How do you audit a smart contract for security vulnerabilities?

Smart contract auditing is a systematic process to identify security vulnerabilities before deployment: (1) Automated static analysis — run automated tools first: Slither (most popular, free, Python-based, detects 80+ vulnerability patterns), Mythril (symbolic execution), Echidna (property-based fuzzing), Foundry's fuzz testing; (2) Manual review — the critical, irreplaceable component. Auditors manually trace every code path, review business logic, check access control, examine all external calls; (3) Vulnerability checklist — systematically check: reentrancy, access control, integer arithmetic, oracle manipulation, flash loan attack surfaces, front-running, signature replay, denial of service, storage collisions in proxies; (4) Test coverage — ensure comprehensive unit tests and integration tests; identify and test edge cases; (5) Economic model review — analyze tokenomics and game theory for exploitable incentives; (6) Invariant testing — define properties that must always be true (e.g., "total balances == totalSupply") and run Echidna/Foundry fuzzer to find violations; (7) Formal verification — use Certora Prover or K Framework for mathematical proof of properties; (8) Multiple auditors — different auditing firms catch different issues; (9) Bug bounty — post-audit, launch a bug bounty (Immunefi) for additional community review. Leading audit firms: Trail of Bits, OpenZeppelin, Consensys Diligence, Spearbit, Code4rena (competitive audits).