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.
Exploitability
AV:NAC:LPR:NUI:RScope
S:UImpact
C:NI:NA:H6.5/CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H