Auditing Foundry Smart Contract Security

Overview

Deployed smart contracts are immutable and custody real funds, so a bug shipped to mainnet cannot be patched — it can only be exploited. Most catastrophic DeFi losses come from a small set of recurring classes: reentrancy, broken access control, oracle/price manipulation, and unchecked arithmetic or external calls.

This skill runs a defense-in-depth, pre-deployment audit of a Foundry project, layering four independent techniques that each catch what the others miss:

1. Static analysis — slither (90+ detectors) and aderyn (Cyfrin, Rust) scan the AST/IR in seconds for known anti-patterns.
2. Symbolic execution — mythril (optional, slow) explores execution paths and SMT-solves for deep arithmetic/reentrancy bugs.
3. Property-based testing — forge test with fuzzing (testFuzz_*) and invariant tests (invariant_* + handler contracts with ghost variables) proves protocol-level properties hold across millions of random sequences.
4. Manual review + key hygiene — a structured checklist (see references/vulnerability-checklist.md) and a secrets/keystore audit so no private key ever lives in plaintext and deployment goes through an encrypted cast keystore (see references/secure-deployment-and-keys.md).

The skill is dev-side and pre-deployment — it is run by the engineer building the contract, not by a SOC after an incident. Findings gate the deploy: any high/critical static finding, failing test, leaked key, or low coverage = FAIL.

When to Use

• Before deploying any Solidity contract to a testnet or mainnet EVM chain.
• When writing or reviewing a Foundry project (foundry.toml, src/, test/, script/).
• When a contract handles value: tokens (ERC-20/721/1155), vaults, staking, AMMs, bridges, governance.
• When adding fuzz or invariant tests, or when coverage of value-moving functions is unknown.
• When wiring deployment scripts — to verify keys are in an encrypted keystore, not .env plaintext.
• When integrating a price oracle, external call, delegatecall, or upgradeable proxy.
• When triaging a Slither/Aderyn report and needing to separate real bugs from false positives.

Prerequisites

• Foundry installed (forge, cast, anvil): curl -L https://foundry.paradigm.xyz | bash && foundryup
• Slither + solc: pip install slither-analyzer and solc-select install <ver> && solc-select use <ver>
• Aderyn (recommended): cargo install aderyn (or npm i -g @cyfrin/aderyn)
• Mythril (optional, slow symbolic exec): pip install mythril
• gitleaks (key-leak scan): see the companion implementing-secret-scanning-with-gitleaks skill
• A Foundry project that compiles (forge build succeeds) — analyzers need build artifacts.
• Solidity ^0.8.x is assumed (built-in overflow checks); pre-0.8 contracts need extra SafeMath review.

Install the Python tools in a virtualenv (recommended on externally-managed distros). Never run analysis against untrusted contract source on a machine with funded wallets unlocked.

Steps

Step 1: Build and sanity-check the project

forge build                    # analyzers require fresh artifacts
forge fmt --check              # style gate (optional)
cat foundry.toml               # note solc version, optimizer, remappings, evm_version

Step 2: Static analysis (fast, run every time)

# Slither — full project (uses foundry.toml + remappings automatically)
slither . --json slither-report.json

# Aderyn — Cyfrin Rust analyzer, complementary detectors
aderyn . -o aderyn-report.json

Or run the bundled orchestrator that runs both, deduplicates, and gates the result:

python3 scripts/agent.py --project . --output audit-report.json

Step 3: Symbolic execution on critical contracts (optional, slow)

# Only on the highest-value contract(s) — Mythril is path-explosive
myth analyze src/Vault.sol --solc-json mythril.config.json --execution-timeout 300 -o json
# or: python3 scripts/agent.py --project . --mythril src/Vault.sol

Step 4: Property-based testing — fuzz + invariants

forge test -vvv                                  # unit + fuzz tests
forge coverage --report summary                  # coverage of value-moving code
forge test --match-test invariant_ -vvv          # invariant suite (handler-based)

Every value-moving contract should have invariant tests with a handler (bounded inputs, ghost variables, targetContract(handler)) — not just unit tests. See references/api-reference.md for the handler pattern, and write a test_RevertWhen_* (with vm.expectRevert) for each access-control guard.

Step 5: Manual review against the checklist

Walk references/vulnerability-checklist.md for every contract: reentrancy (checks-effects-interactions / nonReentrant), access control, oracle manipulation, delegatecall/proxy storage layout, unchecked return values, tx.origin, weak randomness, DoS, front-running/MEV, and ERC-specific pitfalls (approve race, fee-on-transfer, rebasing).

Step 6: Key hygiene & secure deploy

gitleaks detect --no-banner            # no private keys / mnemonics / .env committed
git ls-files | grep -E '\.env$|keystore' && echo "WARN: secrets tracked by git"

# Import the deploy key ONCE into an encrypted keystore — never a plaintext PRIVATE_KEY env
cast wallet import deployer --interactive

# Deploy via the keystore account (testnet first), simulate before --broadcast
forge script script/Deploy.s.sol --account deployer --rpc-url <testnet> --broadcast --verify

See references/secure-deployment-and-keys.md for the full hardening rules (MetaMask hygiene, hardware wallet for mainnet, RPC trust, post-deploy verification).

Step 7: Triage and report

Combine Slither + Aderyn + Mythril + test results, deduplicate by (file, line), drop confirmed false positives, rank by exploitability × financial impact, and map each to its SWC id. The orchestrator emits audit-report.json with a PASS/FAIL gate.

Expected Output

A JSON audit report listing findings with SWC identifiers, severity, tool source, affected contract/function/line, and remediation; plus the test/coverage summary and a single PASS / FAIL deploy gate. FAIL on any high/critical static finding, failing test, leaked secret, or coverage below the configured threshold on value-moving code.