What is Replay Attack?

A replay attack occurs when a valid transaction from one blockchain is rebroadcast and executed on another network sharing the same address space. It most commonly affects users during hard forks.

How does Replay Attack work?

A blockchain splits into two networks with identical histories and addresses.
A user sends a transaction on Chain A.
An attacker copies the signed transaction and broadcasts it to Chain B.
Chain B processes it as valid, causing the user to unintentionally lose funds on the second chain.

Why does Replay Attack matter?

Users can lose funds on a chain they never intended to interact with. Modern blockchains solve this with a unique Chain ID in every transaction via EIP-155, making signatures invalid on other networks.

Key features of Replay Attack

Exploits shared address spaces after a network fork
Requires no private key only the broadcast transaction data
Prevented by EIP-155 chain-specific signatures
Standard protection in 100 percent of major production blockchains today

Examples of Replay Attack

During the Ethereum and Ethereum Classic split in 2016, many users lost funds to replayed transactions. The Bitcoin Cash fork in 2017 faced the same risk until replay protection was added.