Setuid

  • When setuid is set on an executable, the program runs with the permissions of the file owner, not the user executing it.
  • Commonly used for system utilities like passwd, allowing normal users to perform actions requiring root privileges.

Example

  • Normal users cannot access /etc/shadow.
  • passwd modifies /etc/shadow because it has setuid set.
  • s in owner execute → setuid is active
  • Lowercase s → setuid + execute
  • Uppercase S → setuid only, no execute

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Managing Setuid

  • Symbolic method:

    • Add setuid: chmod u+s file
    • Remove setuid: chmod u-s file

  • Octal method:

    • Add setuid: chmod 4775 file (existing 775 + 4000)
    • Remove setuid: chmod 0775 file (subtract 4000)
  • Four-digit octal codes are required to set special permissions.
  • Using three digits can remove existing special permissions.