Replay Attack
An attack where a valid signed transaction or message is captured and submitted again in a context where it is incorrectly accepted.Definition
An attack where a valid signed transaction or message is captured and submitted again in a context where it is incorrectly accepted.Why it matters
Security concepts help users and developers recognize common blockchain attack surfaces and defenses.How it works
When a network forks, the transaction history is mirrored. An attacker waits for a user to broadcast a transaction on the original chain. They then intercept this signed transaction and rebroadcast it onto the forked chain. If the protocol lacks replay protection, the network treats it as a legitimate request and processes it again.Real-world example
When Bitcoin Cash forked from Bitcoin, users who were not careful had their transactions replayed across both chains, leading to accidental loss of Bitcoin Cash.Advantages
- Highlights the importance of cross-chain security
- Forces development of replay protection protocols
- Validates importance of network consensus
Limitations
- Mostly relevant during blockchain forks
- Requires technical awareness to avoid
- Can lead to significant financial loss
Common misconceptions
- People think replay attacks are a general threat at all times, but they are highly specific to hard forks and protocol upgrades.
- Users believe that a transaction is only valid on the chain it was sent to, forgetting that raw data can be broadcast to any compatible network.
Related knowledge
- Phishing — term
- Smart Contract Audit — term
- Sybil Attack — term
Canonical knowledge ID:
glossary:replay-attack