foxguard

name	foxguard
description	Apply foxguard's secure-coding patterns when writing or fixing security-sensitive code. Use proactively when handling user input, shelling out, building SQL, doing file I/O, fetching URLs, generating randomness, or touching crypto. Use when this capability is needed.
metadata	{"author":"0sec-labs"}

Use this guidance when writing or fixing code that touches any of these surfaces. Match the language patterns to what foxguard's rules look for so the auto-scan stays clean.

Command execution

• Never pass concatenated strings to a shell. Use argument lists.
  • Python: subprocess.run(["git", "log", branch], check=True) — never subprocess.run(f"git log {branch}", shell=True).
  • Node: execFile("git", ["log", branch]) — never exec(git log ${branch}).
  • Go: exec.Command("git", "log", branch) — never exec.Command("sh", "-c", "git log "+branch).
• If a shell is genuinely required, validate the input against a strict allowlist regex first.

SQL

• Use parameterized queries everywhere. cursor.execute("SELECT * FROM u WHERE id = ?", (uid,)) — not f-strings or %.
• ORMs are fine; raw query.format(...) / + concat is not.

Path traversal

• Resolve and bound paths: Path(root).resolve() / Path(name).name — strip directory components from untrusted names.
• Reject inputs that contain .., absolute paths, or null bytes before joining.

SSRF

• For outbound HTTP from user-supplied URLs: parse the URL, resolve the host, and reject private/loopback/link-local ranges (10.0.0.0/8, 127.0.0.0/8, 169.254.0.0/16, 172.16.0.0/12, 192.168.0.0/16, ::1, fc00::/7, fe80::/10) and metadata endpoints (169.254.169.254).
• Prefer an allowlist over a denylist when the destinations are known.

Secrets

• Read from env / secrets manager, never from string literals. os.getenv("API_TOKEN") — not API_TOKEN = "sk-...".
• Even in tests, prefer fixtures from env or a vault. High-entropy strings in code will be flagged.
• Never log secrets. Redact before logging.

Randomness

• Security-sensitive: secrets.token_urlsafe, crypto.randomBytes, OsRng, crypto/rand. Never random.random(), Math.random(), rand(), math/rand.
• Non-security (jitter, sampling): the fast PRNGs are fine.

Crypto

• New code: AES-GCM or ChaCha20-Poly1305 for symmetric; Ed25519 for signatures; X25519 for key agreement (until PQ rollout). Never DES, 3DES, RC4, MD5, SHA-1.
• Asymmetric crypto in long-lived security paths: prefer hybrid suites (X25519+ML-KEM) where the toolchain supports it. Use /foxguard:pq-audit to find legacy usage.
• Password hashing: argon2id or bcrypt. Never plain SHA-2 or unsalted hashes.

Deserialization

• Never pickle.load, yaml.load (use safe_load), Marshal.load, or unserialize on untrusted input.
• For JSON, deserialize into a typed schema (pydantic, zod, serde) — don't trust the shape.

When you finish a fix, the PostToolUse hook will re-scan the file and confirm it's clean. If a finding persists, read the rule's description carefully — the remediation is usually one of the patterns above.

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Source: 0sec-labs/foxguard — distributed by TomeVault.