Signature Replay Attacks in Cross-Chain Protocols

In the world of decentralized finance (DeFi), cross-chain protocols enable assets and data to move between different blockchain ecosystems, allowing users to seamlessly interact with various networks. However, while cross-chain systems open new doors for innovation, they also introduce unique security risks—one such risk is the signature replay attack.

Signature replay attacks occur when a valid cryptographic signature, intended for use on one blockchain, is maliciously replayed on a different blockchain where it also remains valid. This type of vulnerability arises when cross-chain protocols fail to properly validate that the signature belongs only to the intended chain or transaction context. If left unchecked, signature replay attacks can lead to unauthorized transfers of assets, duplicate transactions, and serious financial losses.

In this tutorial section, we will explain how signature replay attacks work in cross-chain protocols, walk through an example scenario, and provide mitigation strategies to secure these protocols against replay attacks.

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Understanding Signature Replay Attacks

In blockchain systems, digital signatures are used to verify the authenticity and integrity of a transaction. Each signature is created by a user using their private key and is intended to authorize a specific transaction on a particular blockchain. However, in cross-chain environments, a signature that is valid on one chain may also be valid on another if the system does not differentiate between chains.

In a signature replay attack, an attacker reuses a valid signature intended for one blockchain to perform the same transaction on another blockchain. For example, if a cross-chain protocol accepts the same proof on multiple chains without checking the chain context, an attacker can replay the signature to claim funds or perform unauthorized actions on different networks.

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Example Scenario: Signature Replay in a Cross-Chain Bridge

Let’s walk through a step-by-step example to better understand how a signature replay attack can be exploited in a cross-chain protocol.

Scenario: A Vulnerable Cross-Chain Bridge

1. User Interaction with a Cross-Chain Bridge:

   • Alice uses a cross-chain bridge to transfer her ERC-20 tokens from Ethereum to a Binance Smart Chain (BSC).

   • Alice generates a signature authorizing the transfer of 100 tokens from Ethereum to BSC.

2. Bridge Relayer Sends Proof to BSC:

   • The bridge’s relayer submits the transaction proof, which includes Alice's signature, to the bridge contract on BSC. The bridge contract validates Alice’s signature and transfers 100 tokens to her BSC address.

3. Replay Attack on Polygon:

   • The bridge protocol does not differentiate between chains and accepts Alice’s signature without validating which chain it belongs to.

   • An attacker notices this vulnerability and replays Alice’s signature on the bridge contract on Polygon.

   • Since the signature is valid and the protocol doesn’t check that it was already used on Ethereum, the attacker successfully transfers another 100 tokens to Alice’s Polygon address—effectively duplicating the transaction.

4. Financial Loss:

   • Alice’s original transaction was valid on both BSC and Ethereum, but the replayed signature allowed the attacker to duplicate it on Polygon, resulting in unauthorized token transfers.

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Key Issues Leading to Signature Replay Attacks

1. Lack of Chain Context:

   • The root cause of this vulnerability is that the bridge protocol failed to check whether a signature was already used on a different blockchain. Signatures are often valid across multiple EVM-compatible chains (Ethereum, BSC, Polygon, etc.), making it critical to validate the chain ID or network context when verifying signatures.

2. Reuse of Signatures:

   • If a signature generated for one blockchain is valid on another, attackers can reuse it to execute duplicate transactions across different networks, resulting in the transfer of assets multiple times.

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Mitigation Strategies

To protect cross-chain protocols from signature replay attacks, developers must ensure that every signature is uniquely tied to a specific blockchain or network context. Below are some critical mitigation steps:

1. Chain ID Validation

• The chain ID is a unique identifier for each blockchain. Always include the chain ID as part of the transaction data when generating a signature. This ensures that signatures generated on one chain are not valid on another.