Special File Permissions (setuid, setgid and Sticky Bit)

Three special types of permissions are available for executable files and public directories: setuid, setgid, and sticky bit. When these permissions are set, any user who runs that executable file assumes the ID of the owner (or group) of the executable file.

You must be extremely careful when you set special permissions, because special permissions constitute a security risk. For example, a user can gain superuser capabilities by executing a program that sets the user ID (UID) to 0, which is the UID of root. Also, all users can set special permissions for files that they own, which constitutes another security concern.

You should monitor your system for any unauthorized use of the setuid permission and the setgid permission to gain superuser capabilities. A suspicious permission grants ownership of an administrative program to a user rather than to root or bin. To search for and list all files that use this special permission, see How to Find Files With Special File Permissions.

setuid Permission

When setuid permission is set on an executable file, a process that runs this file is granted access on the basis of the owner of the file. The access is not based on the user who is running the executable file. This special permission allows a user to access files and directories that are normally available only to the owner.

For example, the setuid permission on the passwd command makes it possible for users to change passwords. A passwd command with setuid permission would resemble the following:

-r-sr-sr-x   3 root     sys       28144 Jun 17 12:02 /usr/bin/passwd

This special permission presents a security risk. Some determined users can find a way to maintain the permissions that are granted to them by the setuid process even after the process has finished executing.

The use of setuid permissions with the reserved UIDs (0–100) from a program might not set the effective UID correctly. Use a shell script, or avoid using the reserved UIDs with setuid permissions.

setgid Permission

The setgid permission is similar to the setuid permission. The process's effective group ID (GID) is changed to the group that owns the file, and a user is granted access based on the permissions that are granted to that group. The /usr/bin/mail command has setgid permissions:

-r-x--s--x   1 root   mail     67504 Jun 17 12:01 /usr/bin/mail

When the setgid permission is applied to a directory, files that were created in this directory belong to the group to which the directory belongs. The files do not belong to the group to which the creating process belongs. Any user who has write and execute permissions in the directory can create a file there. However, the file belongs to the group that owns the directory, not to the group that the user belongs to.

You should monitor your system for any unauthorized use of the setgid permission to gain superuser capabilities. A suspicious permission grants group access to such a program to an unusual group rather than to root or bin. To search for and list all files that use this permission, see How to Find Files With Special File Permissions.

Sticky Bit

The sticky bit is a permission bit that protects the files within a directory. If the directory has the sticky bit set, a file can be deleted only by the file owner, the directory owner, or by a privileged user. The root user and the Primary Administrator role are examples of privileged users. The sticky bit prevents a user from deleting other users' files from public directories such as /tmp:

drwxrwxrwt 7  root  sys   400 Sep  3 13:37 tmp

Be sure to set the sticky bit manually when you set up a public directory on a TMPFS file system. For instructions, see Example 7–5.