setgid Permission

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

-r-x--s--x   1 root     mail       63628 Sep 16 12:01 /usr/bin/mail

When setgid permission is applied to a directory, files that were created in this directory belong to the group to which the directory belongs, not 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 user's group ownership.

You should monitor your system for any unauthorized use of the setuid and setgid permissions to gain superuser privileges. To search for and list all of the files that use these permissions, see How to Find Files With setuid Permissions. A suspicious listing grants group ownership of such a program to a user rather than to root or bin.