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signature-vulnerabilities

Signature attack vectors and safe verification patterns for Solidity. Use when implementing permit, meta-transactions, gasless approvals, or any signature-based authentication. Covers replay attacks, EIP-712, nonce management, malleability, and permit front-running.

skills/signature-vulnerabilities/SKILL.md

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# Signature Vulnerabilities

## Replay Attacks

A valid signature can be reused unless explicitly prevented.

### Cross-Chain Replay

A signature valid on Ethereum mainnet can be replayed on Arbitrum, Optimism, etc.

```solidity
// VULNERABLE: no chain ID in signed data
bytes32 hash = keccak256(abi.encodePacked(user, amount, nonce));