Vixie Cron before the 3.0pl1-133 Debian package allows local users to cause a denial of service (use-after-free and daemon crash) because of a force_rescan_user error.
https://salsa.debian.org/debian/cron/commit/40791b93
https://packages.qa.debian.org/c/cron/news/20190311T170403Z.html
https://lists.debian.org/debian-lts-announce/2021/10/msg00029.html
https://lists.debian.org/debian-lts-announce/2019/03/msg00025.html