API Security Best Practices

Overview

Guide developers in building secure APIs by implementing authentication, authorization, input validation, rate limiting, and protection against common vulnerabilities. This skill covers security patterns for REST, GraphQL, and WebSocket APIs.

When to Use This Skill

• Use when designing new API endpoints

• Use when securing existing APIs

• Use when implementing authentication and authorization

• Use when protecting against API attacks (injection, DDoS, etc.)

• Use when conducting API security reviews

• Use when preparing for security audits

• Use when implementing rate limiting and throttling

• Use when handling sensitive data in APIs

How It Works

Step 1: Authentication & Authorization

I'll help you implement secure authentication:

• Choose authentication method (JWT, OAuth 2.0, API keys)

• Implement token-based authentication

• Set up role-based access control (RBAC)

• Secure session management

• Implement multi-factor authentication (MFA)

Step 2: Input Validation & Sanitization

Protect against injection attacks:

• Validate all input data

• Sanitize user inputs

• Use parameterized queries

• Implement request schema validation

• Prevent SQL injection, XSS, and command injection

Step 3: Rate Limiting & Throttling

Prevent abuse and DDoS attacks:

• Implement rate limiting per user/IP

• Set up API throttling

• Configure request quotas

• Handle rate limit errors gracefully

• Monitor for suspicious activity

Step 4: Data Protection

Secure sensitive data:

• Encrypt data in transit (HTTPS/TLS)

• Encrypt sensitive data at rest

• Implement proper error handling (no data leaks)

• Sanitize error messages

• Use secure headers

Step 5: API Security Testing

Verify security implementation:

• Test authentication and authorization

• Perform penetration testing

• Check for common vulnerabilities (OWASP API Top 10)

• Validate input handling

• Test rate limiting

Examples

Example 1: Implementing JWT Authentication

Secure JWT Authentication Implementation

Authentication Flow

1. User logs in with credentials
2. Server validates credentials
3. Server generates JWT token
4. Client stores token securely
5. Client sends token with each request
6. Server validates token

Implementation

1. Generate Secure JWT Tokens

```javascript // auth.js const jwt = require('jsonwebtoken'); const bcrypt = require('bcrypt');

// Login endpoint app.post('/api/auth/login', async (req, res) => { try { const { email, password } = req.body;

// Validate input
if (!email || !password) {
  return res.status(400).json({ 
    error: 'Email and password are required' 
  });
}

// Find user
const user = await db.user.findUnique({ 
  where: { email } 
});

if (!user) {
  // Don't reveal if user exists
  return res.status(401).json({ 
    error: 'Invalid credentials' 
  });
}

// Verify password
const validPassword = await bcrypt.compare(
  password, 
  user.passwordHash
);

if (!validPassword) {
  return res.status(401).json({ 
    error: 'Invalid credentials' 
  });
}

// Generate JWT token
const token = jwt.sign(
  { 
    userId: user.id,
    email: user.email,
    role: user.role
  },
  process.env.JWT_SECRET,
  { 
    expiresIn: '1h',
    issuer: 'your-app',
    audience: 'your-app-users'
  }
);

// Generate refresh token
const refreshToken = jwt.sign(
  { userId: user.id },
  process.env.JWT_REFRESH_SECRET,
  { expiresIn: '7d' }
);

// Store refresh token in database
await db.refreshToken.create({
  data: {
    token: refreshToken,
    userId: user.id,
    expiresAt: new Date(Date.now() + 7 * 24 * 60 * 60 * 1000)
  }
});

res.json({
  token,
  refreshToken,
  expiresIn: 3600
});

} catch (error) { console.error('Login error:', error); res.status(500).json({ error: 'An error occurred during login' }); } }); ```

2. Verify JWT Tokens (Middleware)

```javascript // middleware/auth.js const jwt = require('jsonwebtoken');

function authenticateToken(req, res, next) { // Get token from header const authHeader = req.headers['authorization']; const token = authHeader && authHeader.split(' ')[1]; // Bearer TOKEN

if (!token) { return res.status(401).json({ error: 'Access token required' }); }

// Verify token jwt.verify( token, process.env.JWT_SECRET, { issuer: 'your-app', audience: 'your-app-users' }, (err, user) => { if (err) { if (err.name === 'TokenExpiredError') { return res.status(401).json({ error: 'Token expired' }); } return res.status(403).json({ error: 'Invalid token' }); }

  // Attach user to request
  req.user = user;
  next();
}

); }

module.exports = { authenticateToken }; ```

3. Protect Routes

```javascript const { authenticateToken } = require('./middleware/auth');

// Protected route app.get('/api/user/profile', authenticateToken, async (req, res) => { try { const user = await db.user.findUnique({ where: { id: req.user.userId }, select: { id: true, email: true, name: true, // Don't return passwordHash } });

res.json(user);

} catch (error) { res.status(500).json({ error: 'Server error' }); } }); ```

4. Implement Token Refresh

```javascript app.post('/api/auth/refresh', async (req, res) => { const { refreshToken } = req.body;

if (!refreshToken) { return res.status(401).json({ error: 'Refresh token required' }); }

try { // Verify refresh token const decoded = jwt.verify( refreshToken, process.env.JWT_REFRESH_SECRET );

// Check if refresh token exists in database
const storedToken = await db.refreshToken.findFirst({
  where: {
    token: refreshToken,
    userId: decoded.userId,
    expiresAt: { gt: new Date() }
  }
});

if (!storedToken) {
  return res.status(403).json({ 
    error: 'Invalid refresh token' 
  });
}

// Generate new access token
const user = await db.user.findUnique({
  where: { id: decoded.userId }
});

const newToken = jwt.sign(
  { 
    userId: user.id,
    email: user.email,
    role: user.role
  },
  process.env.JWT_SECRET,
  { expiresIn: '1h' }
);

res.json({
  token: newToken,
  expiresIn: 3600
});

} catch (error) { res.status(403).json({ error: 'Invalid refresh token' }); } }); ```

Security Best Practices

• ✅ Use strong JWT secrets (256-bit minimum)
• ✅ Set short expiration times (1 hour for access tokens)
• ✅ Implement refresh tokens for long-lived sessions
• ✅ Store refresh tokens in database (can be revoked)
• ✅ Use HTTPS only
• ✅ Don't store sensitive data in JWT payload
• ✅ Validate token issuer and audience
• ✅ Implement token blacklisting for logout

Example 2: Input Validation and SQL Injection Prevention

Preventing SQL Injection and Input Validation

The Problem

❌ Vulnerable Code: ```javascript // NEVER DO THIS - SQL Injection vulnerability app.get('/api/users/:id', async (req, res) => { const userId = req.params.id;

// Dangerous: User input directly in query const query = `SELECT * FROM users WHERE id = '${userId}'`; const user = await db.query(query);

res.json(user); });

// Attack example: // GET /api/users/1' OR '1'='1 // Returns all users! ```

The Solution

1. Use Parameterized Queries

```javascript // ✅ Safe: Parameterized query app.get('/api/users/:id', async (req, res) => { const userId = req.params.id;

// Validate input first if (!userId || !/^\d+$/.test(userId)) { return res.status(400).json({ error: 'Invalid user ID' }); }

// Use parameterized query const user = await db.query( 'SELECT id, email, name FROM users WHERE id = $1', [userId] );

if (!user) { return res.status(404).json({ error: 'User not found' }); }

res.json(user); }); ```

2. Use ORM with Proper Escaping

```javascript // ✅ Safe: Using Prisma ORM app.get('/api/users/:id', async (req, res) => { const userId = parseInt(req.params.id);

if (isNaN(userId)) { return res.status(400).json({ error: 'Invalid user ID' }); }

const user = await prisma.user.findUnique({ where: { id: userId }, select: { id: true, email: true, name: true, // Don't select sensitive fields } });

if (!user) { return res.status(404).json({ error: 'User not found' }); }

res.json(user); }); ```

3. Implement Request Validation with Zod

```javascript const { z } = require('zod');

// Define validation schema const createUserSchema = z.object({ email: z.string().email('Invalid email format'), password: z.string() .min(8, 'Password must be at least 8 characters') .regex(/[A-Z]/, 'Password must contain uppercase letter') .regex(/[a-z]/, 'Password must contain lowercase letter') .regex(/[0-9]/, 'Password must contain number'), name: z.string() .min(2, 'Name must be at least 2 characters') .max(100, 'Name too long'), age: z.number() .int('Age must be an integer') .min(18, 'Must be 18 or older') .max(120, 'Invalid age') .optional() });

// Validation middleware function validateRequest(schema) { return (req, res, next) => { try { schema.parse(req.body); next(); } catch (error) { res.status(400).json({ error: 'Validation failed', details: error.errors }); } }; }

// Use validation app.post('/api/users', validateRequest(createUserSchema), async (req, res) => { // Input is validated at this point const { email, password, name, age } = req.body;

// Hash password
const passwordHash = await bcrypt.hash(password, 10);

// Create user
const user = await prisma.user.create({
  data: {
    email,
    passwordHash,
    name,
    age
  }
});

// Don't return password hash
const { passwordHash: _, ...userWithoutPassword } = user;
res.status(201).json(userWithoutPassword);

} ); ```

4. Sanitize Output to Prevent XSS

```javascript const DOMPurify = require('isomorphic-dompurify');

app.post('/api/comments', authenticateToken, async (req, res) => { const { content } = req.body;

// Validate if (!content || content.length > 1000) { return res.status(400).json({ error: 'Invalid comment content' }); }

// Sanitize HTML to prevent XSS const sanitizedContent = DOMPurify.sanitize(content, { ALLOWED_TAGS: ['b', 'i', 'em', 'strong', 'a'], ALLOWED_ATTR: ['href'] });

const comment = await prisma.comment.create({ data: { content: sanitizedContent, userId: req.user.userId } });

res.status(201).json(comment); }); ```

Validation Checklist

• Validate all user inputs
• Use parameterized queries or ORM
• Validate data types (string, number, email, etc.)
• Validate data ranges (min/max length, value ranges)
• Sanitize HTML content
• Escape special characters
• Validate file uploads (type, size, content)
• Use allowlists, not blocklists

Example 3: Rate Limiting and DDoS Protection

Implementing Rate Limiting

Why Rate Limiting?

• Prevent brute force attacks
• Protect against DDoS
• Prevent API abuse
• Ensure fair usage
• Reduce server costs

Implementation with Express Rate Limit

```javascript const rateLimit = require('express-rate-limit'); const RedisStore = require('rate-limit-redis'); const Redis = require('ioredis');

// Create Redis client const redis = new Redis({ host: process.env.REDIS_HOST, port: process.env.REDIS_PORT });

// General API rate limit const apiLimiter = rateLimit({ store: new RedisStore({ client: redis, prefix: 'rl:api:' }), windowMs: 15 * 60 * 1000, // 15 minutes max: 100, // 100 requests per window message: { error: 'Too many requests, please try again later', retryAfter: 900 // seconds }, standardHeaders: true, // Return rate limit info in headers legacyHeaders: false, // Custom key generator (by user ID or IP) keyGenerator: (req) => { return req.user?.userId || req.ip; } });

// Strict rate limit for authentication endpoints const authLimiter = rateLimit({ store: new RedisStore({ client: redis, prefix: 'rl:auth:' }), windowMs: 15 * 60 * 1000, // 15 minutes max: 5, // Only 5 login attempts per 15 minutes skipSuccessfulRequests: true, // Don't count successful logins message: { error: 'Too many login attempts, please try again later', retryAfter: 900 } });

// Apply rate limiters app.use('/api/', apiLimiter); app.use('/api/auth/login', authLimiter); app.use('/api/auth/register', authLimiter);

// Custom rate limiter for expensive operations const expensiveLimiter = rateLimit({ windowMs: 60 * 60 * 1000, // 1 hour max: 10, // 10 requests per hour message: { error: 'Rate limit exceeded for this operation' } });

app.post('/api/reports/generate', authenticateToken, expensiveLimiter, async (req, res) => { // Expensive operation } ); ```

Advanced: Per-User Rate Limiting

```javascript // Different limits based on user tier function createTieredRateLimiter() { const limits = { free: { windowMs: 60 * 60 * 1000, max: 100 }, pro: { windowMs: 60 * 60 * 1000, max: 1000 }, enterprise: { windowMs: 60 * 60 * 1000, max: 10000 } };

return async (req, res, next) => { const user = req.user; const tier = user?.tier || 'free'; const limit = limits[tier];

const key = \`rl:user:\${user.userId}\`;
const current = await redis.incr(key);

if (current === 1) {
  await redis.expire(key, limit.windowMs / 1000);
}

if (current > limit.max) {
  return res.status(429).json({
    error: 'Rate limit exceeded',
    limit: limit.max,
    remaining: 0,
    reset: await redis.ttl(key)
  });
}

// Set rate limit headers
res.set({
  'X-RateLimit-Limit': limit.max,
  'X-RateLimit-Remaining': limit.max - current,
  'X-RateLimit-Reset': await redis.ttl(key)
});

next();

}; }

app.use('/api/', authenticateToken, createTieredRateLimiter()); ```

DDoS Protection with Helmet

```javascript const helmet = require('helmet');

app.use(helmet({ // Content Security Policy contentSecurityPolicy: { directives: { defaultSrc: ["'self'"], styleSrc: ["'self'", "'unsafe-inline'"], scriptSrc: ["'self'"], imgSrc: ["'self'", 'data:', 'https:'] } }, // Prevent clickjacking frameguard: { action: 'deny' }, // Hide X-Powered-By header hidePoweredBy: true, // Prevent MIME type sniffing noSniff: true, // Enable HSTS hsts: { maxAge: 31536000, includeSubDomains: true, preload: true } })); ```

Rate Limit Response Headers

``` X-RateLimit-Limit: 100 X-RateLimit-Remaining: 87 X-RateLimit-Reset: 1640000000 Retry-After: 900 ```

Best Practices

✅ Do This

• Use HTTPS Everywhere - Never send sensitive data over HTTP

• Implement Authentication - Require authentication for protected endpoints

• Validate All Inputs - Never trust user input

• Use Parameterized Queries - Prevent SQL injection

• Implement Rate Limiting - Protect against brute force and DDoS

• Hash Passwords - Use bcrypt with salt rounds >= 10

• Use Short-Lived Tokens - JWT access tokens should expire quickly

• Implement CORS Properly - Only allow trusted origins

• Log Security Events - Monitor for suspicious activity

• Keep Dependencies Updated - Regularly update packages

• Use Security Headers - Implement Helmet.js

• Sanitize Error Messages - Don't leak sensitive information

❌ Don't Do This

• Don't Store Passwords in Plain Text - Always hash passwords

• Don't Use Weak Secrets - Use strong, random JWT secrets

• Don't Trust User Input - Always validate and sanitize

• Don't Expose Stack Traces - Hide error details in production

• Don't Use String Concatenation for SQL - Use parameterized queries

• Don't Store Sensitive Data in JWT - JWTs are not encrypted

• Don't Ignore Security Updates - Update dependencies regularly

• Don't Use Default Credentials - Change all default passwords

• Don't Disable CORS Completely - Configure it properly instead

• Don't Log Sensitive Data - Sanitize logs

Common Pitfalls

Problem: JWT Secret Exposed in Code

Symptoms: JWT secret hardcoded or committed to Git Solution:

// ❌ Bad
const JWT_SECRET = 'my-secret-key';

// ✅ Good
const JWT_SECRET = process.env.JWT_SECRET;
if (!JWT_SECRET) {
throw new Error('JWT_SECRET environment variable is required');
}

// Generate strong secret
// node -e "console.log(require('crypto').randomBytes(64).toString('hex'))"

Problem: Weak Password Requirements

Symptoms: Users can set weak passwords like "password123" Solution:

const passwordSchema = z.string()
.min(12, 'Password must be at least 12 characters')
.regex(/[A-Z]/, 'Must contain uppercase letter')
.regex(/[a-z]/, 'Must contain lowercase letter')
.regex(/[0-9]/, 'Must contain number')
.regex(/[^A-Za-z0-9]/, 'Must contain special character');

// Or use a password strength library
const zxcvbn = require('zxcvbn');
const result = zxcvbn(password);
if (result.score < 3) {
return res.status(400).json({
error: 'Password too weak',
suggestions: result.feedback.suggestions
});
}

Problem: Missing Authorization Checks

Symptoms: Users can access resources they shouldn't Solution:

// ❌ Bad: Only checks authentication
app.delete('/api/posts/:id', authenticateToken, async (req, res) => {
await prisma.post.delete({ where: { id: req.params.id } });
res.json({ success: true });
});

// ✅ Good: Checks both authentication and authorization
app.delete('/api/posts/:id', authenticateToken, async (req, res) => {
const post = await prisma.post.findUnique({
where: { id: req.params.id }
});

if (!post) {
return res.status(404).json({ error: 'Post not found' });
}

// Check if user owns the post or is admin
if (post.userId !== req.user.userId && req.user.role !== 'admin') {
return res.status(403).json({
error: 'Not authorized to delete this post'
});
}

await prisma.post.delete({ where: { id: req.params.id } });
res.json({ success: true });
});

Problem: Verbose Error Messages

Symptoms: Error messages reveal system details Solution:

// ❌ Bad: Exposes database details
app.post('/api/users', async (req, res) => {
try {
const user = await prisma.user.create({ data: req.body });
res.json(user);
} catch (error) {
res.status(500).json({ error: error.message });
// Error: "Unique constraint failed on the fields: (email
)"
}
});

// ✅ Good: Generic error message
app.post('/api/users', async (req, res) => {
try {
const user = await prisma.user.create({ data: req.body });
res.json(user);
} catch (error) {
console.error('User creation error:', error); // Log full error

if (error.code === 'P2002') {
  return res.status(400).json({ 
    error: 'Email already exists' 
  });
}

res.status(500).json({ 
  error: 'An error occurred while creating user' 
});

}
});

Security Checklist

Authentication & Authorization

• Implement strong authentication (JWT, OAuth 2.0)

• Use HTTPS for all endpoints

• Hash passwords with bcrypt (salt rounds >= 10)

• Implement token expiration

• Add refresh token mechanism

• Verify user authorization for each request

• Implement role-based access control (RBAC)

Input Validation

• Validate all user inputs

• Use parameterized queries or ORM

• Sanitize HTML content

• Validate file uploads

• Implement request schema validation

• Use allowlists, not blocklists

Rate Limiting & DDoS Protection

• Implement rate limiting per user/IP

• Add stricter limits for auth endpoints

• Use Redis for distributed rate limiting

• Return proper rate limit headers

• Implement request throttling

Data Protection

• Use HTTPS/TLS for all traffic

• Encrypt sensitive data at rest

• Don't store sensitive data in JWT

• Sanitize error messages

• Implement proper CORS configuration

• Use security headers (Helmet.js)

Monitoring & Logging

• Log security events

• Monitor for suspicious activity

• Set up alerts for failed auth attempts

• Track API usage patterns

• Don't log sensitive data

OWASP API Security Top 10

• Broken Object Level Authorization - Always verify user can access resource

• Broken Authentication - Implement strong authentication mechanisms

• Broken Object Property Level Authorization - Validate which properties user can access

• Unrestricted Resource Consumption - Implement rate limiting and quotas

• Broken Function Level Authorization - Verify user role for each function

• Unrestricted Access to Sensitive Business Flows - Protect critical workflows

• Server Side Request Forgery (SSRF) - Validate and sanitize URLs

• Security Misconfiguration - Use security best practices and headers

• Improper Inventory Management - Document and secure all API endpoints

• Unsafe Consumption of APIs - Validate data from third-party APIs

Related Skills

• @ethical-hacking-methodology

• Security testing perspective

• @sql-injection-testing

• Testing for SQL injection

• @xss-html-injection

• Testing for XSS vulnerabilities

• @broken-authentication

• Authentication vulnerabilities

• @backend-dev-guidelines

• Backend development standards

• @systematic-debugging

• Debug security issues

Additional Resources

• OWASP API Security Top 10

• JWT Best Practices

• Express Security Best Practices

• Node.js Security Checklist

• API Security Checklist

Pro Tip: Security is not a one-time task - regularly audit your APIs, keep dependencies updated, and stay informed about new vulnerabilities!