9.5.2. File Permissions

Once the NFS file system is mounted read/write by a remote host, the only protection each shared file has is its permissions. If two users that share the same user ID value mount the same NFS file system, they can modify each others files. Additionally, anyone logged in as root on the client system can use the su - command to become a user who could access particular files via the NFS share. For more on NFS and user ID conflicts, refer to the chapter titled Managing User Accounts and Resource Access in the Red Hat Enterprise Linux Introduction to System Adminitration.
By default, access control lists (ACLs) are supported by NFS under Red Hat Enterprise Linux. It is not recommended that this feature be disabled. For more about this feature, refer to the chapter titled Network File System (NFS) in the System Administrators Guide.
The default behavior when exporting a file system via NFS is to use root squashing. This sets the user ID of anyone accessing the NFS share as the root user on their local machine to a value of the server's nfsnobody account. Never turn off root squashing.
If exporting an NFS share as read-only, consider using the all_squash option, which makes every user accessing the exported file system take the user ID of the nfsnobody user.