The security mechanism is completely down to permissions on files and programs such as nfsd that run as root (overriding file permissions) and maintain their own security mechanisms. Permissions on the block device files determine whether a user can mount that device. Every file has three sets of permissions (read, write, execute) for three classes of users (owner, group, world), plus a handful of special bits discussed below.
In the 'everything is a file` unix tradition the raw devices and exported volumes appear as special files with file system permissions. Remote mounting remote volumes is a little more complex and is discussed below.
If the user is logged in as the same uid as the owner of the file, then
owner permissions can be used. Each user has a default group, and can be added to group entries in the
/etc/group file. When a user has access to a group the file permissions for the group are used. Otherwise the user has 'world` permissions. ACL's (Access Control Lists) can be used to grant permissions to specific users where they are available.
Programs can override user level security by having the
setuid bit set on their inode entry. This runs the program with the privileges of the owner of the binary rather than the privileges of the logged in user. Examples of such programs are
sudo. These programs have their own security mechanisms; for example
/etc/sudoers that is used to govern permissions.
nfsd work based on uid and gid, which must be in sync on both machines (often traditionally done via NIS). nfsd has a file called
/etc/exports which has the exported file systems and some permissions data. mount and its nfs drivers present credentials which nfsd uses to authenticate the user's right to mount the volume. On a local volume mount uses the file permissions of the block device.