Since it's malicious, it will probably search through all of your desires, pick one of them at random to count as "my utility function", and then reinterpret the body of the wish to maximise that one thing at the expense of all others.

It's malicious and omnipotent. It'll do far worse than that. It'll scan your preferences until it finds a contradiction. Once you have a contradiction you can derive absolutely anything. It would then proceed to calculate your Coherent Extrapolated Volition and minimise it. It may not be obliged to figure out what you actually want but it can certainly do so for the purpose of being spiteful!