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The missing LNK

Peter Ferrie
Virus Bulletin, Sep 2010, pp. 4-6
ISSN 0956-9979
September 2010

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LNK files are everywhere in Windows, so ubiquitous that they are rarely even recognized for what they are: complex structures containing pointers to Portable Executable files and, ultimately, executable code.

Some of the icons that appear in the Control Panel are visible because of LNK files. Many of the entries in the Start Menu and on the Desktop are LNK files. In most cases, the LNK references a file, and specifies an icon to display. When an application is used to view the LNK file, such as browsing a folder using Windows Explorer, the Windows shell parses the format and determines what to display. LNKs are not limited to just files, though. They can be shortcuts to drives such as a shared network location or a floppy disk (as used by the ‘Send To’ menu, for example). The ‘Recent File List’ in Microsoft Office 2007 applications is composed of LNK files.

Overall, LNK files do not pose a direct threat. Of course, some LNK files can point to malicious executables that run when the LNK file is clicked, and some LNK files can point to harmless files and yet still perform malicious actions (such as when the command prompt is executed, but given the instructions to delete some files). Some LNK files can themselves be malicious by virtue of their contents (such as the self-executing LNK file virus from several years ago, where the LNK file carried an actual Portable Executable file, and executed it in a rather roundabout fashion). Then there are the LNK files produced by W32/Stuxnet, which allow the execution of arbitrary code without the need for any user interaction (other than browsing to a folder that contains such a file, with some further clarification below).

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