Shortcut to Windows Update


On January 27, 2022, Malwarebytes Labs shared an article covering new tactics including abusing the Windows Update Client for code executing believed to be the work of Lazarus.

The purpose of this post will be to cover possible detection points for defenders to identify adversaries misusing the Windows Update Client.

Please give the blog post by Ankur Saini and Hossein Jazi a read. The information for the piece of malware this blog will focus on is below:

FilenameFile SizeSHA256Reference
Table 1

Shortcut to Success

Upon opening the malicious document, a chain of code injections occur and numerous files are dropped to disk. Persistence is achieved via the Word document creating a hidden C:\Windows\System32 and dropping a shortcut file (LNK) to the Startup folder.

The purpose of this shortcut file named windowsupdateconf.lnk, is to execute the malicious DLL identified above in Table 1, which is embedded with an additional payload that will communicate with the C2.

wuauclt.exe which resides at C:\Windows\System32 is intended to be used via the command line and accepts a number of options, good luck finding worthwhile documentation though.

More common options that may be seen used with wuauclt.exe are:

  • /detectnow
  • /resetauthorization
  • /reportnow

The following is to achieve code execution in the LNK file’s target path: “C:\Windows\system32\wuauclt.exe” /UpdateDeploymentProvider C:\Wíndows\system32\wuaueng.dll /RunHandlerComServer“.


Image 1: Sysmon Event 1, Process Creation

Looking at the above process creation event output, a few items should jump out to defenders.

  • The Current Directory is the startup folder as opposed to cmd.exe.
  • The command line options “/UpdateDeploymentProvider” and “/RunHandlerComServer” are used to load a DLL. This should be considered suspicious and warrant a closer look.
Image 2: Sysmon CreateRemoteThread detected

As discussed earlier, this particular malware makes use of code injection at numerous points during the execution flow. As we can see in Image 2, wuauclt has injected itself into the explorer.exe process.

After running a few tests in a lab environment, I could not identify a legitimate purpose for wuauclt to inject into explorer.

Identifying the unusual command-line arguments in Splunk is very simple and can be completed in one line. This is an extremely basic rule, and an additional check for the DLL from well-known paths used by adversaries could be used as well.

sysmon EventID 1 AND ParentCommandLine "/UpdateDeploymentProvider" AND "/RunHandlerComServer" AND ParentImage NOT "C:\\Windows\\System32\\cmd.exe"
Image 3: Splunk query for suspicious wuauclt arguments

After reading a recent CrowdStrike blog on Microsoft Protection Logging, or MPLog, I wanted to see what events if any may be detected during the execution of the LNK file.

Unfortunately, Microsoft doesn’t provide a great deal of documentation on the event types in MPLog but in addition to the possible detection areas identified by CrowdStrike, the below event is also worth mentioning.

Image 4: MPLog BM telemetry

The BM or Behavior Monitoring telemetry as the name implies captures suspicious activity on workstations for additional analysis. Your environment may vary, but the only time a BM event was initiated in my lab was when the LNK was run.

This is out of the scope of this blog, but forwarding some of the more useful MPLog events to a SIEM could likely be a useful addition to identifying compromise on a system.

Enough of Sysmon and logs, let’s take a look at the DLL file.

Image 5: capa output of wuaueng.dll

Using Mandiant’s capa tool, we can get a quick overview of the capabilities of the DLL. In the output, capa confirms what we already know, that the DLL file contains an embedded PE file and installs an additional program.

Interesting Strings

– {“message”:”Commit changed details”,”content”:”Q29tcGxldGVkIHN1Y2Nlc3NmdWxseQ==”}

– Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/95.0.4638.69 Safari/537.36

– Accept: application/vnd.github.v3+json

– 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


– metafiles/tmp%.4d%.2d%.2d%.2d%.2d%.2d.txt

I think this is probably a good place to stop my analysis to avoid going deeper down a rabbit hole.

Hope you enjoyed reading!

More Info


Yara Rule

    description = "Detect malicious DLL executed via the Windows Update Client."
    author = "Michael Rippey"
    reference = ""
    date = "2022-02-06"
    hash = "829eceee720b0a3e505efbd3262c387b92abdf46183d51a50489e2b157dac3b1"

        $n1 = "" fullword ascii
        $n2 = "repos/%s/%s/contents/%s" fullword ascii
        $n3 = "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/95.0.4638.69 Safari/537.36" fullword ascii
        $n4 = "metafiles/tmp%.4d%.2d%.2d%.2d%.2d%.2d.txt" fullword ascii

        $e1 = "{\"message\":\"Commit changed details\",\"content\":\"Q29tcGxldGVkIHN1Y2Nlc3NmdWxseQ==\"}" fullword ascii
        $e2 = "CreateRemoteThreNtProtectVirtualWriteProcessMemoRtlCreateUserThr" fullword ascii
        $e3 = "omni callsig" fullword ascii 
        $e4 = " Type Descriptor'" fullword ascii
        $e5 = "$Sectigo Public Code Signing Root R460" fullword ascii
        $e6 = "Sleef" fullword ascii

        uint16(0) == 0x5A4D and 
        (any of ($n*) or 3 of ($e*))
        and filesize < 228KB

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