The CVE text claims arbitrary code execution "via creating a crafted AAC
file" in libx264. libx264 is an H.264 encoder and never parses AAC or any
other container or audio input.
The only reference, x264 issue #75, is a LeakSanitizer report from running
FFmpeg's configure checks with --enable-libx264: when x264_encoder_open()
fails, its fail: path frees the encoder without x264_param_cleanup(), so the
copied parameter strings (a few hundred bytes) leak once. There is no
out-of-bounds access, no use-after-free and no attacker-controlled data
involved. FFmpeg closed the original ticket as invalid and pointed at the
leak in libx264.
The record was assigned by MITRE with no CNA score; the 9.8 comes from
CISA-ADP. Debian rates it unimportant in every suite ("Negligible security
impact, usually only used for short-lived processes"), and no x264 commit
since the report touches this path, so there is nothing to backport.