btrbk is a tool for creating snapshots and remote backups of Btrfs subvolumes. From 0.29.0 until 0.32.7, btrbk's sshfilterbtrbk.sh constructs allowstreammatch with a start anchor but without an end-of-string anchor for the complete command. A user restricted through an authorizedkeys forced command can append a trailing pipe command after a valid btrbk command prefix, bypassing the allowlist and executing arbitrary commands with the privileges of the backup-target SSH account. Deployments that do not use sshfilterbtrbk.sh in authorizedkeys are not affected. This issue is fixed in version 0.32.7.