In FFmpeg 3.2 and 4.1, a denial of service in the subtitle decoder allows attackers to hog the CPU via a crafted video file in Matroska format, because ff_htmlmarkup_to_ass in libavcodec/htmlsubtitles.c has a complex format argument to sscanf.
https://www.debian.org/security/2019/dsa-4449
https://usn.ubuntu.com/3967-1/
https://seclists.org/bugtraq/2019/May/60
https://github.com/FFmpeg/FFmpeg/commit/23ccf3cabb4baf6e8af4b1af3fcc59c904736f21
https://git.ffmpeg.org/gitweb/ffmpeg.git/commit/1f00c97bc3475c477f3c468cf2d924d5761d0982