Symantec has reported that attackers are using the Windows Help file (.hlp) extension to infiltrate systems via social engineering. What is being exploited with the HLP files, and how can enterprises go about protecting users? At what point should these files be blocked?
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Windows Help files have been attacked many times before, and Microsoft has been phasing them out
recently. Windows Help files allow for the potentially undesired execution of code on the local
system by design when opening the HLP file. As Symantec Corp. explained, an attacker could socially
engineer a user to open a malicious HLP file, which could execute commands on the local system
to download malware as the currently logged-in user. If allowed, such malware could allow an
attacker to fully take over a local system. Rather than exploiting an unknown vulnerability in the
software, attackers are just taking advantage of the undesired functionality already present in
Windows Help files.
In many email programs, the icon displayed for a Windows Help file is an innocuous pile of books; users might think it is a file related to a book and not something that could compromise the security of their system, making it more difficult to prevent social engineering attacks. There are only a few reasons a Windows Help file should be transmitted via email or even downloaded from a website, so an effective mechanism to protect users is to block attachments with .hlp extensions or to block URLs of HLP files. Blocking or changing the file association for HLP files could be done on local client systems, which would break the Windows Help file functionality that is necessary for a targeted attack. These blocks could be undone once other controls like anti-malware updates are in place to block malicious HLP files.
This was first published in April 2013