Insufficient sanitization of arguments passed to rsync can bypass the restrictions imposed by rssh, a restricted shell that should restrict users to perform only rsync operations, resulting in the execution of arbitrary shell commands.
https://www.debian.org/security/2019/dsa-4382
https://usn.ubuntu.com/3946-1/
https://security.gentoo.org/glsa/202007-29
https://lists.debian.org/debian-lts-announce/2019/02/msg00007.html