In Libav 12.3, a denial of service in the subtitle decoder allows attackers to hog the CPU via a crafted video file in Matroska format, because srt_to_ass in libavcodec/srtdec.c has a complex format argument to sscanf.
A stack-based buffer overflow in the subtitle decoder in Libav 12.3 allows attackers to corrupt the stack via a crafted video file in Matroska format, because srt_to_ass in libavcodec/srtdec.c misuses snprintf.