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:
- Static analysis —
slither (90+ detectors) and aderyn (Cyfrin, Rust) scan
the AST/IR in seconds for known anti-patterns.
- Symbolic execution —
mythril (optional, slow) explores execution paths and
SMT-solves for deep arithmetic/reentrancy bugs.
- 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.
- 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.