Security Expert

Application Security Expert

> Defensive only: build and harden, don't attack. Treat all input as hostile, enforce authorization on the server for every request, and keep secrets out of code and logs. Default deny.

When to Use

• Writing security-sensitive code (auth, input handling, file/URL/DB access, uploads).
• Reviewing a diff/feature for common vulnerabilities.
• Handling secrets, tokens, sessions, passwords, or dependencies.
• Hardening against the OWASP Top 10.

When NOT to Use

• Building exploits, offensive tooling, or attacks on systems you don't own → decline.
• General correctness bugs → debugging-expert.
• Cluster RBAC/NetworkPolicy specifics → kubernetes-expert.

Core Principles

1. Trust boundaries & input

• All external input is hostile (body, query, headers, files, env, upstream APIs). Validate against an allowlist, then encode/escape for the destination context (HTML, SQL, shell, URL, LDAP).
• Injection: parameterized queries / prepared statements only — never concatenate untrusted data into SQL or shell commands. Avoid eval/dynamic command execution on user input.
• XSS: context-aware output encoding + a strict Content-Security-Policy; avoid innerHTML/dangerouslySetInnerHTML with untrusted data.

2. AuthN vs AuthZ (the #1 real-world gap)

• Authenticate with vetted libraries; hash passwords with argon2/bcrypt (never md5/sha1/plaintext). Cookies HttpOnly+Secure+SameSite; short-lived tokens with rotation.
• Authorize every request on the server with object-level checks (does this user own this resource?). Never trust client-supplied roles, IDs, or hidden fields. Default deny. This is Broken Access Control — OWASP #1.
• Protect state-changing requests from CSRF (tokens or SameSite), and validate redirect targets / server-side fetch URLs against an allowlist to prevent SSRF and open redirects.

3. Secrets & data

• Secrets in env/secret manager, never in code, git, or client bundles. Rotate on exposure. Never log secrets, tokens, passwords, or PII.
• TLS in transit; encrypt sensitive data at rest; minimize collection/retention. Don't leak stack traces or internal details in error responses.

4. Dependencies & supply chain

• Pin and audit deps (npm audit, pip-audit, cargo audit, Dependabot/Renovate); patch known-vuln packages promptly. Verify integrity (lockfiles) and prefer maintained libraries over hand-rolled crypto.

Review checklist

• [ ] Every query parameterized? [ ] Output encoded for its context + CSP?
• [ ] Server-side authz on every endpoint/object? [ ] Default deny?
• [ ] Secrets out of code/logs? [ ] Passwords hashed with argon2/bcrypt?
• [ ] CSRF + SSRF/redirect protections? [ ] Deps audited & pinned?
• [ ] File uploads validated (type/size/path)? [ ] Errors don't leak internals?

Common Mistakes

• String-built SQL/commands → injection; parameterize.
• Authorization only on the client / missing object-level checks → IDOR, privilege escalation.
• Trusting role/userId from the request → spoofing; derive from the authenticated session.
• Secrets in code, env files in git, or logs → leakage.
• Plaintext/weak password hashing → mass compromise on breach.
• Reflecting user input unescaped → XSS. Unvalidated server-side fetch → SSRF.
• Verbose error/stack traces to clients → info disclosure.

Examples

Parameterized query + server-side object authorization

# ❌ injectable
cur.execute(f"SELECT * FROM docs WHERE id = '{doc_id}'")
# ✅ parameterized
cur.execute("SELECT * FROM docs WHERE id = %s", (doc_id,))

def get_document(user, doc_id):
    doc = repo.find(doc_id)
    if doc is None or doc.owner_id != user.id:   # object-level access control
        raise Forbidden()                         # default deny
    return doc

See Also

• nodejs-backend-expert / api-design-expert — secure endpoints, auth flows, rate limits.
• sql-expert — parameterization and least-privilege DB access.
• docker-expert / kubernetes-expert — image scanning, secrets, network policy.
• security-review (built-in) — automated review of pending changes.

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: Miaoge-Ge
• Source: Miaoge-Ge/coding-agent-skills
• License: MIT

Install and usage instructions live in the source repository linked above.