Security Guidelines for Mounting and Unmounting File Systems

The mount command enables you to attach removable file systems and disk or disk partitions to an existing file tree. The mount command uses a file called /etc/fstab, which contains a list of available file systems and their corresponding mount points. Make the /etc/fstab file writable only by root, but readable by others. For more information on mounting file systems, see fstab(4).

Observe the following precautions when mounting a file system or disk:

Create a mount point directory (such as /mnt) on which to mount a new file system. Never mount a file system on a directory that already contains files, because those files will become inaccessible.
The mount point of a mounted file system acquires the permissions and ownership of the file system's root directory.
Set permissions and access control list entries on disk path names to control access to disks.
Use the -r option of the mount command to mount the file system as read-only. You must mount physically write-protected file systems this way.
When mounting a new or foreign file system, assume that the medium is insecure.
- Make sure that the PATH environment variable does not include “.” (the current directory); otherwise, you might run a Trojan horse version of ls or some similar command while examining the new file system.
- Run the fsck command to verify that the file system is not technically corrupted. See fsck(1M).
- Run the ncheck_hfs -s or ncheck_vxfs -s command to scan for setuid and setgid programs and device files, and investigate any suspicious findings. The -s option is intended to discover concealed violations of security policy. For more information, see ncheck_hfs(1M) and ncheck_vxfs(1M).
- Create a directory restricted to root by setting its permissions at 700 (drwx------).
  # mkdir /securefile # chmod 700 /securefile
- Mount the foreign file system read-only at that location:
  # mount -r /dev/disk1 /securefile
- Check all directories for privileged programs, and verify the identity of every program.
- Remount the system read and write permissions and remove any unnecessary setuid and setgid permissions from files that you discovered in the previous step. These precautions are especially important if a user requests that you mount a personal file system.
Only after performing these tests should you unmount the file system and remount it in its desired location.
Be sure to unmount all mounted file systems of a user whose account you are disabling or removing.

For information on files mounted in an NFS environment, see Section .

Security Guidelines for Mounting and Unmounting File Systems

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