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Educational demonstration

Signature Replay Across Deployments, Chains and Encodings

An off-chain authorization scheme without a nonce or a complete EIP-712 domain lets one signature be spent again and again.

Lab target · not a client engagement
Overview

Signature-based authorization is only as safe as what the signature commits to. This demonstration covers three replay variants against one lab ledger, then shows a version that commits to signer, nonce, deadline, chain and contract.

Findings
critical
XV-DEMO-19 · Verified by test

Signed messages have no nonce

Description

The contract verifies the signer but never records the signature as used.

Impact

Anyone holding a signature can resubmit it indefinitely.

Recommendation

Include a per-signer nonce in the signed struct and increment it on use.

high
XV-DEMO-20 · Verified by test

Signature not bound to chain or contract

Description

The signed payload omits chainId and the verifying contract, so it is valid on every deployment.

Impact

Signatures intended for one deployment or chain can be replayed on another.

Recommendation

Use a full EIP-712 domain, computed from block.chainid at verification time.

medium
XV-DEMO-21 · Verified by test

Malleable signatures accepted via raw ecrecover

Description

High-s signatures are accepted, so a second valid encoding exists for each signature.

Impact

Defeats any replay protection keyed on signature bytes.

Recommendation

Reject s values in the upper half of the curve order, and key replay protection on nonces.

Outcome

Measured in the lab: a single 1-ETH transfer signature was spent three times on one vulnerable ledger (original, replay, malleated encoding) and once more on a second deployment.

On the fixed ledger the signature was accepted once; the replay, the malleated encoding, the second deployment, and a signature made for a different chain ID were all rejected. Cross-chain replay is simulated by signing for another chain ID on a single local network.

Reproduce

The lab code and tests for this case study live in labs/web3 of the Xevry website repository.

cd labs/web3npm installnpx hardhat test test/SignatureReplay.test.js

Files: labs/web3/contracts/Signatures.sol · labs/web3/test/SignatureReplay.test.js

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