CVE-2021-3566

Severity: medium (CVSS 5.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.9% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2021-08-05 · updated 2026-06-17
Weakness: CWE-200

Description

Prior to ffmpeg version 4.3, the tty demuxer did not have a 'read_probe' function assigned to it. By crafting a legitimate "ffconcat" file that references an image, followed by a file the triggers the tty demuxer, the contents of the second file will be copied into the output file verbatim (as long as the `-vcodec copy` option is passed to ffmpeg).

Affected products

Debian Linux: 9.0
FFmpeg: before 4.3 — fixed in 4.3

How to fix

Upgrade to a fixed version: FFmpeg 4.3. Check the vendor advisory for mitigations.

References

https://github.com/FFmpeg/FFmpeg/commit/3bce9e9b3ea35c54bacccc793d7da99ea5157532#diff-74f6b92a0541378ad15de9c29c0a2b0c69881ad9ffc71abe568b88b535e00a7f
https://lists.debian.org/debian-lts-announce/2021/08/msg00018.html
https://github.com/FFmpeg/FFmpeg/commit/3bce9e9b3ea35c54bacccc793d7da99ea5157532#diff-74f6b92a0541378ad15de9c29c0a2b0c69881ad9ffc71abe568b88b535e00a7f
https://lists.debian.org/debian-lts-announce/2021/08/msg00018.html

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