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Systems & Data Center · Intermediate

Set User ID (SUID)

A special permission that makes an executable run with the privileges of the file's owner rather than the user running it.

Definition

SUID appears as an s in the owner execute position (rws). It is set with chmod u+s or a leading 4 in octal. The passwd command is SUID root so users can update /etc/shadow. Unnecessary SUID binaries are a privilege-escalation risk.

Example

find / -perm -4000 -type f lists all SUID files for a security audit.

Exam tip

SUID on a root-owned binary is a classic privilege escalation vector; a capital S means the bit is set but execute is not.

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