I was at a talk tonight given by a survivor (Tomi Reichental) of Bergen-Belsen concentration camp. As an aside given to a question asked about the starvation in the camps, the speaker mentioned that although he never directly saw it himself, he had heard from others in later years, that some of the prisoners had turned to cannibalism.
The possibility of cannibalism in the concentration camps has never occurred to me (although now it seems so obvious considering the level of prolonged starvation) and Tomi said himself that he only heard about it second-hand, so what historical evidence is there for cannibalism in any of the Nazi concentration camps?
There is overwhelming evidence of cannibalism in POW camps housing Soviets. Many of these camps were set up without basic supplies, some were literally just barbed wire fenced in plots of land; no barracks. Food was rarely distributed to the millions of recent POWs and some sources cite figures of mortality among prisoners around 70-90% from starvation, exposure and disease. Here is a transcript describing cannibalism in the camps. http://www.cwporter.com/sovcan.htm I'd also recommend checking out "Bloodlands: Europe between Hitler and Stalin" by Timothy Snyder. The book deals heavily with conditions in the camps, I'd quote it but I'm on mobile, hope that helped.
Edit: more sources! From Ivan's War by Catherine Merridale: According to a survivor of the POW camp, Ibrai Tulebaev, 12 men were shot for cannibalism in December, 1941 in his camp. (page 142)
I heard a talk from Dachau survivor Ben Lesser at the Middle Tennessee State University Holocaust Studies Conference in 2013 where he spoke of cannibalism on the train journey to Dachau. He was being transported from another camp (I don't remember which) to Dachau, a hundred or so to a train car. The guards threw in about ten loaves of bread when the prisoners first boarded the train. Mr. Lesser told us he managed to get hold of one of the loaves to feed himself and his cousin, being stabbed in the cheek in the fracas that followed the food being tossed in. He still had the scar. Mr. Lesser had the foresight to save his bread and only give himself and his cousin very small amounts of the bread as the days went by. The journey took a very long time, and during the desperation of those days in the dark train car many of the already starved and sick prisoners died. As this happened the survivors began to eat them as the bread had long since run out. By the time they got to Dachau the Nazis didn't even bother processing the new prisoners as nearly all in the cars were dead. Mr. Lesser said there were maybe 5 in his car that were still alive. Unfortunately after having survived such horror his cousin died of dysentery after they were liberated by the Americans.
I know this doesn't directly answer the question of whether cannibalism occurred within the camps, but I think it might still be the sort of answer you're looking for. I apologise if any of the details are incorrect as I'm going off memory here. I still have a pretty good recollection of it though - it wasn't the sort of story one forgets.