A well-hardened system not only thwarts outside intruders, it also protects against abuse perpetrated (intentionally or accidentally) by any user (whether authorized or unauthorized).
One subsystem that deserves special consideration is the file system. Hardening the file system safeguards the contents and attributes (protection bits, owner, etc.) of every file in the local file system. To secure the local file system, you have to conduct an audit and modify a variety of settings to provide only the minimum access required.
For example, look for inappropriate file and directory permissions, and correct any problems that you find. The most important of these are group- and/or world-writable system executables and directories, and commands that are setuid or setgid. For world-accessible files, change the permissions to be as restrictive as possible. For commands that are setuid or setgid, make sure the command and those permissions are really necessary, and ensure that no unauthorized ones get added.
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