Replay Attack

A replay attack is a form of network security exploit where a valid data transmission is maliciously or fraudulently repeated or delayed. In the context of blockchain, it involves taking a signed transaction intended for one network and broadcasting it to another network where the transaction is also technically valid.

Because the private key signature remains valid across both chains, the transaction can be executed again without the owner's consent, potentially resulting in unauthorized asset transfers. This attack vector is particularly dangerous during chain splits or hard forks, where both chains share the same transaction history and address formats.

Mitigation strategies like EIP-155 effectively neutralize this risk by ensuring transaction signatures are chain-specific. Without such safeguards, users could lose funds simply by interacting with one chain while an attacker replays their actions on another.