Let's consider someone who sits on their buttocks all day, but, in spite of not working hard, gets rewarded anyway.
a particularly lazy coworker who gets a promotion/raise even if they did absolutely nothing to deserve it.
someone who is clueless about a specific task but somehow lucks their way to a satisfactory result (copy-pasting random code from the internet and obtaining a working program)
someone who doesn't mind deadlines but somehow doesn't get burnt when the day of reckoning comes (forgetting to file the tax declaration, but not getting investigated by the tax authority)
These examples might be not entirely related, as undeserved reward and undeserved luck are not exactly the same thing.
However I wonder if there's a chengyu to express such a thing.