What Is a “Fork” and How Does It Relate to the Potential for Replay Attacks?

A blockchain fork is a split in the network's history, resulting in two distinct chains. When a fork occurs, a valid transaction signed on the original chain might also be valid on the new chain, allowing for a "cross-chain replay attack." This is because the signature is for the same transaction data, which exists on both chains.

Solutions like Ethereum's EIP-155 prevent this by including a unique chain ID in the signed transaction data.