The 4755 permission in Linux is a specific file permission set that combines standard read, write, and execute permissions with a special attribute known as the Set User ID (SUID) bit. This permission is crucial for certain system utilities but also carries significant security implications.
Understanding the 4755 Permission
The four-digit octal number 4755 represents the following:
- First Digit (4): This is the special permission bit for Set User ID (SUID).
- Second Digit (7): Represents the permissions for the owner of the file.
- Third Digit (5): Represents the permissions for the group that owns the file.
- Fourth Digit (5): Represents the permissions for others (users not in the owner's group).
Let's break down each component:
The SUID Bit (4)
The Set User ID (SUID) bit is a powerful special permission. When this bit is set on an executable file (like a binary program), it means that any user who executes this program will run it with the effective permissions of the owner of the program, rather than their own permissions.
As noted, this particular permission allows a user to execute a binary program as though they were the owner of that program even though they are not. This is incredibly useful for system commands that need elevated privileges to perform their function but are executed by regular users.
Standard Permissions (755)
The subsequent 755 represents the standard read (r), write (w), and execute (x) permissions for the owner, group, and others:
| Digit | Permission Type | Numeric Value | Meaning |
|---|---|---|---|
| 7 | Owner | 4+2+1 | Read, Write, and Execute (rwx) |
| 5 | Group | 4+0+1 | Read and Execute (r-x) |
| 5 | Others | 4+0+1 | Read and Execute (r-x) |
Practical Implications and Examples
The 4755 permission is most famously used for critical system commands. The most well-known example of this is the passwd command.
- Example:
passwdcommand: When a regular user wants to change their password, they run thepasswdcommand. For this command to successfully update the user's password, it needs to write to the/etc/shadowfile, which contains encrypted password information and is only writable by therootuser for security reasons.- By having the SUID bit set (
4755or similar, usually4755or4711depending on specific permissions), thepasswdexecutable runs with the effective user ID of its owner, which is typicallyroot. This allows it to modify/etc/shadowsecurely on behalf of the user, without granting the user directrootprivileges.
- By having the SUID bit set (
Security Considerations
While essential for system functionality, using the SUID bit comes with significant security considerations:
- Use with Caution: As the reference suggests, "You should use this option with caution." Setting the SUID bit on an executable that is not carefully written and thoroughly audited can be a major security vulnerability.
- Privilege Escalation: If an SUID program has any exploitable flaws (e.g., buffer overflows, command injection vulnerabilities), an attacker could exploit these flaws to execute arbitrary code with the permissions of the file owner (often
root), leading to privilege escalation. - Minimizing Risk: SUID should only be set on trusted, essential system binaries. Developers must ensure SUID programs drop privileges when not needed and handle all inputs securely.
How to Identify and Set 4755 Permissions
You can identify files with the SUID bit set using the ls -l command. When the SUID bit is set, the x (execute) permission for the owner will appear as an s (lowercase 's' if execute permission is also set, uppercase 'S' if execute permission is not set).
ls -l /usr/bin/passwd
# Example output: -rwsr-xr-x 1 root root 68200 May 20 2023 /usr/bin/passwdNotice the s in place of the x in the owner's permissions (rws). This indicates the SUID bit is set.
To set the 4755 permission on a file (e.g., myprogram), you would use the chmod command:
chmod 4755 myprogramThis command sets the SUID bit and grants read, write, and execute permissions to the owner, and read and execute permissions to the group and others.
For more information on Linux file permissions and the chmod command, you can refer to resources like Linux File Permissions Explained or the chmod man page.
