GStreamer before 1.18.4 might access already-freed memory in error code paths when demuxing certain malformed Matroska files.
https://www.debian.org/security/2021/dsa-4900
https://security.gentoo.org/glsa/202208-31
https://lists.debian.org/debian-lts-announce/2021/04/msg00027.html
https://gstreamer.freedesktop.org/security/sa-2021-0002.html