init_tmp in TeeJee.FileSystem.vala in Timeshift before 20.03 unsafely reuses a preexisting temporary directory in the predictable location /tmp/timeshift. It follows symlinks in this location or uses directories owned by unprivileged users. Because Timeshift also executes scripts under this location, an attacker can attempt to win a race condition to replace scripts created by Timeshift with attacker-controlled scripts. Upon success, an attacker-controlled script is executed with full root privileges. This logic is practically always triggered when Timeshift runs regardless of the command-line arguments used.
https://usn.ubuntu.com/4312-1/
https://github.com/teejee2008/timeshift/releases/tag/v20.03
https://github.com/teejee2008/timeshift/commit/335b3d5398079278b8f7094c77bfd148b315b462