In FreeBSD 13.0-STABLE before n245118 12.2-STABLE before r369552 11.4-STABLE before r369560 13.0-RC5 before p1 12.2-RELEASE before p6 and 11.4-RELEASE before p9 a superuser inside a FreeBSD jail configured with the non-default allow.mount permission could cause a race condition between the lookup of \..\ and remounting a filesystem allowing access to filesystem hierarchy outside of the jail.