operating systems question:

Consider a computer running in the user mode. It will switch to the monitor mode whenever an interrupt or trap occurs, jumping to the address determined from the interrupt vector.

(a) A smart, but malicious, user took advantage of a certain serious loophole in the computer's protection mechanism, by which he could make run his own user program in the monitor mode! This can cause disastrous effects. What could have he possibly done to achieve this? What disastrous effects could it cause?

(b) Suggest a remedy for the loophole.