API Security Testing 101: Protecting Your Data from Vulnerabilities

Data is vital to everything we do in the modern world. When it comes to data, we cannot ignore APIs. They act as the internet’s functional backbone, helping in the smooth transfer of data between servers, apps, and devices. APIs must be protected from risks and vulnerabilities because they are used at every step. This is where security testing for APIs comes in. Ignoring this could be costly because it could compromise the privacy of sensitive user data, disrupt business operations, and harm your company’s reputation with customers.

Introduction to API Security

API security testing simulates the behavior of cyber attackers. It involves sending incorrect inputs, requesting unauthorized access ,replicating injection or brute-force attacks, allowing you to identify and remediate vulnerabilities before a real hacker does it for you.

In this blog, you’ll learn:

• Learn why API security testing is important

• A demo-ready example to help apply best practices

• Tools like Keploy helps you to test your API in ease with Security

APIs present unique dangers because of the direct system-to-system access and automation-friendly interfaces APIs enable. If you understand these risks beforehand you can build an effective defense.
So let’s dive into the different types of risks that APIs pose.

1. Broken Object-Level Authentication (BOLA)

   APIs can allow a malicious user to obtain user-specific information if there is no access control to every individual object by their ID (e.g., /users/123), allowing sensitive personal user information to be compromised.

2. Broken Authentication

   Poor token management, such as tokens that do not expire or weak credentials make it difficult to manage tokens securely. Brute force attacks are much easier in this environment.

3. Excessive Data Exposure / Mass Assignment

   When an API provides responses with too much unfiltered data, it can reveal how the app works behind the scenes, allowing attackers to access sensitive data or take actions that shouldn’t have been possible.

4. No Rate Limits

   When an API does not limit the number of requests someone can send, it is easy to overload the API or perform brute force attacks by sending many repeated requests.

5. Security Misconfiguration

   These common mistakes create easy routes for attackers, such as using outdated API versions, open CORS policies, and debugging endpoints.

6. Injection Attacks

   With no input validation in place, malicious SQL, NoSQL, or script commands can be injected into the system, leading to data corruption or complete compromise of the system.

7. Insufficient Logging & Monitoring

   Without logging requests to API endpoints or raising alerts. Threats can go unnoticed, making it hard to catch a security breach in time.

What Is API Security Testing?

API security testing is a proactive simulation of cyberattacks to your API, such as malformed inputs, brute-force attacks, and authentication bypasses, so that security weaknesses can be determined and remediated before attackers can exploit them.

This builds resilience by testing:

• The codebase against known defects (via SAST)

• The running API under attack (via DAST)

• The API behavior under actual use (via IAST, etc.)

• The resistance to logic attacks or chained exploits (via Pen Testing)

In conclusion, it is not just testing, it is a security first way of thinking that helps you ensure your API is robust, compliant, and consistent.

How API Security Testing Works

API security testing is a structured, multi-step approach designed to systematically identify and fix vulnerabilities.

1. Scope & Discovery

Identify all API endpoints internal, external, or third party and rank them based on exposure and sensitivity using OpenAPI specifications, documentation or logs.

2. Threat modeling

Identify and map attack vectors against the OWASP API Top 10, broken or lack of authentication, injection, data exposure, insufficient rate limit; etc.

3. Automated scanning

• SAST: Identify injection flaws, secrets and configuration issues statically by scanning source code.

• DAST: Identify runtime problems by hitting running API’s with crafted requests.

4. Manual penetration testing

Testers who are professionals simulate as closely as possible real attacks, manual testing allows testers to target business logic and follow chains of vulnerabilities allowing for a much wider scope than the forms of testing discussed above.

5. Analyze & report

Go through the findings to understand severity (CVSS), reproduce findings, and list (unless stated otherwise) simple remediation steps for technical teams to work wih.

6. Fix & retest

Once patches are released:

• Automated scanning

• Manual Validations

How is API Security Different From General Application Security?

API security may be thought of as part of application security. However, there is a different mindset and testing methodology required for APIs. Here is a breakdown of the differences:

What They Are Protecting

• Application security protects the user interface, session management, client-side attacks like XSS or CSRF.

• API security protects backend services that communicate directly in between systems (without user interface interaction).

Attack Surface

• Applications are limited to attacks through a UI, forms, inputs, and session-based attacks.

• APIs are exposed to attacks via endpoints, payloads, headers, tokens, and sometimes even business logic directly.

Authentication and Access Control

• Applications will rely on session authentication flows (cookies, login flows, etc.).

• APIs rely on token-based authentication (JWT, OAuth, API keys, etc.) which introduces its risks (token leakage, absence of scope validation, absence of expiration).

Testing Methodology

• Application security testing focuses on UI behavior and user flow.

• API security testing focuses on sending raw HTTP requests, creating malformed payloads, bypassing authentication and abusing the business logic of the application.

Risk of Automation

• Applications have a UI layer that a user has to interact with and exposes the steps of a flow.

• APIs are machine friendly, provide direct access and are not limited by UI. This makes them less restrictive and increases the risk of bots and scripting abuse.

Types of API Security Tests

To ensure resilient APIs, it’s important to layer different types of security testing. Each method reveals different weaknesses from vulnerability in code, runtime issues on environments etc.

What are API testing Tools and Frameworks

API testing tools and frameworks are software tools that developers use to test if their APIs are working. By sending HTTP requests to the API endpoints, it verifies the responses against the expected outcomes including status codes, response times, and data formats.

Types of API Testing Tools

API Security Testing Best Practices

API security vulnerabilities cost companies millions in breaches. Here are 5 essential practices with simple demos to secure your APIs effectively.

1. Implement Authentication & Authorization

Why it matters: 61% of data breaches involve compromised credentials.

Quick Demo – JWT Authentication:

javascript
// Simple JWT middleware
const jwt = require('jsonwebtoken');

const authenticateToken = (req, res, next) => {
  const token = req.headers['authorization']?.split(' ')[1];

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

  jwt.verify(token, process.env.JWT_SECRET, (err, user) => {
    if (err) return res.status(403).json({ error: 'Invalid token' });
    req.user = user;
    next();
  });
};

// Protected route
app.get('/api/profile', authenticateToken, (req, res) => {
  res.json({ userId: req.user.id, role: req.user.role });
});

Test it:

bash
# Should fail without token
curl http://localhost:3000/api/profile
# Expected: 401 Unauthorized

# Should work with valid token
curl -H "Authorization: Bearer YOUR_JWT_TOKEN" http://localhost:3000/api/profile

2. Validate All Input

Why it matters: Input validation prevents 90% of injection attacks.

Quick Demo – Input Sanitization:

javascript
const validator = require('validator');

const validateUser = (req, res, next) => {
  const { email, username } = req.body;

  // Basic validation
  if (!email || !validator.isEmail(email)) {
    return res.status(400).json({ error: 'Valid email required' });
  }

  if (!username || username.length < 3) {
    return res.status(400).json({ error: 'Username must be 3+ characters' });
  }

  // Sanitize input
  req.body.email = validator.normalizeEmail(email);
  req.body.username = validator.escape(username);

  next();
};

app.post('/api/users', validateUser, (req, res) => {
  // Safe to process - input is validated
  createUser(req.body);
  res.json({ message: 'User created' });
});

Test it:

bash
# Test malicious input
curl -X POST http://localhost:3000/api/users \
  -H "Content-Type: application/json" \
  -d '{"email":"test","username":"<script>alert(1)</script>"}'
# Expected: 400 Validation error

3. Add Rate Limiting

Why it matters: Rate limiting prevents 99% of brute force attacks.

Quick Demo – Basic Rate Limiting:

javascript
const rateLimit = require('express-rate-limit');

// General API rate limit
const apiLimiter = rateLimit({
  windowMs: 15 * 60 * 1000, // 15 minutes
  max: 100, // 100 requests per window
  message: { error: 'Too many requests, try again later' }
});

// Strict rate limit for sensitive endpoints
const authLimiter = rateLimit({
  windowMs: 15 * 60 * 1000,
  max: 5, // Only 5 login attempts per 15 minutes
  message: { error: 'Too many login attempts' }
});

app.use('/api/', apiLimiter);
app.use('/api/auth/', authLimiter);

Test it:

bash
# Test rate limiting (run multiple times quickly)
for i in {1..10}; do curl http://localhost:3000/api/auth/login; done
# Expected: First few succeed, then 429 Too Many Requests

4. Follow OWASP Guidelines

Why it matters: OWASP API Top 10 covers 95% of common vulnerabilities.

Quick Demo – Prevent Data Exposure:

javascript
// BAD: Exposes sensitive data
app.get('/api/users/:id', (req, res) => {
  const user = getUserById(req.params.id);
  res.json(user); // Returns password, tokens, etc.
});

// GOOD: Return only necessary data
app.get('/api/users/:id', authenticateToken, (req, res) => {
  const user = getUserById(req.params.id);

  // Only return safe fields
  const safeUser = {
    id: user.id,
    username: user.username,
    email: user.email,
    createdAt: user.createdAt
  };

  res.json(safeUser);
});

Common OWASP Issues to Test:

• Broken authentication

• Excessive data exposure

• Lack of rate limiting

• Broken access control

5. Automate Security Testing

Why it matters: Manual testing misses 70% of vulnerabilities.

Quick Demo – Basic Security Tests:

javascript
// test/security.test.js
const request = require('supertest');
const app = require('../app');

describe('API Security Tests', () => {
  test('should require authentication', async () => {
    const response = await request(app).get('/api/profile');
    expect(response.status).toBe(401);
  });

  test('should reject malicious input', async () => {
    const response = await request(app)
      .post('/api/users')
      .send({ username: '<script>alert(1)</script>' });
    expect(response.status).toBe(400);
  });

  test('should enforce rate limits', async () => {
    // Make multiple requests quickly
    const requests = Array(10).fill().map(() => 
      request(app).post('/api/auth/login')
    );

    const responses = await Promise.all(requests);
    const blocked = responses.some(r => r.status === 429);
    expect(blocked).toBe(true);
  });
});

Run Security Tests:

bash
# Add to package.json
"scripts": {
  "test:security": "jest test/security.test.js",
  "security-audit": "npm audit --audit-level=high"
}

# Run tests
npm run test:security
npm run security-audit

How Keploy Makes Your API Testing More Secure

In this blog, we are discussing API security, right? What if the platform provided a way to make API testing more secure and implement all the best practices? Yes, you can now test your APIs without writing any tests and ensure 100% security. Sounds interesting?

Go to: app.keploy.io

The interesting part is, if your APIs have any authentication, you can integrate all of those through a simple, intuitive UI.

Once everything is set, Keploy API Testing agent starts creating APIs without you writing any test cases. Once the test suites are created, you can run and test them. Here’s the catch:

Concerned about privacy? No worries! You can use our local private agent for running your test suites, ensuring your data stays within your control. Whether you choose the local private agent or the hosted agent, Keploy offers flexible options to suit your needs.

To know more Keploy API Testing agent: https://keploy.io/docs/running-keploy/api-test-generator/

Related Resources

For developers looking to enhance their API security testing strategy, explore these useful guides:

• Complete API Testing Guide – Fundamentals of API testing methodologies

• REST Assured Alternatives – Compare 20 powerful testing tools including Keploy

• No-Code API Testing – Simplify testing workflows with automation

• Test Mock Data Best Practices – Create secure and realistic test data

• API Performance Testing – Optimize API speed and security simultaneously

Conclusion

APIs are an important part of the digital systems we use today, but as more APIs are used, the number of attacks and security issues also increases. Securing APIs through regular testing is now a must and should be seen as a basic need. API security testing helps you find problems early, protect user data, and prevent costly attacks. Tools like Keploy make it easier by turning real traffic into useful test cases. Adding security testing to your software development process and following trusted standards like the OWASP API Top 10 lets you build safer APIs while keeping your team’s speed and productivity. Good API security protects your business and builds trust with your users.

FAQs

1. Are all cyberattacks preventable via API security testing?

   API security testing helps reduce risk by identifying vulnerabilities before they can be exploited by attackers. However, it should be part of a comprehensive security plan that includes incident response and monitoring.

2. How often should I test my API security?

   Manual testing should be done every three months, while automated testing should be integrated into your CI/CD pipeline, and more extensive testing should be conducted for major API changes.

3. How do penetration testing and API security testing differ?

   API security testing automatically scans for known vulnerabilities. Penetration testing involves experts simulating real attacks to find complex vulnerabilities.

4. How Secure is Keploy API Testing agent?

   Keploy is built with security-first principles and is compliant with major industry standards:

   • ✅ SOC 2

   • ✅ ISO 27001

   • ✅ GDPR

   • ✅ HIPAA

   Your data and test traffic are handled securely, with the option to run Keploy entirely within your network using our self-hosted agent or BYO LLM infrastructure.

5. What’s the difference between manual and automated API security testing?

   Automated testing (SAST, Keploy replay testing, DAST) should be done with every code change or CI build to catch issues early. Manual testing, like quarterly or post-release penetration testing, finds more complex exploits and logic errors.