Malicious .SLK Files

External sources have identified the limited use of maliciously crafted Symbolic Link files (.SLK) in phishing campaigns. Spreadsheet applications are used to launch and execute the malicious PowerShell command contained inside. This infection vector does not require the use of macros, making standard detection less likely.

At this time, .SLK files do not appear to be commonly incorporated into phishing campaigns. There is a Medium confidence rating that this delivery method will be widely adopted by threat actors in the near future.

What we’re doing about it

• The Threat Intelligence team is monitoring phishing related activity for noteworthy changes.

What you should do about it

• Attachments with the .SLK ending should be treated with extra caution.
• Block files with the .SLK file extension through SPAM filters.
• Ensure users are well informed about current threats through awareness programs and training.

Additional information

Symbolic Link files using the “.SLK” file extension are used to transfer data between spreadsheets and other databases. These files are primarily used by Microsoft Office but are also recognized by other products in Windows, Mac, Linux, iOS and Android [1].

Protected view does not appear to be an option when opening .SLK files directly. Fortunately, there are multiple security notifications before a malicious .SLK file can be executed; the end user must agree to at least three separate prompts. These prompts limit the likelihood of success but do not eliminate the chance that a user may unwittingly permit the malicious activity.

[1] https://fileinfo.com/extension/slk