Linux File Permissions Deep Dive: SUID, SGID, and Sticky Bit

Every Linux user knows rwx (Read, Write, Execute). But operating a secure server requires understanding the "Special Permissions" that govern how users interact with shared files and system binaries.

The Basics: UGO and Octal

Permissions apply to User (Owner), Group, and Others.

• Read (4): View file contents or list directory.
• Write (2): Modify file or create/delete inside directory.
• Execute (1): Run binary or enter directory.

chmod 755 file means:

• User: 7 (4+2+1) = Read + Write + Exec
• Group: 5 (4+0+1) = Read + Exec
• Others: 5 (4+0+1) = Read + Exec

1. SUID (Set User ID)

What: When executed, the file runs as the Owner, not the current user. Why: Passwords. /usr/bin/passwd needs to modify /etc/shadow (root only), but users need to run it to change their own password. SUID allows my user to run passwd as root temporarily. Risk: If a script has SUID and creates a shell, anyone running it becomes root.

chmod u+s filename
# Look for 's' in execute spot: -rwsr-xr-x

2. SGID (Set Group ID)

Files: Executed with the permissions of the Group. Directories: New files created inside inherit the Group ownership of the directory, not the user's primary group. Use Case: Shared folders. Development teams often use this so everyone can edit each other's files in /var/www.

chmod g+s directoryname

3. Sticky Bit

What: Only the Owner (and root) can delete files inside this directory. Use Case: /tmp. Everyone has write access (can create files), but you shouldn't be able to delete another user's temporary files.

chmod +t /tmp
# Look for 't' at the end: drwxrwxrwt

Security Best Practices

• Never use 777: It allows anyone to modify or inject malicious code.
• Audit SUID files: find / -perm /4000 lists all SUID files. Check for binaries that shouldn't grant root access (like vim or less).
• Use Groups: Instead of opening permissions to "Others", create a specific group for access.

Mastering these bits separates a Linux User from a Systems Administrator.