1. Securing Files and Verifying File Integrity in Oracle Solaris 11.4
  2. Controlling Access to Files
  3. Using UNIX Permissions to Protect Files
  4. Special File Permissions Using setuid, setgid and Sticky Bit
  5. setuid Permission

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   1 root     sys       62K Jun 14 14:14 /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.

Note:

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.