NFS (Network File System) Security.

NFS is a very widely-used file sharing protocol. It allows servers running nfsd and
mountd to "export" entire file systems to other machines using NFS filesystem support
built in to their kernels (or some other client support if they are not Linux machines).
mountd keeps track of mounted file systems in /etc/mtab, and can display them with
showmount.

Many sites use NFS to serve home directories to users, so that no matter what machine
in the cluster they login to, they will have all their home files.
There is some small amount of security allowed in exporting file systems. You can
make your nfsd map the remote root user (uid=0) to the nobody user, denying them
total access to the files exported. However, since individual users have access to their
own (or at least the same uid) files, the remote root user can login or su to their account
and have total access to their files. This is only a small hindrance to an attacker
that has access to mount your remote file systems.
If you must use NFS, make sure you export to only those machines that you really
need to. Never export your entire root directory; export only directories you need to
export.
See the NFS HOWTO for more information on NFS, available at
http://metalab.unc.edu/mdw/HOWTO/NFS-HOWTO.html