Btrbk before 0.31.2 allows command execution because of the mishandling of remote hosts filtering SSH commands using ssh_filter_btrbk.sh in authorized_keys.
github.com/digint/btrbk/blob/master/ChangeLog
github.com/digint/btrbk/commit/58212de771c381cd4fa05625927080bf264e9584
lists.debian.org/debian-lts-announce/2021/09/msg00002.html
lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/BP2T32JMENJFRP2HWXR7FTTZVRTTPECL/
lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/LM7GLTUN5YS4KE2RNBX732EAMVVGNEX3/