sys/nfsclient/nfs_vfsops.c in the NFS client in the kernel in FreeBSD 7.2 through 8.1-PRERELEASE, when vfs.usermount is enabled, does not validate the length of a certain fhsize parameter, which allows local users to gain privileges via a crafted mount request.
http://www.exploit-db.com/exploits/14003
http://www.exploit-db.com/exploits/14002
http://securitytracker.com/id?1024039
http://security.FreeBSD.org/advisories/FreeBSD-SA-10:06.nfsclient.asc