Su Must Be Suid To Work Properly Jun 2026

: The primary purpose of su is to allow users to assume the identity of another user, usually to gain elevated privileges. If su were not SUID, users would not be able to switch to a different user account (such as root) because they wouldn't have the necessary permissions to do so.

Because of this risk, modern implementations of su (and its more secure cousin, sudo ) take extensive precautions:

Consider what would happen if su was a standard binary without the SUID bit: su must be suid to work properly

The SUID bit is a special permission bit on executable files that allows users to execute the file with the permissions of the file's owner. When the SUID bit is set on a program, users executing the program do so with the permissions of the program's owner, not their own. This allows system administrators to control which users can perform specific tasks that require higher privileges.

If my_tool tries to read a file owned by root (UID 0) with permissions set to 600 (read/write for owner only), the kernel checks the process EUID. It sees 1000, compares it to the file's owner (0), and immediately denies access. : The primary purpose of su is to

In a standard Linux environment, when a user executes a binary, the resulting process generally inherits the User ID (UID) of the user who launched it. This is the core of the security model:

The logic applied to su is identical to that of sudo . sudo must also be SUID root. When the SUID bit is set on a

Attempt to use su :

When a user launches an SUID binary:

Note: sudo itself has SUID enabled on most systems for similar reasons (to read /etc/sudoers and validate users).