# Security And Hardening

> Hardens code against vulnerabilities. Use when handling user input, authentication, data storage, or external integrations. Use when building any feature that accepts untrusted data, manages user sessions, or interacts with third-party services.

- Skill: `oscarqjh/security-and-hardening` (Agent Skill)
- Install (CLI): `npx skillmds@latest add oscarqjh/security-and-hardening`
- Raw SKILL.md: https://api.skillmd.com/api/skills/oscarqjh/security-and-hardening/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: AI & ML
- Author: oscarqjh (https://skillmd.com/u/oscarqjh)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/oscarqjh/security-and-hardening

---


# Security and Hardening

## Overview

Security-first development practices for web applications. Treat every external input as hostile, every secret as sacred, and every authorization check as mandatory. Security isn't a phase — it's a constraint on every line of code that touches user data, authentication, or external systems.

**Related:** `super-agent-skills:threat-modeling` identifies WHAT to protect against (design time). This skill ensures the protection is correctly IMPLEMENTED (build time).

## When to Use

- Building anything that accepts user input
- Implementing authentication or authorization
- Storing or transmitting sensitive data
- Integrating with external APIs or services
- Adding file uploads, webhooks, or callbacks
- Handling payment or PII data
- Adding or updating third-party dependencies
- Auditing packages for security or license compliance
- Building applications that use LLMs or AI models

## The Three-Tier Boundary System

### Always Do (No Exceptions)

- **Validate all external input** at the system boundary (API routes, form handlers)
- **Parameterize all database queries** — never concatenate user input into SQL
- **Encode output** to prevent XSS (use framework auto-escaping, don't bypass it)
- **Use HTTPS** for all external communication
- **Hash passwords** with bcrypt/scrypt/argon2 (never store plaintext)
- **Set security headers** (CSP, HSTS, X-Frame-Options, X-Content-Type-Options)
- **Use httpOnly, secure, sameSite cookies** for sessions
- **Run `npm audit`** (or equivalent) before every release

### Ask First (Requires Human Approval)

- Adding new authentication flows or changing auth logic
- Storing new categories of sensitive data (PII, payment info)
- Adding new external service integrations
- Changing CORS configuration
- Adding file upload handlers
- Modifying rate limiting or throttling
- Granting elevated permissions or roles

### Never Do

- **Never commit secrets** to version control (API keys, passwords, tokens)
- **Never log sensitive data** (passwords, tokens, full credit card numbers)
- **Never trust client-side validation** as a security boundary
- **Never disable security headers** for convenience
- **Never use `eval()` or `innerHTML`** with user-provided data
- **Never store sessions in client-accessible storage** (localStorage for auth tokens)
- **Never expose stack traces** or internal error details to users

## OWASP Top 10 Prevention

### 1. Injection (SQL, NoSQL, OS Command)

```typescript
// BAD: SQL injection via string concatenation
const query = `SELECT * FROM users WHERE id = '${userId}'`;

// GOOD: Parameterized query
const user = await db.query('SELECT * FROM users WHERE id = $1', [userId]);

// GOOD: ORM with parameterized input
const user = await prisma.user.findUnique({ where: { id: userId } });
```

### 2. Broken Authentication

```typescript
// Password hashing
import { hash, compare } from 'bcrypt';

const SALT_ROUNDS = 12;
const hashedPassword = await hash(plaintext, SALT_ROUNDS);
const isValid = await compare(plaintext, hashedPassword);

// Session management
app.use(session({
  secret: process.env.SESSION_SECRET,  // From environment, not code
  resave: false,
  saveUninitialized: false,
  cookie: {
    httpOnly: true,     // Not accessible via JavaScript
    secure: true,       // HTTPS only
    sameSite: 'lax',    // CSRF protection
    maxAge: 24 * 60 * 60 * 1000,  // 24 hours
  },
}));
```

### 3. Cross-Site Scripting (XSS)

```typescript
// BAD: Rendering user input as HTML
element.innerHTML = userInput;

// GOOD: Use framework auto-escaping (React does this by default)
return <div>{userInput}</div>;

// If you MUST render HTML, sanitize first
import DOMPurify from 'dompurify';
const clean = DOMPurify.sanitize(userInput);
```

### 4. Broken Access Control

```typescript
// Always check authorization, not just authentication
app.patch('/api/tasks/:id', authenticate, async (req, res) => {
  const task = await taskService.findById(req.params.id);

  // Check that the authenticated user owns this resource
  if (task.ownerId !== req.user.id) {
    return res.status(403).json({
      error: { code: 'FORBIDDEN', message: 'Not authorized to modify this task' }
    });
  }

  // Proceed with update
  const updated = await taskService.update(req.params.id, req.body);
  return res.json(updated);
});
```

### 5. Security Misconfiguration

```typescript
// Security headers (use helmet for Express)
import helmet from 'helmet';
app.use(helmet());

// Content Security Policy
app.use(helmet.contentSecurityPolicy({
  directives: {
    defaultSrc: ["'self'"],
    scriptSrc: ["'self'"],
    styleSrc: ["'self'", "'unsafe-inline'"],  // Tighten if possible
    imgSrc: ["'self'", 'data:', 'https:'],
    connectSrc: ["'self'"],
  },
}));

// CORS — restrict to known origins
app.use(cors({
  origin: process.env.ALLOWED_ORIGINS?.split(',') || 'http://localhost:3000',
  credentials: true,
}));
```

### 6. Sensitive Data Exposure

```typescript
// Never return sensitive fields in API responses
function sanitizeUser(user: UserRecord): PublicUser {
  const { passwordHash, resetToken, ...publicFields } = user;
  return publicFields;
}

// Use environment variables for secrets
const API_KEY = process.env.STRIPE_API_KEY;
if (!API_KEY) throw new Error('STRIPE_API_KEY not configured');
```

## Input Validation Patterns

### Schema Validation at Boundaries

```typescript
import { z } from 'zod';

const CreateTaskSchema = z.object({
  title: z.string().min(1).max(200).trim(),
  description: z.string().max(2000).optional(),
  priority: z.enum(['low', 'medium', 'high']).default('medium'),
  dueDate: z.string().datetime().optional(),
});

// Validate at the route handler
app.post('/api/tasks', async (req, res) => {
  const result = CreateTaskSchema.safeParse(req.body);
  if (!result.success) {
    return res.status(422).json({
      error: {
        code: 'VALIDATION_ERROR',
        message: 'Invalid input',
        details: result.error.flatten(),
      },
    });
  }
  // result.data is now typed and validated
  const task = await taskService.create(result.data);
  return res.status(201).json(task);
});
```

### File Upload Safety

```typescript
// Restrict file types and sizes
const ALLOWED_TYPES = ['image/jpeg', 'image/png', 'image/webp'];
const MAX_SIZE = 5 * 1024 * 1024; // 5MB

function validateUpload(file: UploadedFile) {
  if (!ALLOWED_TYPES.includes(file.mimetype)) {
    throw new ValidationError('File type not allowed');
  }
  if (file.size > MAX_SIZE) {
    throw new ValidationError('File too large (max 5MB)');
  }
  // Don't trust the file extension — check magic bytes if critical
}
```

## Triaging npm audit Results

| Severity | Reachable in prod? | Action |
|----------|-------------------|--------|
| Critical/High | Yes | Fix immediately |
| Critical/High | No (dev-only, unused path) | Fix soon, not a blocker |
| Moderate | Yes | Fix next release cycle |
| Low | — | Fix during regular dependency updates |

Key question: Is the vulnerable function actually called in your code path? When deferring a fix, document the reason and set a review date.

## Rate Limiting

```typescript
import rateLimit from 'express-rate-limit';

// General API rate limit
app.use('/api/', rateLimit({
  windowMs: 15 * 60 * 1000, // 15 minutes
  max: 100,                   // 100 requests per window
  standardHeaders: true,
  legacyHeaders: false,
}));

// Stricter limit for auth endpoints
app.use('/api/auth/', rateLimit({
  windowMs: 15 * 60 * 1000,
  max: 10,  // 10 attempts per 15 minutes
}));
```

## Secrets Management

```
.env files:
  ├── .env.example  → Committed (template with placeholder values)
  ├── .env          → NOT committed (contains real secrets)
  └── .env.local    → NOT committed (local overrides)

.gitignore must include:
  .env
  .env.local
  .env.*.local
  *.pem
  *.key
```

**Before committing:** Run `git diff --cached | grep -i "password\|secret\|api_key\|token"` to catch accidentally staged secrets.

For detailed security checklists and pre-commit verification steps, see `references/security-checklist.md`.

## Supply Chain Security

Dependencies are attack surface. Every package you install runs with your application's privileges.

### Before Adding a Dependency

Ask five questions before `npm install`:

1. **Do we need it?** Can the standard library or existing deps solve this?
2. **Is it maintained?** Last commit within 6 months? Active issue responses?
3. **Is it trusted?** Check download count, maintainer count, GitHub stars. Low numbers on a critical package = risk.
4. **Is it safe?** Run `npm audit` / `pip audit` / `cargo audit` before and after adding.
5. **Is the license compatible?** GPL in a proprietary project = legal problem. Check with `license-checker` or equivalent.

### Typosquatting Detection

Before installing, verify the exact package name on npm/PyPI — don't trust autocomplete (`lodash` not `1odash`, `@babel/core` not `@bable/core`).

### AI-Specific Vulnerabilities

When building applications that use LLMs:
- **Prompt injection:** Treat all user input that reaches an LLM as untrusted. Never concatenate user input directly into system prompts.
- **Output validation:** LLM output is untrusted data. Validate and sanitize before using in SQL, HTML, or system commands.
- **Model supply chain:** Verify model checksums. Don't load models from untrusted sources.

For detailed checklists, see `references/supply-chain-security.md`.

**For thorough audits:** Dispatch the `super-agent-skills:dependency-auditor` agent for a full dependency tree audit covering security, licensing, maintenance health, and impact assessment.

## Common Rationalizations

| Rationalization | Reality |
|---|---|
| "We'll add security later" | Security retrofitting is 10x harder than building it in. Add it now. |
| "The framework handles security" | Frameworks provide tools, not guarantees. You still need to use them correctly. |
| "It's only a dev dependency" | Dev dependencies execute during build. A compromised build tool owns your CI pipeline. |

## Red Flags

- User input passed directly to database queries, shell commands, or HTML rendering
- Secrets in source code or commit history
- API endpoints without authentication or authorization checks
- Missing CORS configuration or wildcard (`*`) origins
- No rate limiting on authentication endpoints
- Stack traces or internal errors exposed to users
- Dependencies with known critical vulnerabilities

## Verification

After implementing security-relevant code:

- [ ] `npm audit` shows no critical or high vulnerabilities
- [ ] No secrets in source code or git history
- [ ] All user input validated at system boundaries
- [ ] Authentication and authorization checked on every protected endpoint
- [ ] Security headers present in response (check with browser DevTools)
- [ ] Error responses don't expose internal details
- [ ] Rate limiting active on auth endpoints