Replay Attack

An attack in which a valid signed transaction on one network is broadcast on a different network where it is also accepted, moving funds without the user's knowledge of the second spend. Replay attacks are most common after hard forks that produce two chains with shared transaction history. Chains defend against this with replay protection: chain-specific identifiers (chain ID in Ethereum, SIGHASH_FORKID in Bitcoin Cash) that invalidate signatures on other chains.