Selfish Mining
Selfish mining is a block-withholding strategy where miners hide valid blocks to gain disproportionate rewards, undermining network fairness.
Definition
Selfish mining is a strategy where a miner or mining pool finds a valid block but does not publish it right away. Instead, the miner keeps the block private and tries to build a secret chain ahead of the public one. The goal is to make honest miners waste work and earn extra block rewards.
How It Works
In normal proof-of-work mining, miners broadcast a valid block as soon as they find it. Other nodes verify it, add it to the blockchain, and everyone starts the next block.
In selfish mining, the attacker hides a newly found block and keeps mining privately on top of it. Honest miners cannot see the hidden block, so they continue mining on the public chain.
If the selfish miner finds another block first, the private chain grows a lead. If honest miners find a competing block at the same height, the selfish miner publishes the hidden block to create a race. Whether honest miners build on the selfish block depends on a propagation factor, often called γ (gamma). A higher γ means the selfish block reaches more nodes first, increasing the chance that the honest block becomes an orphan block with no reward.
When the private lead grows to two or more blocks, the selfish miner can safely publish one private block for every honest block found, maintaining their advantage while draining honest miners of income.
The Profitability Threshold
A key finding from Eyal and Sirer’s 2013 analysis is that selfish mining becomes profitable when a pool controls more than one-third of the network’s hash rate — roughly α > 33%. This threshold is lower than the 51% often assumed for attacks, and it drops further with a propagation advantage (high γ).
In practice, this means even a moderately large pool could earn disproportionately by withholding blocks. After the paper’s publication, several major pools publicly committed to limiting their share of hash rate to avoid this risk, showing how the theoretical result shaped real-world pool behavior.
Why It Matters
Selfish mining turns mining from a simple race into a strategic game. A large pool may earn extra revenue by withholding blocks, while smaller miners lose income from blocks pushed off the main chain.
For miners, the main risk is centralization. If one pool gains an advantage, others may join it to avoid lower payouts, making mining pool centralization worse. More concentration makes threats such as a 51 percent attack easier to coordinate.
Countermeasures
Networks defend against selfish mining through several strategies. Uniform tie-breaking rules help by ensuring that when two blocks compete at the same height, each has an equal chance of being extended — removing the selfish miner’s ability to bias the race. Faster block relay protocols reduce γ by propagating blocks more quickly, shrinking the attacker’s timing advantage. Hash rate distribution across many pools lowers the chance that any single pool reaches the profitability threshold.