Detection: Windows System Binary Proxy Execution Compiled HTML File URL In Command Line
Description
The following analytic identifies hh.exe (HTML Help) execution of a Compiled HTML Help (CHM) file from a remote url. This particular technique will load Windows script code from a compiled help file. CHM files may contain nearly any file type embedded, but only execute html/htm. Upon a successful execution, the following script engines may be used for execution - JScript, VBScript, VBScript.Encode, JScript.Encode, JScript.Compact. Analyst may identify vbscript.dll or jscript.dll loading into hh.exe upon execution. The "htm" and "html" file extensions were the only extensions observed to be supported for the execution of Shortcut commands or WSH script code. During investigation, identify script content origination. Review reputation of remote IP and domain. Some instances, it is worth decompiling the .chm file to review its original contents. hh.exe is natively found in C:\Windows\system32 and C:\Windows\syswow64.
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Implementation
To successfully implement this search, you need to be ingesting logs with the process name, parent process, and command-line executions from your endpoints. If you are using Sysmon, you must have at least version 6.0.4 of the Sysmon TA.
Known False Positives
Although unlikely, some legitimate applications may retrieve a CHM remotely, filter as needed.
Associated Analytic Story
Risk Based Analytics (RBA)
| Risk Message | Risk Score | Impact | Confidence |
|---|---|---|---|
| An instance of $process_name$ was identified on endpoint $dest_device_id$ by user $dest_user_id$ contacting a remote destination. | 90 | 90 | 100 |
References
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https://github.com/redcanaryco/atomic-red-team/blob/master/atomics/T1218.001/T1218.001.md
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https://gist.github.com/mgeeky/cce31c8602a144d8f2172a73d510e0e7
Version: 4