A flaw was found in mbsync before v1.3.6 and v1.4.2, where an unchecked pointer cast allows a malicious or compromised server to write an arbitrary integer value past the end of a heap-allocated structure by issuing an unexpected APPENDUID response. This could be plausibly exploited for remote code execution on the client.
https://www.openwall.com/lists/oss-security/2021/06/07/1
https://security.gentoo.org/glsa/202208-15
https://lists.debian.org/debian-lts-announce/2022/07/msg00001.html
https://github.blog/2021-06-10-privilege-escalation-polkit-root-on-linux-with-bug/
https://bugzilla.redhat.com/show_bug.cgi?id=1967397