9.3. Set-User-ID and Set-Group-ID Programs

A set-user-ID program allows a process to gain privileges it would not normally have, by setting the process’s effective user ID to the same value as the user ID (owner) of the executable file. A set-group-ID program performs the analogous task for the process’s effective group ID. (The terms set-user-ID program and set-group-ID program are sometimes abbreviated as set-UID program and set-GID program.)

Like any other file, an executable program file has an associated user ID and group ID that define the ownership of the file. In addition, an executable file has two special permission bits: the set-user-ID and set-group-ID bits. (In fact, every file has these two permission bits, but it is their use with executable files that interests us here.) These permission bits are set using the chmod command. An unprivileged user can set these bits for files that they own. A privileged user (CAP_FOWNER) can set these bits for any file. Here’s an example: