CVE-2021-3566

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).