Selfish Mining Re-Examined [pdf]

Selfish mining is a strategy, or exploit, miners can employ against their competitors to reduce their profitability by selectively withholding valid mined blocks rather than always publishing them immediately.

Long term this can drive honest miners out of the network and concentrate power into the hands of miners willing to form cartels or who can rent large quantities of hash power.

A preemptive response to those who would say PoS is superior for this reason: PoW is an armor and selfish mining is a flaw in that armor - PoS does not have this flaw because it does not have that armor. There is no cost to fabricating disruptive chains in PoS and the security landscape is built up around that fact - if you use selfish mining to compare the two you are likely to misunderstand the differences between the consensus mechanisms.

One notable solution is a consensus protocol which does not orphan all work on the addition of a new block - that is the fundamental tool which allows selfish miners to render real competing work useless. Deriving work from transactions can fix the orphan problem, but that alone is just one piece a fully secure blockchain consensus.