Microsoft Sentinel - Hunting for Single Character Filenames

 Hackers, the good ones and the evil ones, are lazy.  In the interest of fewer keystrokes they will use filenames with single characters (e.g. "f.exe" or "2.ps1").  So this short but sweet query will help you find potentially problematic executables: 

       

let allowlist = datatable (DName:string)
['because you are going to have exceptions','Put em here'];
let allowarray =
allowlist
| summarize make_set(DName);
DeviceProcessEvents
| where FileName matches regex @'^.\.[eE][xX][eE]' or FileName matches regex @'^.\.[pP][sS][0-9]' or FileName matches regex @'^.\.[cC][mM][dD]' or FileName matches regex @'^.\.[cC][oO][mM]'
| where SHA1 !in (allowarray)
| project AccountName, DeviceName, SHA1, FileName, FolderPath, InitiatingProcessAccountName, InitiatingProcessAccountUpn

 

I was reminded of this when reading this tweet from @GossiTheDog and @h2jazi, but please note that the above query will NOT detect that TrikBot technique.  For that you could leverage DeviceFileEvents for creation of the single character filename, or perhaps go this route:

 

       
DeviceProcessEvents
| where InitiatingProcessFileName contains "rundll32"
| where InitiatingPRocessCommandLine contains "sml" 

... but I'm certainly open to better approaches.  The above query seems to only cause about one false positive a month, and I can live with that.

Hope this is helpful.  Happy holidays everyone!


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