Florianbuetow Claude Code Harden

Security Hardening

Proactive security improvement suggestions. Unlike vulnerability scanners that find what is broken, this skill identifies what could be better -- defense-in-depth measures, missing security headers, insufficient input validation, absent rate limiting, and other hardening opportunities that reduce the blast radius of future vulnerabilities.

Supported Flags

Read ../../shared/schemas/flags.md for the full flag specification.

Workflow

Step 1: Identify Technology Stack

Scan the codebase to determine:

1. Web framework(s): Express, Django, Flask, Spring, Rails, Next.js, ASP.NET, FastAPI, etc.
2. Deployment target: Container, serverless, VM, PaaS (from Dockerfile, serverless.yml, etc.).
3. Reverse proxy/CDN: Nginx, Apache, Cloudflare, AWS ALB (from config files).
4. Database(s): SQL, NoSQL, cache layers.
5. Authentication mechanism: Session, JWT, OAuth, SAML.
6. Existing security middleware: Helmet, django-security, Spring Security, etc.

Step 2: Check Security Headers

Verify the application sets these HTTP response headers (or that a reverse proxy / CDN handles them):

Note: If a reverse proxy config (nginx.conf, etc.) is present and sets these headers, do not flag them as missing from application code.

Step 3: Review CORS Configuration

Check for overly permissive CORS settings:

1. Access-Control-Allow-Origin: * on authenticated endpoints.
2. Reflecting the Origin header without validation.
3. Access-Control-Allow-Credentials: true with wildcard origins.
4. Overly broad allowed methods or headers.
5. Missing Vary: Origin when origin is dynamic.

Step 4: Assess Input Validation

For each entry point discovered (or in scope):

1. Schema validation: Are request bodies validated against a schema (Joi, Zod, Pydantic, JSON Schema)?
2. Type coercion: Are string inputs properly typed before use?
3. Length limits: Are string lengths bounded? Are array sizes limited?
4. Allowlist vs denylist: Is validation positive (allowlist) rather than negative (denylist)?
5. Nested input: Are deeply nested objects limited to prevent DoS?
6. File validation: Are uploaded files validated beyond extension (magic bytes, size limits)?

Step 5: Check Rate Limiting

Identify endpoints that should have rate limiting:

1. Authentication endpoints: Login, registration, password reset, MFA verification.
2. API endpoints: Especially those that are computationally expensive or return sensitive data.
3. File upload endpoints: To prevent storage exhaustion.
4. Search/query endpoints: To prevent enumeration and DoS.

Check if rate limiting is implemented and whether limits are reasonable.

Step 6: Review Error Handling and Information Disclosure

1. Stack traces: Are stack traces exposed in production error responses?
2. Verbose errors: Do error messages reveal internal paths, versions, or database details?
3. Error differentiation: Do auth errors distinguish between "user not found" and "wrong password" (enables enumeration)?
4. Default error pages: Are framework default error pages replaced?
5. Debug mode: Is debug mode disabled in production configuration?

Step 7: Assess Security Logging

Check that these security-relevant events are logged:

1. Authentication events: Login success/failure, logout, password changes.
2. Authorization failures: Access denied events with user and resource context.
3. Input validation failures: Rejected requests with sanitized details.
4. Administrative actions: Config changes, user management, privilege changes.
5. Sensitive data access: Audit trail for PII/financial data reads.

Verify logs do NOT contain: passwords, tokens, credit card numbers, SSNs, or other sensitive data.

Step 8: Check Defensive Coding Patterns

1. Fail-closed defaults: Do authorization checks default to deny?
2. Secure defaults: Are new configurations secure by default?
3. Least privilege: Do database connections, API keys, and service accounts use minimal permissions?
4. Timeout configuration: Do HTTP clients, database connections, and external calls have timeouts?
5. Resource limits: Are memory/CPU-intensive operations bounded?
6. Dependency security: Is npm audit / pip audit / equivalent run in CI?

Step 9: Report

Output hardening suggestions grouped by category.

Output Format

Hardening suggestions are advisory and use a lighter format than vulnerability findings.

## Security Hardening Report

### Summary
- Hardening suggestions: N
- By priority: N HIGH, N MEDIUM, N LOW
- Categories covered: headers, cors, validation, rate-limiting, logging, error-handling, config

### HIGH Priority

#### [H-001] Missing Content-Security-Policy header
**Category**: Headers | **Effort**: Low
**Location**: src/middleware/security.ts
**Current**: No CSP header set
**Recommended**: Add strict CSP via helmet
```js
app.use(helmet.contentSecurityPolicy({
  directives: {
    defaultSrc: ["'self'"],
    scriptSrc: ["'self'"],
    styleSrc: ["'self'", "'unsafe-inline'"],
    imgSrc: ["'self'", "data:"],
  }
}));

MEDIUM Priority

...

LOW Priority

...

When hardening gaps represent actual vulnerabilities (e.g., CORS misconfiguration allowing credential theft), emit a formal finding using `../../shared/schemas/findings.md`.

Finding ID prefix: **HARD** (e.g., `HARD-001`).

- `metadata.tool`: `"harden"`
- `references.cwe`: Varies by suggestion (e.g., `CWE-693` Protection Mechanism Failure, `CWE-16` Configuration)

## Pragmatism Notes

- Hardening is contextual. An internal admin tool has different requirements than a public API.
- Do not recommend CSP for a CLI tool or rate limiting for a batch job.
- If a CDN or reverse proxy handles headers, note that rather than flagging missing headers in app code.
- Prioritize suggestions that are easy to implement with high security impact.
- Acknowledge when existing security measures are already good. Not every review needs findings.
- Some frameworks (Next.js, Rails) include secure defaults. Credit what is already done well.

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