How Does the ‘Longest Chain Rule’ in Proof-of-Work Facilitate the Selfish Mining Attack?

The longest chain rule dictates that the network always accepts the longest valid chain of blocks as the true, canonical history. Selfish miners exploit this by secretly extending their private chain, creating a longer chain than the honest network's.

When the selfish miner reveals their longer chain, the honest network is forced to switch to it, invalidating the blocks mined by honest participants. This rule is the core vulnerability.