What Is a “Selfish Mining” Attack and How Does It Relate to 51 Percent Control?

Selfish mining is a strategy where a mining pool secretly mines blocks and withholds them from the public network. They release their private chain only when it is long enough to cause the public chain to be orphaned.

This allows the selfish miners to earn a disproportionately high share of the block rewards compared to their actual hashrate. While it does not require 51 percent control, a larger hashrate (e.g. over 33 percent) makes the attack much more profitable and successful.

It is an attack on the fairness of block reward distribution, not directly on transaction immutability.