Selfish Mining

Selfish mining is an attack strategy in which a miner who finds a new block keeps it secret and continues mining privately, releasing withheld blocks strategically to waste competitors' work and capture more than a fair share of rewards. Instead of broadcasting a block immediately, the selfish miner builds a hidden chain; when honest miners publish a block, the attacker reveals their longer private chain, orphaning the honest work.

For example, suppose a selfish miner with 30% of the network's hash rate finds two blocks in a row and withholds both. When the rest of the network finds one block, the attacker publishes their two-block chain, which the network adopts as longest — the honest miners' block and its reward are discarded, while the attacker keeps both of theirs. Repeated over time, this lets the attacker earn a larger fraction of blocks than their share of hash rate would suggest, and research first formalized by Eyal and Sirer in 2013 showed the strategy can be profitable well below 51% of the network.

A common misconception is that attacks on proof-of-work chains require majority hash rate; selfish mining shows meaningful manipulation is theoretically possible with much less. In practice, no sustained selfish mining attack has been convincingly observed on Bitcoin — the strategy is risky, detectable through unusual orphan rates, and would damage confidence in the very coin the attacker earns — but it remains a foundational result in blockchain security analysis.