The parse_arguments function in options.c in rsyncd in rsync before 3.1.3 does not prevent multiple --protect-args uses, which allows remote attackers to bypass an argument-sanitization protection mechanism.
https://usn.ubuntu.com/3543-1/
https://security.gentoo.org/glsa/201805-04
https://lists.debian.org/debian-lts-announce/2021/11/msg00028.html
https://lists.debian.org/debian-lts-announce/2019/03/msg00027.html
https://lists.debian.org/debian-lts-announce/2018/01/msg00021.html