What is selfish mining?

Selfish mining is a strategy where a miner or group of miners hides newly found blocks instead of sharing them right away. They then release those blocks at carefully chosen moments, trying to make other miners’ work go unpaid and collect a larger share of rewards than their computing power would normally earn. It is a real mining attack, but it is not a guaranteed way to make more money.

How withholding blocks creates an advantage

A block is a batch of transactions that miners compete to add to the blockchain, the network’s shared transaction record. Normally, a miner announces a valid block immediately so everyone can start building on it.

A selfish miner does something different:

• They find a block and keep it private.
• They continue mining on top of that hidden block.
• Other miners, unaware of it, keep working on the public chain.
• The selfish miner releases some or all of the hidden blocks when doing so could give their version an advantage.

Nodes—the computers checking the blockchain—choose between valid competing chains using the network’s rules. In Monero, the deciding factor is cumulative mining difficulty, meaning the total amount of mining work represented by a chain.

If the selfish miner’s chain wins, competing blocks may become stale blocks: valid blocks that are left outside the accepted chain. Their miners receive no block reward.

Why it can pay—and why it can fail

The goal is not necessarily to earn more coins immediately. It is to capture a larger percentage of accepted blocks than the attacker’s percentage of total mining power.

For example, someone with 30% of the mining power would normally expect roughly 30% of rewards over a long period. Selfish mining aims to push that reward share higher. This is an illustration, not a claim that 30% is enough in every network.

There is no universal success threshold. Results depend on:

• How much mining power the attacker controls.
• How quickly their blocks reach other miners.
• Which competing block miners choose when two versions arrive.
• The withholding strategy and the network’s difficulty rules.

The attacker also takes a risk. If other miners get ahead, hidden blocks can become worthless. A higher share of rewards does not automatically mean higher profit after electricity and other costs.

How this differs from a 51% attack

Selfish mining does not inherently require majority control. Under some conditions, a miner with less than half the network’s mining power can gain an advantage through withholding.

A 51% attack usually refers to an attacker controlling most mining power, giving them a stronger ability to replace recent blockchain history or prevent transactions from being included.

Selfish mining primarily targets the distribution of mining rewards. It does not, by itself, let someone forge your signature, spend coins from your wallet, or create coins outside the network’s rules.

What this means for Monero and browser mining

Monero uses RandomX, a mining algorithm designed to favor ordinary computer processors. That hardware preference does not eliminate the possibility of block-withholding strategies.

For you as a browser miner, selfish mining is not a useful performance setting. You generally contribute work through a service or pool—a group that combines mining power—and do not control when network blocks are published.

Also, a delayed pool payout is not evidence of selfish mining. Payout schedules and withholding newly found network blocks are different things.

The bottom line

Selfish mining tries to turn secret blocks and careful timing into an unfair reward advantage. It can harm honest miners, but it carries risks and does not guarantee profit or control over the network.