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APIs are the backbone of modern applications, but they're also prime targets for attacks. From authentication vulnerabilities to injection flaws, a poorly secured API can expose sensitive data and cripple your service. I've seen production APIs taken down by preventable mistakes, and it's never pretty. This guide covers essential security practices every developer should implement—rate limiting, authentication, input validation, and more. Whether you're building a REST or GraphQL API, these principles will keep your endpoints safe.

API Security Best Practices

1. Why API Security Matters
2. Authentication and Authorization
3. Protecting Against Common Attacks
   1. SQL Injection and NoSQL Injection
   2. Cross-Site Scripting (XSS)
   3. CSRF and Token Security
4. Rate Limiting and Throttling
5. Input Validation and Sanitization
6. Monitoring and Logging

1. Why API Security Matters

APIs expose your application's core functionality to the internet. Without proper security, attackers can steal data, bypass authentication, or bring your service down entirely. The cost of a breach—in downtime, reputation damage, and regulatory fines—far exceeds the effort of securing APIs upfront.

Security isn't optional. It's a baseline requirement for any production API.

2. Authentication and Authorization

Authentication verifies who the user is. Use JWT tokens or OAuth 2.0 for stateless auth. Store tokens securely (httpOnly cookies or secure storage) and set short expiration times.

// Express.js example
app.use((req, res, next) => {
  const token = req.headers.authorization?.split(' ')[1];
  if (!token) return res.status(401).send('Unauthorized');
  
  jwt.verify(token, SECRET, (err, user) => {
    if (err) return res.status(403).send('Forbidden');
    req.user = user;
    next();
  });
});

Authorization checks what the user can do. Implement role-based access control (RBAC) or permissions to restrict sensitive endpoints.

3. Protecting Against Common Attacks

3.1 SQL Injection and NoSQL Injection

Never concatenate user input into queries. Use parameterized queries or ORMs:

// Bad: vulnerable to SQL injection
const query = `SELECT * FROM users WHERE id = ${userId}`;

// Good: parameterized query
const query = 'SELECT * FROM users WHERE id = ?';
db.execute(query, [userId]);

3.2 Cross-Site Scripting (XSS)

Sanitize output and escape HTML. For APIs returning JSON, XSS is less common but still sanitize data before storing.

3.3 CSRF and Token Security

Use CSRF tokens for state-changing operations. For JWT, validate the signature and check expiration on every request.

4. Rate Limiting and Throttling

Rate limiting prevents abuse and DDoS attacks. Limit requests per IP or user:

// Express rate-limit example
const rateLimit = require('express-rate-limit');

const limiter = rateLimit({
  windowMs: 15 * 60 * 1000,  // 15 minutes
  max: 100                    // limit each IP to 100 requests per window
});

app.use('/api/', limiter);

Use exponential backoff for repeated failures and consider implementing API keys for tracking.

5. Input Validation and Sanitization

Validate all inputs—type, length, format, and range. Use libraries like Joi, Zod, or express-validator:

const { body, validationResult } = require('express-validator');

app.post('/user', [
  body('email').isEmail(),
  body('age').isInt({ min: 18, max: 120 })
], (req, res) => {
  const errors = validationResult(req);
  if (!errors.isEmpty()) {
    return res.status(400).json({ errors: errors.array() });
  }
  // Process valid data
});

Never trust client input. Sanitize, validate, and enforce constraints on the server.

6. Monitoring and Logging

Log all authentication failures, rate limit violations, and suspicious activity. Use tools like Sentry, DataDog, or CloudWatch to monitor API health.

Set up alerts for anomalies—sudden traffic spikes, repeated 401s, or unusual endpoints being hit.

• Log requests but never log sensitive data (passwords, tokens).
• Use structured logging (JSON format) for easier parsing.
• Rotate logs regularly and archive securely.

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API security is an ongoing process, not a one-time setup. Stay updated on vulnerabilities, audit your code regularly, and treat security as a first-class concern. Have you dealt with an API breach? Share your lessons in the comments—we all learn from each other's experiences.

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