The nullfs implementation in sys/fs/nullfs/null_vnops.c in the kernel in FreeBSD 8.3 through 9.2 allows local users with certain permissions to bypass access restrictions via a hardlink in a nullfs instance to a file in a different instance.
http://www.securitytracker.com/id/1029015
http://www.freebsd.org/security/advisories/FreeBSD-SA-13:13.nullfs.asc
http://www.debian.org/security/2013/dsa-2769
http://svnweb.freebsd.org/base?view=revision&revision=255442