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.
https://lists.debian.org/debian-lts-announce/2021/09/msg00002.html
https://github.com/digint/btrbk/commit/58212de771c381cd4fa05625927080bf264e9584