Wednesday, May 31, 2017

Use of LNK Files...Again

I've discussed LNK file, and more recently, here.  Windows shortcut files are an artifact on Windows systems that have been available for so long that there are likely a great many analysts who understand the category this artifact falls into, but may not know a great deal about the nature of the artifact itself.  That is to say that it's likely that a good many training and certification courses tend to gloss over some of the more esoteric aspects of the Windows shortcut file as an artifact.

Most analysts likely understand the Windows shortcut, or "LNK" file to be an artifact of "user knowledge of files", as the most commonly understood activity that leads to the automatic creation of these files is a user double-clicking on a file via the Windows Explorer shell.

We also know that understanding the format of these files can be very important, as well.  The LNK file format is used as the basis for individual streams within JumpLists, and there are several Registry keys that contain value data that follows the LNK file format.  In fact, LNK files are compose, at least in part, of shell now we're getting into the format of the individual building blocks of many artifacts on Windows systems.

Let's take a step back for a moment, and consider the most common means of "producing" Windows shortcut files.  From a standpoint of developing and interpreting evidence, let's say that a user plugs a USB device into a laptop, opens the contents of the device in Windows Explorer, and then double-clicks an MS Word document.  At that point, a shortcut file is created in the user's Recent folder that contains not only a series of shell items that comprise the path to the file, but also a string that refers to the file, as well.  However, the LNK file also contains information about that system on which it was created, and it's this aspect of the file format that is missed or forgotten, due the fact that in most cases, this embedded information isn't very interesting.  However, if an LNK file is not created as an artifact of, say, a malware infection, but is instead used by an attacker to infect other systems, then the LNK file will contain information about the system on which it was developed, which would NOT be the victim system.  This is when the contents of the LNK file become very interesting.

TrendMicro's TrendLabs recently published a blog article that gives a very good example of when these files can be extremely interesting.  The article describes the reported increase in use of LNK files as weaponized attachments by the group identifed as "APT 10".  I find this absolutely fascinating, not because something that has been part of Windows from the very beginning has been turned against users and employed as an attack vector, but because of the information and metadata that seems to be left on the floor because these files simply are not parsed or dug least not to any extent that is discussed publicly.  It seems that most organizations may miss or discount the value of Windows shortcut file contents, and not incorporate them into their overall intelligence picture.

This JPCERT blog post describes "Evidence of Attackers' Development Environment Left in Shortcut Files", and this NViso blog post discusses, "Tracking Threat Actors Through .LNK Files".  These are great at describing what can be done, providing illustration and example.