There are some torrrents showing up with .lnkextension (ex: movie.mp3.lnk, tvshow.mkv.lnk...) and automated software (Sonarr, Radarr, Lidarr, qBittorrent RSS Downloader) could pick those torrents (but not import).
These (fake) torrents include a .lnk file that executes a script on your Windows
HOW TO exclude from download on qBittorrent.
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Go to Options -> Downloads
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Enable "Exclude file names"
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Add patterns:
(one by line)
*.mp4.lnk
*.mp3.lnk
*.mkv.lnk
*.torrent.lnk
Or exclude all together: *.lnk
Example on VirusTotal https://www.virustotal.com/gui/file/e74f64df6ebaf3a1b6e3f42591eb6e87d2ac2828eb5a99fd8d3d82c140137fc9/detection
When I read the title, I was thinking of something sophisticated such as hidden executable streams inside the MKV container (IIRC, it's possible to append binary data other than audio, video or subtitles specifically inside a MKV). The ".lnk" trick only works in Windows and, even there, it's easy to prevent: Windows Explorer > Options > Advanced > find and check "Always show extensions for files" (i can't really remember the exact label for this option as I'm not a Windows user, but something like this will be there).
I believe you uncheck "Hide extensions for known file types"
Exactly! Thanks! I couldn't point the exact label, I've been using Linux for years in a daily basis so I forgot most of the Windows shortcuts/options.
Even then, that setting doesn't unhide the ".lnk" file extension, that requires a registry edit: https://www.askvg.com/tip-how-to-show-file-extensions-of-shortcuts-lnk-url-pif-in-windows-explorer/
Although shortcuts are pretty easy to spot in the first place unless you just double-click things without paying attention lol