Special permissions SUID, SGID and sticky bit

SUID (Set owner User ID up on execution)

• SUID is a special permission assigned to a file.
• Allow user to run a program with the permissions of the file owner
• e.g.
  • -rwsr-xr-x 1 root root 30768 Nov 24  2015 /usr/bin/passwd
  • -rwsr-xr-x 1 root root 38520 Mar 22  2017 /bin/ping
• setup suid, two ways
  • chmod u+s file.txt
  • chmod 4750 file.txt
• S vs s
  • S means no executable permission, then run
    • chmod u_x file.txt

SGID

• execute a file with the group ownership of that file
• setup sgid, two ways
  • chmod g+s xxx
  • chmod 2775 xxx

Sticky Bit

• set on file or directory
• allow root or owner to delete or rename the file
• e.g.
  • drwxrwxrwt 10 root root 4096 Feb  7 05:30 /tmp
• setup, two ways
  • chmod +t xxx
  • chmod 1777 xxx