In a recent security scan of our site, we found a number of systems which are exporting NFS file systems for "everyone". This is an important security risk because such exports allow access to the contents of the file systems to anyone on the Internet, which can result in compromise of the system and user accounts in any number of ways. Please examine your system for any unrestricted NFS exports (the default) by issuing the "showmount -e" command, or examining /etc/exports (/etc/dfs/sharetab for System V), for any entries which do not specifiy the qualifiers -access, -rw or -ro. The following actions are recommended, in order of preference:

1) Remove any unnecessary mount points;
2) Restrict access to mount points to those clients which legitimately need access by using the -access or -rw qualifiers and specifying the host names of the clients;
3) For mount points which must be accessible to any client, use the -ro qualifier to prevent unauthorized modifications to files;

See the man page for exports for more details.