Thursday, April 01, 2021

LNK Files, Again

 I ran across SharpWebServer via Twitter recently...the first line of the file states, "A Red Team oriented simple HTTP & WebDAV server written in C# with functionality to capture Net-NTLM hashes." I thought this was fascinating because it ties directly to a technique MITRE refers to as "Forced Authentication".  What this means is that a threat actor can (and has...we'll get to that shortly) modify Windows shortcut/LNK files such that the iconfilename field points to an external resource. What happens is that when LNK file is launched, Explorer will reach out to the external resource and attempt to authenticate, sending NTLM hashes across the wire.  As such, SharpWebServer is built to capture those hashes.

What this means is that a threat actor can gain access to an infrastructure, and as has been observed, use various means to maintain persistence...drop backdoors or RATs, create accounts on Internet-facing systems, etc.  However, many (albeit not all) of these means of persistence can be overcome via the judicious use of AV, EDR monitoring, and a universal password change.

Modifying the iconfilename field of an LNK file is a means of persisting beyond password changes, because even after passwords are change, the updated hashes will be sent across the wire.

Now, I did say earlier that this has been used before, and it has.  CISA Alert TA18-074A includes a section named "Persistence through LNK file manipulation". 

Note that from the alert, when looking at the "Contents of enu.cmd", "Persistence through LNK file manipulation", and "Registry Modification" sections, we can see a pretty comprehensive set of toolmarks associated with this threat actor.  This is excellent intrusion intelligence, and should be incorporated into any and all #DFIR parsing, enrichment and decoration, as well as threat hunting.

However, things are even better! This tweet from bohops illustrates how to apply this technique to MSWord docs.

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