You may be prompted to download a "missing codec" or "player upgrade" to view the video. These are almost always malicious installers that can infect your system.

Clicking links for such files often redirects to sites designed to steal personal information or flood your browser with intrusive ads.

Ensure the file is actually an .mp4 and not an executable like .mp4.exe , which is a common trick to hide malicious programs. How to Create and Open an MP4 File

If you have already interacted with the file, use a reputable antivirus or a website security checker to scan your device for hidden threats.

If you must open unknown media, use updated, sandboxed players like VLC which patch known vulnerabilities quickly.

If you encounter this file on social media, via direct message, or on a suspicious website, do not click it .