery is the Journaling. The NTFS Journal is kept inside NTFS Metadata in a file called $LOGFILE. This file is stored in the MFT entry number 2 and every time there is a change in the NTFS Metadata, there is a transaction recorded in the $LOGFILE. These transactions are recorded to be possible to redo or undo file system operations. After the transaction has been logged then the file system can perform the change. When the change is done, another transaction is logged in the form of a commit. The $LOGFILE allows the file system to recover from metadata inconsistencies such as transactions that don’t have a commit. The size of the $LOGFILE can be consulted and changed using chkdsk /l and per default is 65536 KB. Why would $LOGFILE be important for our investigation? Because the $LOGFILE keeps record of all operations that occurred in the NTFS volume such as file creation, deletion, renaming, copy, etc. Therefore, we might find relevant evidence in there.
    • Tools: Parse it and analyze it with LogFileParser from Joakim Schicht
  • NTFS $UsnJrnl
    • Description: The change journal contains a wealth of information that shouldn’t be overlooked. Another interesting aspect of the change journal is that allocates space and deallocates as it grows and records are not overwritten unlike the $LogFile. This means we can find old journal records in unallocated space on a NTFS volume. How to obtain those? Luckily, the tool USN Record Carver written by PoorBillionaire can carve journal records from binary data and thus recover these records
    • Tools: Parse and analyze it with UsnJrnl2Csv from Joakim Schicht or from unallocated space with USN Record Carver from PoorBillionaire.

Scenario 2: Which account did the attacker used to log into the system when he placed “kas.exe” on the file system?

  • Windows Event Logs
    • Description: The Windows Event logs record activities about the operating system and its applications. What is logged depends on the audit features that are turned thus impacting the information that one can obtain. From a forensic perspective the Event Logs capture a wealth of information. The main three Windows Event Logs are Application, System, and Security and on Windows Vista and beyond they are saved on %System32%\winevt\Logs in a binary format. For example the Event id’s 4624, 4625 might give us answers.
    • Tools: Parse it and Analyze it with PLASO/Log2timeline, LibEvtx-utils from Joakim Schicht , python-evtx from William Ballenthin or Event Log Explorer. You likely get better results if in your environment if you have consistent and enhanced audit policy settings defined that track both success and failures. In case the attacker  deletes the Windows Event Logs, there is the possibility to recover Windows Event Log records from the pagefile.sys or from unallocated space, from Volume Shadow copies or even the system Memory. You could use EVTXtract from Willi Ballenthin to attempt to recover Event logs from raw data.

Scenario 3: Attacker executed the “kas.exe” binary. Which artifacts might record this evidence?

  • Windows Prefetch / Superfetch
    • Description: To improve customer experience, Microsoft introduced a memory management technology called Prefetch. This functionality was introduced into Windows XP and Win-dows 2003 Server. This mechanism analyses the applications that are most frequently used and preloads them in advance in order speed the operating system booting and application launching. On Windows Vista, Microsoft enhanced the algorithm and introduced SuperFetch which is an improved version of Prefetch. The Prefetch files are stored in %SYSTEMROOT%\Prefetch directory and have a .pf extension. The Superfetch files have a .db extension. Prefetch files keep track of programs that have been executed in the system even if the original file is no longer present. In addition Prefetch files can tell you when the program was executed, how many times and from which path.
    • Tools: PLASO/log2timeline, Windows-Prefetch-Parser from Adam Witt, Prefetch Parser from Eric Zimmerman. For Superfetch you could use SuperFetch tools.
  • ShimCache either from Registry or from Kernel Memory
    • Description: Microsoft introduced the ShimCache in Windows 95 and it remains today a mechanism to ensure backward compatibility of older binaries into new versions of Microsoft op-erating systems. When new Microsoft operating systems are released some old and legacy application might break. To fix this Microsoft has the ShimCache which acts as a proxy layer between the old application and the new operating system. A good overview about what is the ShimCache is available on the Microsoft Blog on an article written by Tim Newton “Demystifying Shims – or – Using the App Compat Toolkit to make your old stuff work with your new stuff“. The interesting part is that from a forensics perspective the ShimCache is valuable because the cache tracks metadata for binary that was executed and stores it in the ShimCache.
    • Tools: From Kernel memory, you can parse it and analyze it with Volatility ShimCache and ShimCacheMem plugin. From the Registry you can use ShimCacheParser https://github.com/mandiant/ShimCacheParser. You can also use RegRipper from Harlan Carvey or AppCompatCacheParser from Eric Zimmerman. In addition, to analyze ShimCache artifacts at scale you can use AppCompatProcessor from Mattias Bevilacqua,
  • AMCache
    • Description: On Windows 8, Amcache.hve replaced the RecentFileCache.bcf file, a registry file used in Windows 7 as part of the Application Experience and Compatibility feature to ensure compatibility of existing software between different versions of Windows. Similar to its predecessor, Amcache.hve is a small registry hive that stores a wealth of information about recently run applications and programs, including full path, file timestamps, and file SHA1 hash value. Amcache.hve is commonly found at the following location: C:\Windows\AppCompat\Programs\Amcache.hve. The Amcache.hve file is standard within the Windows 8 operating system, but has been found to exist on Windows 7 systems as well.
    • Tools: To read the amcache HIVE you could use RegRipper or Willi Ballenthin stand-alone script or Eric Zimmerman AmcacheParser. To analyze AMCache artifacts at scale you can use AppCompatProcessor from Mattias Bevilacqua,
  • Windows Event Logs. 
    • The Windows Event logs – for example id 4688 – could track binary execution if you have the proper audit settings or you use Sysmon.

Scenario 4: The execution of “kas.exe” dropped three files on disk that used DLL Search Order Hijacking to achieve persistence and install the malicious payload. Which artifacts might help identifying this technique?

Identifying evidence of DLL Search Order hijacking is not easy if no other leads are available. Likely you need a combination of artifacts. The following artifacts / tools might help.

  • NTFS MFT, INDX, $LogFile, $UsnJrnl.
  • Prefetch / SuperFetch.
  • ShimCache either from Registry or from Kernel Memory.
  • AMCache.
  • Windows Event Logs could track process execution and give you leads if you have the proper audit settings or you use Sysmon
  • Volatility to perform memory analysis.
  • RegRipper – One thing you could try, among many others that this powerful tool allows,is to identify different persistence mechanism that could have resulted as part of the DLL Search Order Hijacking technique.
  • AppCompatProcessor to analyze ShimCache and AMCache at scale combined with with PlugX signatures.

Scenario 5: The PlugX dropped files have the NTFS timestamps manipulated i.e., It copies the timestamps obtained from the operating system filename ntdll.dll to set the timestamps on the dropped files. What artifacts could be used to detect this?

The time modification will cause a discrepancy between the NTFS $STANDART_INFORMATION and $FILENAME timestamps. You could combine the NTFS artifacts with the execution artifacts to spot such anomalies.  Other technique you could use is with AppCompat Processor which has the Time Stomp functionality that will search for appcompat entries outside of the Windows,  System and SysWOW64 folders with last modification dates matching a list of known operating system files.

Scenario 6: Attacker used the PlugX controller to Invoke a command shell and execute Windows built-in commands. Are there any artifacts left behind that could help understand commands executed?

  • ShimCache either from Registry or from Kernel Memory.
  • Memory analysis with Volatility and look for Process creation, Console History, cmdscan or consoles plugin.
  • The Windows Event logs could track process execution if you have the proper audit settings or you use Sysmon.

Scenario 7: Attacker established a persistence mechanism either using a Service or Registry Key. 

  • Producing a timeline of the Registry would help identify the last modification dates of the registry keys. You could use RegRipper from Harlan Carvey or RECmd from Eric Zimmerman. The Windows Event logs would also help in case the there was a service created on the operating system. For example Event ID 7009, 7030, 7035, 7036, 7040, 7023 or 7045 could help. In addition, to list the services and its properties you could perform memory analysis with Volatility or use RegRipper.

Scenario 8: The attacker accessed the Active Directory database using the “ntdsutil.exe” command. What could be used to detect this activity?

  • As we saw previously, command execution could be identified using ShimCache either from Registry or from Kernel Memory. Because “ntdsutil.exe” would be executed on a Server system, Prefetch won’t help here because its not enabled on Server systems. One of the most usefull artifacts would be the Windows Event logs but you need to have the right settings so it could track binary execution and the interactions with the Active Directory. One thing that might help in case the memory image has been acquired not long after the attacker activity is to perform memory analysis and creating a timeline of the artifacts with Volatility might help identifying the process creation and its parent(s). In addition, you might get interesting leads just by running strings (little and  big endian) on the pagefile.sys. Other than that, the execution of “ntdsutil.exe” the way it was executed on the scenario, leaves behind artifacts on the NTFS metadata.

That’s it for today. With this article I presented a quick listing on some artifacts and tools that can help you perform forensic analysis on a system and help you answer your investigative questions. Many other tools and artifacts would be available depending on the attacker activities, for example if the attacker logged into a system interactively, but the ones listed might give you a starting point and might help you understand what happened and when. One thing that would greatly complement the findings of a system forensic analysis the network data such as the ones that comes from Firewall, Router, IDS or Proxy logs or any other kind of networking logs you might have. Specially if attacker is using a C2 and is clearing evidence such as the threat group that used a file named “a.bat” to clean several artifacts as illustrated on the “Paranoid PlugX” article written by Tom Lancaster and Esmid Idrizovic from Unit 42.

Happy hunting and If you have dealt with a security incident where PlugX was used, please leave your comments about the tools or techniques you used to detect it.

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Malware Analysis – PlugX – Part 2

Following my previous article on PlugX, I would like to continue the analysis but now use the PlugX controller to mimic some of the steps that might be executed by an attacker. As you know the traditional steps of an attack lifecycle follow, normally, a predictable sequence of events i.e., Reconnaissance, initial compromise, establish foothold, escalate privileges, internal reconnaissance, move laterally, maintain persistence, complete mission. For sake of brevity I will skip most of the steps and will focus on the lateral movement.I will use the PlugX controller and C2 functionality to simulate an attacker that established a foothold inside an environment and obtained admin access to a workstation. Following that, the attacker moved laterally to a Windows Domain Controller. I will use the PlugX controller to accomplish this scenario and observe how an attacker would operate within a compromised environment.

As we saw previously, the PlugX controller interface allows an operator to build payloads, set campaigns and define the preferred method for the compromised hosts to check-in and communicate with the controller. In the PlugX controller, English version from Q3 2013, an operator can build the payload using two techniques. One is using the “DNS Online” technique which allows the operator to define the C2 address e.g, an URL or IP address, that will be used by the payload to speak with the C2. The other method, is the “Web Online”, which allows the operator to tell the payload from where it should fetch the C2 address. This method allows the operator to have more control over the campaign. The following diagram illustrates how the “Web Online” technique works.

Why do I say this technique would allow an attacker to have more control? Consider the case that an organization was compromised by a threat actor that use this PlugX technique. In case the C2 is discovered, the impacted organization could block the IP or URL on the existing boundary security controls as a normal reaction to the concerns of having an attacker inside the network. However, the attacker could just change the C2 string and point it to a different system. In case the organization was not able to scope the incident and understand the TTP’s (Tools, Tactics and Procedures) then the attacker would still maintain persistence in the environment. This is an example that when conducting incident response, among other things, you need to have visibility into the tools and techniques the attacker is using so you could properly scope the incident and perform effective and efficient containment and eradication steps. As an example of this technique, below is a print screen from a GitHub page that has been used by an unknown threat actor to leverage this method.

So, how to leverage this technique on the PlugX builder? The picture below shows how the operator could create a payload that uses the “Web Online” technique. The C2 address would be fetched from a specified site e.g. a Pastebin address, which on its turn would redirect the payload to the real C2 address. The string “DZKSAFAAHHHHHHOCCGFHJGMGEGFGCHOCDGPGNGDZJS” in this case is used to decode to the real C2 address which is “www.builder.com”. On the “PlugX: some uncovered points” article, Fabien Perigaud writes about how to decode this string. Palo Alto Unit42 gives another example of this technique on the “Paranoid PlugX” article. The article “Winnti Abuses GitHub for C&C Communications” from Cedric Pernet ilustrates an APT group leveraging this technique using GitHub.

For sake of simplicity, in this article, I’m going to use the DNS Online technique using “www.builder.com” as C2 address. Next, on the “First” tab I specify the campaing ID and the password used by the payload to connect to the C2.

Next, on the Install tab I specify the persistence settings, in this case I’m telling the payload to install a service and I can specify different settings including where to deploy the binaries, the service name and service description. In addition, I can specify that if the Service persistence mechanism fails due to some reason the payload should install the persistence mechanism using the Registry and I can specify which HIVE should be used.

Then, In the inject tab I specify which process will be injected with the malicious payload. In this case I choose “svchost.exe”. This will make PlugX start a new instance of “svchost.exe” and then inject the malicious code into svchost.exe process address space using process hollowing technique.

Other than that, the operator could define a schedule and determine which time of the week the payload should communicate with the C2. Also the operator could define the Screen Recording capability that will take screenshots at a specific frequency and will save them encrypted in a specific folder.

Last settings on the “option” tab allow the operator to enable the keylogger functionality and specify if the payload should hide it self and also delete itself after execution.

Finally, after all the settings defined, the operator can create/download the payload in different formats. An executable, binary form (Shellcode), or an array in C that can then be plugged in another delivery mechanism e.g, PowerShell or MsBuild. After deploying and installing the payload on a system, that system will check-in into the PlugX controller and an operator can call the “Manager” to perform the different actions. In this example I show how an attacker, after having compromised a system, uses the C2 interface to:

  • Browse the network

  • Access remote systems via UNC path

  • Upload and execute a file e.g., upload PlugX binary

  • Invoke a command shell and perform remote commands e.g., execute PlugX binary on a remote system

Previous pictures illustrate actions that the attacker could perform to move laterally and, for example, at some point in time, access a domain controller via UNC path, upload the PlugX payload to a directory of its choice and execute it. In this case the pictures show that the PlugX payload was dropped into c:\PerfLogs\Admin folder and then was executed using WMI. Below example shows the view from the attacker with two C2 sessions. One for one workstation and another for a domain controller.

Having access to a domain controller is likely one of the goals of the attacker so he can obtain all the information he needs about an organization from the Active Directory database.

To access the Active Directory database, the attacker could, for example, run the “ntdsutil.exe” command to create a copy of the “NTDS.dit” file using Volume Shadow Copy technique. Then, the attacker can access the database and download it to a system under his control using the PlugX controller interface. The picture below illustrates an attacker obtained the relevant data that was produced using the “ntdsutil.exe” command.

Finally, the attacker might delete the artifacts that were left behind on the file system as consequence of running “ntdsutil.exe”.

So, in summary, we briefly looked at the different techniques a PlugX payload could be configured to speak with a Command and Controller. We built, deploy and install a payload. Compromised a system and obtain a perspective from PlugX operator. We move laterally to a domain controller and installed the PlugX payload and then used a command shell to obtain the Active Directory database. Of course, as you noted, the scenario was accomplished with an old version of the PlugX controller. Newer versions likely have many new features and capabilities. For example, the print screen below is from a PlugX builder from 2014 (MD5: 534d28ad55831c04f4a7a8ace6dd76c3) which can create different payloads that perform DLL Search order hijacking using Lenovo’s RGB LCD Display Utility for ThinkPad (tplcdclr.exe) or Steve Gibson’s Domain Name System Benchmarking Utility (sep_NE.exe). The article from Kaspersky “PlugX malware: A good hacker is an apologetic hacker” outlines a summary about it.

That’s it! With this article we set the table for the next article focusing on artifacts that might helps us uncover the hidden traits that were left behind by the attacker actions performed during this scenario. Stay tuned and have fun!

 

 

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Digital Forensics – Artifacts of interactive sessions

In this article I would like to go over some of the digital forensic artifacts that are likely to be useful on your quest to find answers to investigative questions. Specially, when conducting digital forensics and incident response on security incidents that you know the attacker performed its actions while logged in interactively into a Microsoft Windows systems. Normally, one of the first things I look is the Windows Event logs. When properly configured they are a treasure trove of information, but in this article, I want to focus on artifacts that can be useful even if an attacker attempts to cover his tracks by deleting the Event Logs.

Let’s start with ShellBags!

To improve the customer experience, Microsoft operating systems stores folder settings in the registry. If you open a folder, resize its dimensions, close it and open it again, did you notice that Windows restored the view you had? Yep, that’s ShellBags in action. This information is stored in the user profile hive “NTUSER.dat” within the directory “C:\Users\%Username%\” and in the hive “UsrClass.dat” which is stored at “%LocalAppData%\Microsoft\Windows”. When a profile is loaded into the registry, both hives are mounted into the HKEY_USERS and then then linked to the root key HKEY_CURRENT_USER and HKEY_CURRENT_USER\Software\Classes respectively. If you are curious, you can see where the different files are loaded by looking at the registry key “HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\hivelist”. On Windows XP and 2003 the ShellBags registry keys are stored at HKEY_USERS\{SID}\Software\​Microsoft\Windows\Shell\ and HKEY_USERS\{SID}\Software\​Microsoft\Windows\ShellNoRoam\.  On Windows 7 and beyond the ShellBags registry keys are stored at “HKEY_USERS\{SID}_Classes\​Local Settings\Software\​Microsoft\Windows\Shell\”.

Why are ShellBags relevant?

Well, this particular artifact allows us to get visibility about the intent or knowledge that a user or an attacker had when accessing or browsing directories and, when. Even if the directory does no longer exists. For example, an attacker that connects to a system using Remote Desktop and accesses a directory where his toolkit is stored. Or an unhappy employee that accesses a network share containing business data or intellectual property weeks before his last day and places this information on a USB drive. ShellBags artifacts can help us understand if such actions were performed. So, when you obtain the NTUSER.dat and UsrClass.dat hives you could parse it and then placed events into a timeline.  When corroborated with other artifacts, the incident response team can reconstruct user activities that were performed interactively and understand what happened and when.

Which tools can we use to parse ShellBags?

I like to use RegRipper from Harlan Carvey, ShellBags Explorer from Eric Zimmerman or Sbags from Willi Ballenthin. The below picture shows an example of using Willi’s tool to parse the ShellBags information from the NTUSER.dat and UsrClass.dat hives. As an example, this illustration shows that the attacker accessed several network folders within SYSVOL and also accessed “c:\Windows\Temp” folder.

To give you context, why I’m showing you this particular illustration of accessing the SYSVOL folder, is because they contain Active Directory Group Policy preference files that in some circumstances might contain valid domain credentials that can be easily decrypted. This is a known technique used by attackers to obtain credentials and likely to occur in the beginning of an incident. Searching for passwords in files such as these are simple ways for attackers to get credentials for service or administrative accounts without executing credential harvesting tools.

Next artifact on our list, JumpLists!

Once again, to increase the customer experience and accelerate the workflow, Microsoft introduced on Windows 7 the possibility to allow a user to access a list of recently used applications and files. This is done by enabling the feature to store and display recently opened programs and items in the Start Menu and the taskbar. There are two files that store JumpLists information. One is the {AppId}.automaticDestination-ms and the other is {AppId}.customDestination-ms where {AppId} corresponds to a 16 hex string that uniquely identifies the application and is calculated based on application path CRC64 with a few oddities. These files are stored in the folder “C:\Users\%USERNAME%\AppData\​Roaming\Microsoft\Windows\​Recent\AutomaticDestinations” and “C:\Users\%USERNAME%\AppData\​Roaming\Microsoft\Windows\​Recent\CustomDestinations”. The folder AutomaticDestinations contain files {16hexchars}.automaticDestination-ms and these files are generated by common operating system applications and stored in a in Shell Link Binary File Format known as [MS-SHLLINK] that are encapsulated Inside a Compound File Binary File Format known as MS-CFB or OLE. The folder CustomDestinations contain files {16hexchars}.customDestination-ms and these files are generated by applications installed by the user or scripts there were executed and stored in Shell Link Binary File Format known as [MS-SHLLINK].

Why are JumpLists relevant?

Just like like ShellBags, this artifact allows us to get visibility about the intent or knowledge an attacker had when opening a particular file, launching a particular application or browsing a specific directory during the course of an interactive session. For example, consider an attacker that is operating on a compromised system using Remote Desktop and launches a browser, the JumpList associated with it will contains the most visited or the recently closed website. If the attacker is pivoting between system using the Terminal Services client, the JumpList shows the system that was specified as an argument. If an attacker dumped credentials from memory and saved into a text file and opened it with Notepad, the JumpList will show evidence about it. Essentially, the metadata stored on these JumpList files can be parsed and will show you a chronological list of Most Recently Used (MRU) or Most Frequently Used (MFU) files opened by the user/application. Among other things, the information contains the Standard Information timestamps from the list entry and the time stamps from the file at the time of opening. Furthermore, it shows the original file path and sizes. This information, when placed into a timeline and corroborated with another artifact can give us a clear picture of the actions performed.

Which tools can we use to parse JumpLists?

JumpListsView from NIRSOFT, JumpLister from Mark Waon or JumpLists Explorer from Eric Zimmerman. Below an example of using Eric’s tool to parse the JumpLists files. More specifically the JumpList file that is associated with Notepad. As an example, this illustration shows that an attacker opened the file “C:\Windows\Temp\tmp.txt”with Notepad. It shows when the file was created and the MFT entry. Very useful.

Next artifact, LNK files!

Again, consider an attacker operating on a compromised system using a Remote Desktop session where he dumped the credentials to a text file and then double clicked on the file. This action will result in the creation of the corresponding Windows shortcut file (LNK file). LNK files are Windows Shortcuts. Everyone that has used Windows has created a shortcut of your most favorite folder or program. However, the Windows operating system behind the scenes also keeps track of recently opened files by creating LNK files within the directory “C:\Documents and Settings\%USERNAME%\Recent\”.   The LNK files, like JumpLists, are stored in Shell Link Binary File Format known as [MS-SHLLINK]. When parsed, the LNK file, contains metadata that, among other things, shows the target file Standard Information timestamps, path, size and MFT entry number. This information is maintained even if the target file does no longer exists on the file system. The MFT entry number can be valuable in case the file was recently deleted and you would like to attempt to recover by carving it from the file system.

Which tools can we use to parse .LNK files?

Joachim Metz has an utility that to parse the information from the Windows Shortcut files. The utility is installed by default on SIFT workstation. In the illustration below, while analyzing a disk image, we could see that there are several .LNK files created under a particular profile. Knowing that this profile has been used by an attacker you could parse the files. In this case parsing, when parsing the file “tmp.lnk” file we can see the target file “C:\Windows\Temp\tmp.txt”, its size and when was created.

Next artifact, UserAssist!

The UserAssist registry key keeps track of the applications that were executed by a particular user. The data is encoded using ROT-13 substitution cipher and maintained on the registry key HKEY_USERS\{SID}\Software\​Microsoft\Windows\CurrentVersion​\Explorer\UserAssist.

Why is UserAssist relevant?

Consider an attacker operating on a compromised system where he launched “cmd.exe” to launch other Windows built-in commands, or opened the Active Directory Domains and Trusts Snap-in “domain.msc” to gather information about a particular domain, or launched a credential dumper from an odd directory. This action will be tracked by the UserAssist registry key. The registry key will show information about which programs have been executed by a specific user and how frequently. Due to the nature of how timestamps are maintained on registry ie., only the last modified timestamp is kept, this artifact will show when was the last time that a particular application was launched.

Which tools can we use to parse the UserAssist registry keys?

Once again RegRipper from Harlan Carvey is a great choice. Another tool is UserAssist from Didier Stevens. Other method that I often use is to use log2timeline using Windows Registry plugin and then grepping for the UserAssist parser. In this example, we can see that an attacker while operating under a compromised account, executed “cmd,exe”, “notepad.exe”and “mmc.exe”. Now combining these artifacts with the Shellbags, JumpLists and .LNK files, I can start to interpret the results.

Next artifact, RDP Bitmap Cache!

With the release of RDP 5.0 on Windows 2000, Microsoft introduced a persistent bitmap caching mechanism that augmented the bitmap RAM cache. With this mechanism, when you do a Remote Desktop connection, the bitmaps can get stored on disk and are available for the RDP client, allowing it to load them from disk instead of waiting on the latency of the network connection. Of course this was developed with low bandwidth network connections in mind. On Windows 7 and beyond the cache folder is located on “%USERPROFILE%\AppData\Local\Microsoft\Terminal Server Client\Cache\ ” and there two types of cache files. One that contains a .bmc extension and a newer format that was introduced on Windows 7 that follows the naming convention of “cache{4-digits}.bin’. Both files have tiles of 64×64 pixels. The .bmc files support different bits per pixel ranging from 8-bits to 32-bits. The .bin files are always 32-bits and have more capacity and a file can store up to 100Mb of data.

Why are RDP Bitmap cache files relevant?

If an attacker is pivoting between systems in a particular environment and is leveraging Remote Desktop then, on the system where the connection is initiated you could find the bitmap cache that was stored during the attacker Remote Desktop session. After reconstructing the bitmaps, that translate what was being visualized by the attacker, it might be possible to reconstruct the bitmap puzzle and observe what was seen by the attacker while performing the Remote Desktop connections to the compromised systems. A great exercise for people who like puzzles!

Which tools can we use to parse RDP Bitmap Cache files?

Unfortunately, there aren’t many tools available. ANSSI-FR released a RDP Bitmap Cache parser that you could use to extract the bitmaps from the cache files. There was a tool called BmcViewer that was available on a now defunct website and is great tool to parse the .bmc files. The tool doesn’t support the .bin files. If you know how to code, an interesting project might be to develop a parser that allows you to puzzle the tiles.

Finally, combining these artifacts with our traditional file system metadata timeline and other artifacts such as ShimCache, would allows us to further uncover more details. Below an illustration of parsing ShimCache from a memory image using Volatility and the ShimCacheMem plugin written by Fred House. We could see that there are some interesting files. For example “m64.exe” and looking at the adjacent entries we can see that it shows the execution of “notepad.exe”, “p64.exe” and “reg.exe”. Searching for those binaries on our file system timeline uncovers that for example m64.exe is Mimikatz.

That’s it for today! As I wrote in the beginning, the Windows Even Logs are a treasure trove of information when properly configured but If an attacker attempts to cover his tracks by deleting the Event Logs, there are many other artifacts to look for. Combine the artifacts outlined in this article with File system metadata, ShimCache, AMCache, RecentDocs, Browser History, Prefetch, WorldWheelQuery, ComDlg32, RunMRU, and many others and you likely will end up having a good understanding of what happened and when. Happy hunting!

References:
PS: Due to the extensive list of references I decided to attach a text file with links: references. Without them, this article won’t be possible.

Luttgens, J., Pepe, M., Mandia, K. (2014) Incident Response & Computer Forensics, 3rd Edition
Carvey, H. (2011) Windows Registry Forensics: Advanced Digital Forensic Analysis of the Windows Registry, Second Edition
SANS 508 – Advanced Computer Forensics and Incident Response

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Analysis of a Master Boot Record – EternalPetya

NoPetya or EternalPetya has kept the security community pretty busy last week. A malware specimen that uses a combined arms approach and maximizes its capabilities by using different techniques to sabotage business operations. One aspect of the malware that raised my interest was the ability to overwrite the Master Boot Record (MBR) and launch a custom bootloader. This article shows my approach to extract the MBR using digital forensic techniques and then analyze the MBR using Bochs. Before we roll up our sleeves let’s do a quick review on how the MBR is used by today’s computers during the boot process.

The computers that rely on BIOS flash memory instead of the new EFI standard, when they boot, the BIOS code is executed and, among other things, the code performs a series of routines that perform hardware checks i.e., Power-On-Self-Tests (POST). Then, the BIOS attempts to find a bootable device. If the bootable device is a hard drive, the BIOS reads the sector 1, track 0, head 0 and if contains a valid MBR (valid means that the sector ends with bytes 0xAA55) it will load that sector into a fixed memory location. By convention the code will be loaded into the real-mode address 0000:7c00. Then, the instruction pointer register is transferred into that memory location and the CPU will start executing the MBR code. What happens next is dependable on the MBR implementation code i.e., different operating systems have different MBR code  Nonetheless, the code needs to fit in the 512-bytes available at disk sector. The MBR follows a standard and its structure contains executable code, the partition table (64-bytes) with the locations of the primary partitions and finally 2-bytes with 0xAA55 signature. This means only 446-bytes are available to implement a MBR. In the Microsoft world, when the MBR code is executed, its role is to find an active partition, read its first sector, which contains the VBR code, load it into memory and transfer execution into it. The VBR is a bootloader program that will find the Windows BootMgr program and executes it All this happens in 16-bits real-mode.

Now, that we have a brief overview about the boot process, how can we extract and analyze the MBR? More specifically the MBR that is used by EternalPetya? Well, we infect a victim machine on a controlled and isolated environment. We know that EternalPetya main component is a DLL and we can launch it and infect a Windows machine by running “rundll32.exe petya.dll, #1 10”. Our setup consisted of 2 Virtual Machines. One running with Windows 7 and another running REMnux. We created a snapshot of the victim machine before the infection. Then executed the malware. Following that, we waited 10 minutes for the infection to complete. The scheduled task created by the malware restarted the operating system and a ransom note appeared. Then, I shutdown the Windows 7 virtual machine and used vmware-vdiskmanager.exe utility to create a single VMDK file from the disk state before and after the infection. Next, I moved the VMDK files to a Linux machine where I used QEMU to convert the VMDK images to RAW format.

Following that I could start the analysis and look at the MBR differences. The picture below illustrates the difference between the original MBR and the EternalPetya MBR. On the right side you have the EternalPetya MBR, the first 147 bytes (0x00 through 0x92) contain executable code.  The last 66 bytes (0x1be through 0x1fd) contain the partition table and are equal to the original MBR.

So, we are interested in the code execution instructions. We can start by extracting the MBR into a binary file and convert it to assembly instructions. This can be done using radare, objdump or ndisasm. Now, this is the hard part of the analysis. Read the assembly instructions and understand what it does. We can look at the instructions and perform static analysis but we can also perform dynamic analysis by running the MBR code, combining both worlds we will have better understanding – or at least we can try.

To perform dynamic analysis of the MBR code we will use Bochs. Bochs is an open source, fully fledged x86 emulator. Originally written by Kevin Lawton in 1994 is still being actively maintained today and last April version 2.6.9 was released. Bochs brings a CLI and GUI debugger and is very useful to debug our MBR code.  In addition, Bochs can be integrated with IDA PRO and Radare.  You can download Bochs from here.  In our case, we want to use Bochs to dynamically debug our MBR code. For that we need a configuration file called Bochsrc which is used to specify different options such as the disk image and also the CHS parameters for the disk. This article from Hex-Rays contains a how-to on how to integrate Bochs with IDA PRO. At the end of the article there is the mbr_Bochs.zip file that you can download. We will use these files to use Bochs standalone or combined with IDA PRO in case you have it. The Bochsrc file that comes with the ZIP file contains options that are deprecated on the newer Bochs version. The picture below shows the Bochsrc options I used. The Bochs user guide documents this file well.

Then you can try your configuration setup and launch Bochs. If you have IDA PRO then you can use this guide starting from step 6 in order to integrate it with IDA PRO. If all is set up, the debugging session will open and stop at the first instruction from its own BIOS code at memory address F000:FFF0.  Why this address? You can read this and many other low level things in the outstanding work from Daniel B. Sedory.

Uncomment the last line from the Bochsrc configuration file, to tell Bochs to use the Enhanced Debugger. For further references, you can read the “How to DEBUG System Code using The Bochs Emulator on a Windows™ PC” article. Start Bochs again and the GUI will show up. You can load the stack view by pressing F2. Then set a breakpoint where the MBR code will be loaded by issuing the command “lb 0x7c00” and then the “c” to continue the debugging session.

Now we can look at the code, step into the instructions, inspect the registers and stack. After some back and forth with the debugger I created the following listing with my interpretation of some of the instructions.

Bottom line, the MBR code will perform a loop that uses BIOS interrupt 0x13 function 0x42 to read data starting at sector 1 of the hard-drive. The loop copies 8880 (0x22af) bytes of data into memory location 0x8000. When the copy is done, the execution is transferred to the address 0x8000 by performing a far jump and the malicious bootloader is executed. The malicious bootloader code has been uploaded by Matthieu Suiche to Virus Total here. You can also extract it from the hard drive by extracting the sector 1 through 18 or better using the commands from the following picture. Then you can perform static and dynamic analysis.

The 16-bits bootloader code is harder to analyze than the MBR code but it is based on the Petya ransomware code from 2016. In this great article, from Hasherezade, she analyzes both Petya and EternalPetya bootloader using IDA PRO. When you use Bochs integrated with IDA PRO disassembler and debugger, the analysis is more accessible due to the powerful combination.

That’s it for today – Entering the world of real-mode execution on x86 is quite interesting. Analyzing code that relies on BIOS services such as software interrupts to perform different operations like reading from disk or writing to screen or, accessing the memory through segments is revealing. What we learned today might be a starting point to start looking at bootkits that are beneath the operating system and subvert the MBR boot sequence.

Have fun!

References:

Windows Internals, Sixth Edition, Part 2 By: Mark E. Russinovich, David A. Solomon, and Alex Ionescu

Various articles written by Daniel B. Sedory : http://starman.vertcomp.com/index.html

http://standa-note.blogspot.ch/2014/11/debugging-early-boot-stages-of-windows.html

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Threat Hunting in the Enterprise with AppCompatProcessor

Last April, at the SANS Threat Hunting and IR Summit, among other things, there was a new tool and technique released by Matias Bevilacqua. Matias’s presentation was titled “ShimCache and AmCache enterprise-wide hunting, evolving beyond grep” and he released the tool AppCompatProcessor. Matias also wrote a blog post “Evolving Analytics for Execution Trace Data” with more details.

In this article, I want to go over a quick exercise on how to use this tool and expand the existing signatures. First, let me write that, in case you have a security incident and you are doing enterprise incident response or you are performing threat hunting as part of your security operations duties, this is a fantastic tool that you should become familiar with and have on your toolkit. Why? Because it allows the security teams to digest, parse and analyze, at scale, two forensic artifacts that are very useful. The forensic artifacts are part of the Windows Application Experience and Compatibility features and are known as ShimCache and the AMCache.

To give you more context, the ShimCache can be obtained from the registry and from it we can obtain information about all executable binaries that have been executed in the system since it was rebooted. Furthermore, it tracks its size and the last modified date. In addition, the ShimCache tracks executables that have not been executed but were browsed for example through explorer.exe. This makes a valuable source of evidence for example to track executables that were on the system but weren’t executed – consider an attacker that used a directory on a system to move around his toolkit. The AMCache is stored on a file and from it we can retrieve information for every executable that run on the system such as the PATH, last modification time and created, SHA1 and PE properties. You can read more about those 2 artifacts in the article I wrote last year.

So, I won’t go over on how to acquire this data at scale – feel free to share you technique in the comments – but, AppCompatProcessor digests data that has been acquired by ShimCacheParser.py, Redline and MIR but also consumes raw ShimCache and AMCache registry hives. I will go directly to the features.At the time of this writing the tool version is 0.8 and one of the features I would like to focus today is the search module. This module allows us to search for known bad using regex expressions. The search module was coded with performance in mind, which means the regex searches are quite fast. By default, the tool includes more than 70 regex signatures for all kinds of interesting things an analyst will look for when perform threat hunting. Signatures include searching for dual usage tools like psexec , looking for binaries in places where they shouldn’t normally be, commonly named credential dumpers, etc. The great thing is that you can easily include your own signatures. Just add a regex line with your signature!

For this exercise, I want to use the search module to search for binaries that are commonly used by the PlugX backdoor family and friends. This backdoor is commonly used by different threat groups on targeted attacks. PlugX is also refered as KORPLUG, SOGU, DestroyRAT and is a modular backdoor that is designed to rely on the execution of signed and legitimated executables to load malicious code. PlugX, normally has three main components, a DLL, an encrypted binary file and a legitimated executable that is used to load the malware using a technique known as DLL search order. I won’t go discuss the details about PlugX in this article but you can read the White Paper “PlugX – Payload Extraction” done by Kevin O’Reilly from Context, the presentation about Plugx at Black Hat ASIA in 2014 given by Takahiro Haruyama and Hiroshi Suzuki, the analysis done by the Computer Incident Response Center Luxembourg and the Ahnlab threat report. With this and other reports you could start compiling information about different PlugX payloads. However, Adam Blaszczyk from Hexacorn, already did that job and wrote an article where he outlines different PlugX payloads seen in the wild.

Ok, with this information, we start creating the PlugX regex signatures. Essentially we will be looking for the signed and legitimate executables but in places where they won’t normaly be. The syntax to create a new regex signature is simple and you can add your own signatures to the existing AppCompatSearch.txt file or just create a new file called AppCompatSearch-PlugX.txt which will be consumed automatically by the tool. The figure below shows the different signatures that I produced. . At the time of this writing, this is still work in progress but is a starting point.

Next step, launch AppCompatProcessor against our data set using the newly created signatures. The following picture shows how the output of the search module looks like. In this particular case the search produced 25 hits and a nicely presented summary of the hits is displayed on a histogram. The raw dumps of the hits are saved on the file called Output.txt.  As an analyst or investigator, you would look at the results and verify which ones would be worth to further investigate and which ones are false positives. For this exercise, there was a hit that triggered on the file “c:\Temp\MsMpEng.exe”. This file is part of the Windows Defender suite but could be used by PlugX as part of DLL search order hijack technique. Basically, the attacker will craft a malicious DLL named MpSvc.dll and will place that in the same directory as the MsMpEng.exe file and execute MsMpEng.exe. The DLL would need to be crafted in a special way but that is what PlugX specializes in. This will load the attacker code.

Following these findings, we would want to look at the system that triggered the signature and view all the entries. The picture below shows this step where we use the dump module. The output shows all the ShimCache entries for this particular system. The entries are normally sorted in order of execution from bottom to top, and in this case, adjacent to the “c:\Temp\MsMpEng.exe” file there are several windows built-in commands that were executed and a file named “c:\Temp\m64.exe”. This is what Matias calls a strong temporal execution correlation. This is indicative that an attacker obtained access to the system, executed several windows built-in commands and and executed a file called “m64.exe” which likely is Mimikatz or a cousin. 

Following those leads, you might want to obtain those binaries from the system and perform malware analysis in order to extract indicators of compromise such as the C&C address, look at other artifacts such Windows Event Logs, UsnJournal, memory, etc.. and have additional leads. In addition, you might want to further use AppCompatProcessor to search for the “m64.exe” file and also use the tstack module, to search across all the data set for binaries that match the date of those two binaries. With these findings, among other things, you would need to scope the incident by understanding which systems the attacker accessed, find new investigation leads and pivot on the findings. AppCompatProcessor is a tool that helps doing that. This kind of finding would definitely trigger your incident response processes and procedures.

That’s it, hopefully, AppCompatProcessor will reduce the entry barrier for your security operations center or incident response teams to start performing threat hunting in your environment and produce actionable results. If you find this useful, contribute with your threat hunting signatures in AppCompatProcessor GitHub repo and Happy Hunting!

 

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Intro to Linux Forensics

This article is a quick exercise and a small introduction to the world of Linux forensics.  Below, I perform a series of steps in order to analyze a disk that was obtained from a compromised system that was running a Red Hat operating system. I start by recognizing the file system, mounting the different partitions, creating a super timeline and a file system timeline. I also take a quick look at the artifacts and then unmount the different partitions. The process of how to obtain the disk will be skipped but here are some old but good notes on how to obtain a disk image from a VMware ESX host.

When obtaining the different disk files from the ESX host, you will need the VMDK files. Then you move them to your Lab which could be simple as your laptop running a VM with SIFT workstation. To analyze the VMDK files you could use the “libvmdk-utils” package that contain tools to access data store in VMDK files. However, another approach would be to convert the VMDK file format into RAW format. In this way, it will be easier to run the different tools such as the tools from The Sleuth Kit – which will be heavily used – against the image. To perform the conversion, you could use the QEMU disk image utility. The picture below shows this step.

Following that, you could list the partition table from the disk image and obtain information about where each partition starts (sectors) using the “mmls” utility. Then, use the starting sector and query the details associated with the file system using the “fsstat” utility. As you could see in the image, the “mmls” and “fsstat” utilities are able to identify the first partition “/boot” which is of type 0x83 (ext4). However, “fsstat” does not recognize the second partition that starts on sector 1050624.

This is due to the fact that this partition is of type 0x8e (Logical Volume Manager). Nowadays, many Linux distributions use LVM (Logical Volume Manager) scheme as default. The LVM uses an abstraction layer that allows a hard drive or a set of hard drives to be allocated to a physical volume. The physical volumes are combined into logical volume groups which by its turn can be divided into logical volumes which have mount points and have a file system type like ext4.

With the “dd” utility you could easily see that you are in the presence of LVM2 volumes. To make them usable for our different forensic tools we will need to create device maps from the LVM partition table. To perform this operation, we start with “kpartx” which will automate the creation of the partition devices by creating loopback devices and mapping them. Then, we use the different utilities that manage LVM volumes such as “pvs”, “vgscan” abd “vgchange”. The figure below illustrates the necessary steps to perform this operation.

After activating the LVM volume group, we have six devices that map to six mount points that make the file system structure for this disk. Next step, mount the different volumes as read-only as we would mount a normal device for forensic analysis. For this is important to create a folder structure that will match the partition scheme.

After mounting the disk, you normally start your forensics analysis and investigation by creating a timeline. This is a crucial step and very useful because it includes information about files that were modified, accessed, changed and created in a human readable format, known as MAC time evidence (Modified, Accessed, Changed). This activity helps finding the particular time an event took place and in which order.

Before we create our timeline, noteworthy, that on Linux file systems like ext2 and ext3 there is no timestamp about the creation/birth time of a file. There are only 3 timestamps. The creation timestamp was introduced on ext4. The book “The Forensic Discovery 1st Edition”from Dan Farmer and Wietse Venema outlines the different timestamps:

  • Last Modification time. For directories, the last time an entry was added, renamed or removed. For other file types, the last time the file was written to.
  • Last Access (read) time. For directories, the last time it was searched. For other file types, the last time the file was read.
  • Last status Change. Examples of status change are: change of owner, change of access permission, change of hard link count, or an explicit change of any of the MACtimes.
  • Deletion time. Ext2fs and Ext3fs record the time a file was deleted in the dtime stamp.the filesystem layer but not all tools support it.
  • Creation time: Ext4fs records the time the file was created in crtime stamp but not all tools support it.

The different timestamps are stored in the metadata contained in the inodes. Inodes are the equivalent of MFT entry number on a Windows world. One way to read the file metadata on a Linux system is to first get the inode number using, for example, the command “ls -i file” and then you use “istat” against the partition device and specify the inode number. This will show you the different metadata attributes which include the time stamps, the file size, owners group and user id, permissions and the blocks that contains the actual data.

Ok, so, let’s start by creating a super timeline. We will use Plaso to create it. For contextualization  Plaso is a Python-based rewrite of the Perl-based log2timeline initially created by Kristinn Gudjonsson and enhanced by others. The creation of a super timeline is an easy process and it applies to different operating systems. However, the interpretation is hard. The last version of Plaso engine is able to parse the EXT version 4 and also parse different type of artifacts such as syslog messages, audit, utmp and others. To create the Super timeline we will launch log2timeline against the mounted disk folder and use the Linux parsers. This process will take some time and when its finished you will have a timeline with the different artifacts in plaso database format. Then you can convert them to CSV format using “psort.py” utility. The figure below outlines the steps necessary to perform this operation.

Before you start looking at the super timeline which combines different artifacts, you can also create a traditional timeline for the ext file system layer containing data about allocated and deleted files and unallocated inodes. This is done is two steps. First you generate a body file using “fls” tool from TSK. Then you use “mactime” to sort its contents and present the results in human readable format. You can perform this operation against each one of the device maps that were created with “kpartx”. For sake of brevity the image below only shows this step for the “/” partition. You will need to do it for each one of the other mapped devices.

Before we start the analysis, is important to mention that on Linux systems there is a wide range of files and logs that would be relevant for an investigation. The amount of data available to collect and investigate might vary depending on the configured settings and also on the function/role performed by the system. Also, the different flavors of Linux operating systems follow a filesystem structure that arranges the different files and directories in a common standard. This is known as the Filesystem Hierarchy Standart (FHS) and is maintained here. Its beneficial to be familiar with this structure in order to spot anomalies. There would be too much things to cover in terms of things to look but one thing you might want to run is the “chkrootkit” tool against the mounted disk. Chrootkit is a collection of scripts created by Nelson Murilo  and Klaus Steding-Jessen that allow you to check the disk for presence of any known  kernel-mode and user-mode rootkits. The last version is 0.52 and contains an extensive list of known bad files.

Now, with the supertimeline and timeline produced we can start the analysis. In this case, we go directly to timeline analysis and we have a hint that something might have happened in the first days of April.

During the analysis, it helps to be meticulous, patience and it facilitates if you have comprehensive file systems and operating system artifacts knowledge. One thing that helps the analysis of a (super)timeline is to have some kind of lead about when the event did happen. In this case, we got a hint that something might have happened in the beginning of April. With this information, we start to reduce the time frame of the (super)timeline and narrowing it down. Essentially, we will be looking for artifacts of interest that have a temporal proximity with the date.  The goal is to be able to recreate what happen based on the different artifacts.

After back and forth with the timelines, we found some suspicious activity. The figure below illustrates the timeline output that was produced using “fls” and “mactime”. Someone deleted a folder named “/tmp/k” and renamed common binaries such as “ping” and “ls” and files with the same name were placed in the “/usr/bin” folder.

This needs to be looked further. Looking at the timeline we can see that the output of “fls” shows that the entry has been deleted. Because the inode wasn’t reallocated we can try to see if a backup of the file still resides in the journaling. The journaling concept was introduced on ext3 file system. On ext4, by default, journaling is enabled and uses the mode “data=ordered”. You can see the different modes here. In this case, we could also check the options used to mount the file system. To do this just look at “/etc/fstab”. In this case we could see that the defaults were used. This means we might have a chance of recovering data from deleted files in case the gap between the time when the directory was deleted and the image was obtained is short. File system journaling is a complex topic but well explained in books like “File system forensics” from Brian Carrier. This SANS GCFA paper from Gregorio Narváez also covers it well.  One way you could  attempt to recover deleted data is using the tool “extundelete”.  The image below shows this step.

The recovered files would be very useful to understand more about what happened and further help the investigation. We can compute the files MD5’s, verify its contents and if they are known to the NSLR database or Virustotal. If it’s a binary we can do strings against the binary and deduce functionality with tools like “objdump” and “readelf”. Moving on, we also obtain and look at the different files that were created on “/usr/sbin” as seen in the timeline. Checking its MD5, we found that they are legitimate operating system files distributed with Red Hat. However, the files in “/bin” folder such as “ping” and “ls” are not, and they match the MD5 of the files recovered from “/tmp/k”.  Because some of the files are ELF binaries, we copy them to an isolated system in order to perform quick analysis. The topic of Linux ELF binary analysis would be for other time but we can easily launch the binary using “ltrace -i” and “strace -i” which will intercept and record the different function/system calls. Looking at the output we can easily spot that something is wrong. This binary doesn’t look the normal “ping” command, It calls the fopen() function to read the file “/usr/include/a.h” and writes to a file in /tmp folder where the name is generated with tmpnam(). Finally, it generates a segmentation fault. The figure below shows this behavior.

Provided with this information, we go back and see that this file “/usr/include/a.h” was modified moments before the file “ping” was moved/deleted. So, we can check when this “a.h” file was created – new timestamp of ext4 file system – using the “stat” command. By default, the “stat” doesn’t show the crtime timestamp but you can use it in conjunction with “debugfs” to get it. We also checked that the contents of this strange file are gibberish.

So, now we know that someone created this “a.h” file on April 8, 2017 at 16:34 and we were able to recover several other files that were deleted. In addition we found that some system binaries seem to be misplaced and at least the “ping” command expects to read from something from this “a.h” file. With this information we go back and look at the super timeline in order to find other events that might have happened around this time.  As I did mention, super timeline is able to parse different artifacts from Linux operating system. In this case, after some cleanup we could see that we have artifacts from audit.log and WTMP at the time of interest. The Linux audit.log tracks security-relevant information on Red Hat systems. Based on pre-configured rules, Audit daemon generates log entries to record as much information about the events that are happening on your system as possible. The WTMP records information about the logins and logouts to the system.

The logs shows that someone logged into the system from the IP 213.30.114.42 (fake IP) using root credentials moments before the file “a.h” was created and the “ping” and “ls” binaries misplaced.

And now we have a network indicator. Next step we would be to start looking at our proxy and firewall logs for traces about that IP address. In parallel, we could continue our timeline analysis to find additional artifacts of interest and also perform in-depth binary analysis of the files found, create IOC’s and, for example, Yara signatures which will help find more compromised systems in the environment.

That’s it for today. After you finish the analysis and forensic work you can umount the partitions, deactivate the volume group and delete the device mappers. The below picture shows this steps.

Linux forensics is a different and fascinating world compared with Microsoft Windows forensics. The interesting part (investigation) is to get familiar with Linux system artifacts. Install a pristine Linux system, obtain the disk and look at the different artifacts. Then compromise the machine using some tool/exploit and obtain the disk and analyze it again. This allows you to get practice. Practice these kind of skills, share your experiences, get feedback, repeat the practice, and improve until you are satisfied with your performance. If you want to look further into this topic, you can read “The Law Enforcement and Forensic Examiner’s Introduction to Linux” written by Barry J. Grundy. This is not being updated anymore but is a good overview. In addition, Hal Pomeranz has several presentations here and a series of articles written in the SANS blog, specially the 5 articles written about EXT4.

 

References:
Carrier, Brian (2005) File System Forensic Analysis
Nikkel, Bruce (2016) Practical Forensic Imaging

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