> ## Documentation Index
> Fetch the complete documentation index at: https://docs.theblockchainlibrary.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Replay Attack

> An attack where a valid signed transaction or message is captured and submitted again in a context where it is incorrectly accepted.

# 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](/generated/v2/glossary/phishing) — term
* [Smart Contract Audit](/generated/v2/glossary/smart-contract-audit) — term
* [Sybil Attack](/generated/v2/glossary/sybil-attack) — term

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**Canonical knowledge ID:** `glossary:replay-attack`