Suid

Suid, short for set-user-ID on execution, is a Unix-like filesystem permission where an executable runs with the privileges of its owner rather than the user who started it. When a program with the set-user-ID on execution bit set runs, the process’s effective user ID becomes the owner of the file, typically root. This allows ordinary users to perform specific privileged actions without granting broad root access.

The principal purpose of the suid bit is to enable privileged operations that need to be performed on behalf of a user. Common examples include password-changing utilities, which need to modify protected system files. The feature is designed to be used only for trusted programs; its misuse can create serious security risks.

In permission notation, the suid bit appears as an s in the user (owner) execute field. For example, a file with permissions -rwsr-xr-x has the suid bit set. If the execute bit for the owner is not set but the suid bit is, it may be shown as an uppercase S. The suid bit corresponds to the octal value 4000.

Security considerations: SUID programs can be targets for privilege escalation if they contain vulnerabilities or are misconfigured. To reduce risk, suid should be limited to trusted executables, regularly audited, and avoided on scripts. When possible, alternatives such as sudo or capability-based access should be used to grant necessary privileges.

See also: SGID, sticky bit, chmod, sudo, capabilities.