Pull ups, quite possibly the king of upper body exercises, also blast your abs when done correctly. To increase the work being done by your abs and core muscles during the pull up follow these cues; Slow down, eliminate all momentum from the move and hold your body in a single straight line throughout the exercise (likely at a slightly tilted angle!)
It’s one of the reasons I like bodyweight exercises like push ups and pull ups so much - they work much more than just the target muscles - in a simple and joint-friendly way!