In ProFTPD through 1.3.8b before cec01cc, supplemental group inheritance grants unintended access to GID 0 because of the lack of supplemental groups from mod_sql.
https://github.com/proftpd/proftpd/issues/1830
https://github.com/proftpd/proftpd/commit/cec01cc0a2523453e5da5a486bc6d977c3768db1