A denial of service in the subtitle decoder in FFmpeg 3.2 and 4.1 allows attackers to hog the CPU via a crafted video file in Matroska format, because handle_open_brace in libavcodec/htmlsubtitles.c has a complex format argument to sscanf.
https://usn.ubuntu.com/3967-1/
https://github.com/FFmpeg/FFmpeg/commit/273f2755ce8635d42da3cde0eeba15b2e7842774
https://git.ffmpeg.org/gitweb/ffmpeg.git/commit/894995c41e0795c7a44f81adc4838dedc3932e65