The vfs_hang_addrlist function in sys/kern/vfs_export.c in the NFS server implementation in the kernel in FreeBSD 8.3 and 9.x through 9.1-RELEASE-p5 controls authorization for host/subnet export entries on the basis of group information sent by the client, which allows remote attackers to bypass file permissions on NFS filesystems via crafted requests.
http://www.securityfocus.com/bid/61484
http://www.freebsd.org/security/advisories/FreeBSD-SA-13:08.nfsserver.asc
http://svnweb.freebsd.org/base?view=revision&revision=244226