CVE-2021-3566

Name: CVE-2021-3566
Creator: NVD / NIST
Published: 2021-08-05T21:15:12.783+00:00
License: https://creativecommons.org/publicdomain/zero/1.0/

MEDIUMCVSS 5.5/10EPSS 0.88%

Last modified Jun 17, 2026

CVE-2021-3566 is a medium-severity vulnerability rated 5.5/10 on the CVSS scale. 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).. EPSS estimates a 0.88% chance of exploitation in the next 30 days.

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).

Metrics

CVSS 3.1

5.5/10

CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:N/A:N

EPSS Probability

0.88%

54.5th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

CWE-200

Affected Software

Vendor	Product	Versions
Ffmpeg	Ffmpeg	< 4.3
Debian	Debian Linux	9.0

References

https://github.com/FFmpeg/FFmpeg/commit/3bce9e9b3ea35c54bacccc793d7da99ea5157532#diff-74f6b92a0541378ad15de9c29c0a2b0c69881ad9ffc71abe568b88b535e00a7fPatch, Third Party Advisory
https://lists.debian.org/debian-lts-announce/2021/08/msg00018.htmlMailing List, Third Party Advisory
https://github.com/FFmpeg/FFmpeg/commit/3bce9e9b3ea35c54bacccc793d7da99ea5157532#diff-74f6b92a0541378ad15de9c29c0a2b0c69881ad9ffc71abe568b88b535e00a7fPatch, Third Party Advisory
https://lists.debian.org/debian-lts-announce/2021/08/msg00018.htmlMailing List, Third Party Advisory

Timeline

Published: Aug 5, 2021
Last Modified: Jun 17, 2026
Status: Modified

Frequently Asked Questions

What is CVE-2021-3566?

How severe is CVE-2021-3566?

CVE-2021-3566 has a CVSS score of 5.5/10 (MEDIUM severity). The EPSS model estimates a 0.88% probability of exploitation in the next 30 days.

How do I fix CVE-2021-3566?

Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.

Are you affected by CVE-2021-3566?

Run a free Strix scan to check your systems for this vulnerability.

Scan your code now

Source: NVD / NIST