Digital Forensics – DLL Search Order

Following our series of posts on Digital Forensics we will continue our journey about analyzing our compromised system. In the last two articles we covered Windows Prefetch and Shimcache. Among other things, we wrote that Windows Prefetch and ShimCache artifacts are useful to find evidence about executed files and executables that were on the system but weren’t execute. While doing our investigation and looking at these artifacts, the Event Logs and the SuperTimeline, we found evidence that REGEDIT.EXE was executed. In addition, from the Prefetch artifacts we saw this execution invoked a DLL called CLB.DLL from the wrong path. On Windows operating systems CLB.DLL is located under %SYSTEMROOT%\System32.  In this case CLB.DLL was invoked from %SYSTEMROOT%.

However, when we looked inside the %SYSTEMROOT% folder and we could not find any traces of the CLB.DLL file. This raised the following questions:

  • How did this file got loaded from the wrong PATH?
  • Did file got deleted by the attacker?

Let’s answer the first question.

Inside PE files there is a structure called Import Address Table (IAT) that contains the addresses of the library routines that are imported from DLL’s. When an application is launched the operating system will check this table to understand which routines are needed and from which DLL’s. For example, when I execute REGEDIT.EXE the binary has a set of dependencies needed in order to execute.  To see this dependencies, you can look at the IAT. On Windows you could use dumpbin.exe /IMPORTS or on REMNUX you could use pedump as illustrated below.

dllsearchorder-regiat

But from where will this DLL’s be loaded from? The operating system will locate the required DLL’s by searching a specific set of directories in a particular order. This is known as the DLL Search Order and is explained here. This mechanism can and has been abused frequently by attackers to plant a malicious DLL inside a directory that is part of the DLL Search Order mechanism. This will trick the Windows operating system to load the malicious DLL instead of the real one.  The DLL Search Order by default on Windows XP and above is the following:

  • The directory from which the application loaded.
  • The current directory.
  • The system directory.
  • The 16-bit system directory.
  • The Windows directory.
  • The directories that are listed in the PATH environment variable.

Not all DLL’s will be found using the DLL Search Order. There is a mechanism known as KnownDLLs Registry Key which contains a list of important DLL’s that will be invoked without consulting the DLL Search Order. This key is stored in the registry location HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\Session Manager\KnownDLLs.

Throughout the years Microsoft patched some of the problems with DLL Search Order mechanism and also introduced some improvements. One is the Safe DLL Search Order Registry which changes the order and moves the search of “The Current Directory” to the bottom making harder for the attacker without admin rights to plant a DLL in a place that will be searched first. This feature is controlled by the registry key HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\Session Manager\SafeDllSearchMode.

Bottom-line, this technique is known as DLL pre-loading, side-loading or hijacking and is an attack vector used to takeover a DLL and escalate privileges or achieve persistence by taking advantage of the way DLL’s are searched. This technique can be pulled off by launching an executable that is not in %SYSTEMROOT%\System32 – like our REGEDIT.EXE – or by leveraging weak Directory Access Control Lists (DACLS) and dropping a malicious DLL in the appropriate folder. In addition, for this technique to work the malicious DLL should contain the same exported functions and functionality has the hijacked DLL or work as proxy in order to ensure the executed program will run properly.  The picture below shows the routines that are exported by the malicious DLL. As you could see these are the same functions like the ones required by REGEDIT.EXE from the CLB.DLL.

dllsearchorder-iat

To further understand the details, you might want to read a write-up on leveraging this technique to escalate privileges described by Parvez Anwar here and to achieve persistence described by Nick Harbour here. Microsoft also gives guidance to developers on how to make applications more resistant to this attacks here.

Considering the REGEDIT.EXE example we can see from where the DLL’s are loaded on a pristine system using Microsoft Windows debugger like CDB.EXE.  Here we can see that CLB.DLL is loaded from %SYSTEMROOT%\System32.

dllsearchorder-regedit

We have now a understanding about how that DLL file might have been loaded. DLL sideloading is a clever technique that load malicious code and is often used and abused to either escalate privileges or to achieve persistence. We found evidences of it using the Prefetch artifacts but without Prefetch e.g., a Windows Server, this won’t be so easy to find and we might need to rely on other sources of evidence like we saw on previous articles. Based on the evidence we observed we consider that the attacker used DLL sideloading technique to hijack CLB.DLL and execute malicious code when invoking REGEDIT.EXE. However, we could not find this DLL file on our system. We will need to look deeper and use different tools and techniques that help us find evidence about it and answer the question we raised in the begging. This will be the topic of the upcoming article!

 

References:
Luttgens, J., Pepe, M., Mandia, K. (2014) Incident Response & Computer Forensics, 3rd Edition
Carvey, H. (2014) Windows Forensic Analysis Toolkit, 4th Edition
Russinovich, M. E., Solomon, D. A., & Ionescu, A. (2012). Windows internals: Part 1
Russinovich, M. E., Solomon, D. A., & Ionescu, A. (2012). Windows internals: Part 2

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