what does it mean to be setuid root?

Question

I am reading Tanenbaum's Modern Operating Systems 3e. He says "Suppose that the program being attacked [with malicious code] is SETUID root in UNIX (or has Administrator power in Windows). The [malicious code inserted with a buffer overflow attack] can now make a couple of system calls to convert the attacker's shell file on the disk into SETUID root, so that when it is executed it has superuser power."

What does it mean to be SETUID root? Does he mean that the program has root permissions? Why does he say that the program is setuid root?

This somewhat of a language question, but to a security noob with only basic linux/unix that passage in the book is hard to understand. What's he talking about?

Answer 1

SETUID means that the executable can run under different permissions than the user who has executed it.

Certain applications are set like this so the users can run with low permissions but a specific application they need to run with higher permissions can be.

SETUID 0 or root means that when the program is run it is as if root ran it - which leaves you open to all the usual security risks of running something at high permission levels - it could potentially do anything to your system - so generally the number of applications that require SETUID should be minimised on a Unix box. Most security audits of Unix machines include looking for SETUID and SETGID files that are world writeable as they present a high risk.