API Security Testing: Complete Guide for 2026

Tools & Methods   January 14, 2026

Introduction

APIs (Application Programming Interfaces) have become the backbone of modern software architecture. From mobile apps to microservices, organisations rely on APIs to connect systems, share data, and deliver functionality. But this connectivity comes with risk.

In 2026, APIs represent one of the most common attack vectors in web applications. According to recent data, API-related incidents have increased by 400% since 2020, with breaches at major organisations often traced back to insecure API endpoints.

This guide covers everything you need to know about API security testing in 2026, from basic concepts to advanced penetration testing techniques. Whether you’re a developer implementing APIs or a security professional assessing them, you’ll learn how to identify and fix common API vulnerabilities before attackers exploit them.

Why API Security Testing Matters in 2026

APIs expose business logic and data access in ways that traditional web applications don’t. A single misconfigured endpoint can expose:

  • Customer data: Personal information, payment details, account credentials
  • Business logic flaws: Price manipulation, privilege escalation, unauthorised actions
  • Internal systems: Database access, admin functions, backend services
  • Third-party integrations: Partner systems, payment gateways, cloud services

The shift to microservices architecture and API-first development has expanded the attack surface dramatically. Modern applications may expose dozens or hundreds of API endpoints, each requiring proper security controls.

Types of APIs and Their Security Challenges

REST APIs

REST (Representational State Transfer) remains the most popular API architecture in 2026. REST APIs use HTTP methods (GET, POST, PUT, DELETE) to perform operations on resources identified by URLs.

Common security challenges:

  • Broken object level authorisation (BOLA/IDOR)
  • Excessive data exposure in responses
  • Missing rate limiting
  • Mass assignment vulnerabilities
  • Insecure HTTP methods (TRACE, OPTIONS exposing information)

GraphQL APIs

GraphQL has grown significantly, offering clients flexible query capabilities. This flexibility creates unique security challenges.

Common security challenges:

  • Query depth and complexity attacks (DoS)
  • Introspection enabled in production
  • Authorisation logic at field level (easy to miss)
  • Batch query attacks
  • Information disclosure through error messages

SOAP and XML-Based APIs

While less common in new projects, SOAP APIs remain prevalent in enterprise and legacy systems.

Common security challenges:

  • XML external entity (XXE) injection
  • XML bomb attacks
  • WSDL information disclosure
  • WS-Security implementation flaws

gRPC and Modern Protocols

gRPC and similar modern protocols are gaining adoption for high-performance scenarios.

Common security challenges:

  • Limited security tooling compared to REST
  • Binary protocol obscurity doesn’t equal security
  • Authentication and authorisation implementation complexity
  • Streaming endpoint vulnerabilities

The OWASP API Security Top 10

The OWASP API Security Project maintains the industry’s most recognised list of API vulnerabilities. Understanding these is essential for effective API security testing.

API1:2023 – Broken Object Level Authorisation (BOLA)

This is the most common and impactful API vulnerability. It occurs when an API doesn’t properly verify that the authenticated user should have access to the specific object being requested.

Example scenario:

GET /api/users/123/transactions

If the API only checks that the user is authenticated but not whether they own user ID 123, any logged-in user could access any other user’s transactions by changing the ID.

Testing approach: 1. Identify object references in endpoints (IDs, GUIDs, slugs) 2. Test with multiple user accounts 3. Attempt to access objects belonging to other users 4. Try sequential ID enumeration 5. Test both read and write operations

API2:2023 – Broken Authentication

Authentication flaws allow attackers to compromise tokens, passwords, or session identifiers to assume other users’ identities.

Common issues:

  • Weak password policies
  • Credential stuffing vulnerabilities (no rate limiting)
  • Predictable or weak tokens
  • Tokens that don’t expire
  • Insecure token storage
  • Missing multi-factor authentication

Testing approach: 1. Analyse authentication mechanisms (JWT, OAuth, API keys, sessions) 2. Test password reset workflows for account takeover 3. Attempt credential stuffing with rate limit testing 4. Decode and analyse tokens (JWT structure, claims, signatures) 5. Test token lifecycle (expiration, refresh, revocation)

API3:2023 – Broken Object Property Level Authorisation

This occurs when APIs expose more object properties than intended, either through excessive data exposure (reading) or mass assignment (writing).

Reading example – Excessive data exposure:

GET /api/users/me
Response:
{
  "id": 123,
  "name": "John Smith",
  "email": "john@example.com",
  "internal_notes": "VIP customer, discount approved",
  "credit_score": 750
}

Writing example – Mass assignment:

PUT /api/users/me
Request:
{
  "name": "John Smith",
  "is_admin": true  // User shouldn't be able to set this
}

Testing approach: 1. Compare API responses with UI display (extra fields?) 2. Capture and analyse full response objects 3. Test PATCH/PUT with additional parameters 4. Try setting admin flags, pricing fields, status fields 5. Fuzz parameter names based on naming conventions

API4:2023 – Unrestricted Resource Consumption

APIs without proper rate limiting or resource constraints can be abused for denial of service or cost exploitation.

Testing approach: 1. Test rate limiting on authentication endpoints 2. Attempt large batch requests 3. For GraphQL, test query depth and breadth limits 4. Test file upload size limits 5. Measure response times for complex queries

API5:2023 – Broken Function Level Authorisation

Similar to BOLA but at the function level. Users can access administrative or privileged functions they shouldn’t have access to.

Example:

POST /api/admin/users/delete

A standard user might be able to call this endpoint if the API doesn’t verify administrative privileges.

Testing approach: 1. Identify admin or privileged endpoints 2. Test with low-privilege accounts 3. Try guessing admin endpoint patterns 4. Test HTTP method switching (GET to POST, etc.) 5. Analyse API documentation for privileged functions

API Security Testing Methodology

Phase 1: Discovery and Reconnaissance

Identify API endpoints:

  • Review client-side code (mobile apps, web apps)
  • Analyse JavaScript files for API calls
  • Check robots.txt, sitemap.xml
  • Use browser developer tools to capture requests
  • Review API documentation (Swagger, OpenAPI specs)
  • Check subdomain enumeration (api.example.com, api-dev.example.com)

Map API structure:

  • List all endpoints and HTTP methods
  • Identify authentication mechanisms
  • Note request/response formats (JSON, XML, etc.)
  • Document parameter types and expected values
  • Identify rate limiting behaviour

Phase 2: Authentication and Authorisation Testing

This is typically where the highest-impact vulnerabilities are found.

Authentication testing: 1. Test credential validation (weak passwords, account lockout) 2. Analyse token generation and structure 3. Test token expiration and refresh mechanisms 4. Attempt token replay attacks 5. Test for hardcoded or default credentials 6. Check for authentication bypass techniques

Authorisation testing: 1. Create test users with different privilege levels 2. Test horizontal privilege escalation (user A accessing user B’s data) 3. Test vertical privilege escalation (regular user performing admin actions) 4. Test each endpoint with multiple user contexts 5. Test unauthenticated access to “authenticated” endpoints

Phase 3: Input Validation and Injection Testing

APIs often lack the input validation present in traditional web forms.

Test for:

  • SQL injection in query parameters
  • NoSQL injection (MongoDB, etc.)
  • Command injection in file processing endpoints
  • XML injection (XXE, XPath)
  • LDAP injection
  • Template injection
  • Server-side request forgery (SSRF)

Testing approach: 1. Identify all input points (path, query, headers, body) 2. Fuzz with injection payloads 3. Observe error messages for information disclosure 4. Test different encoding methods 5. Try multiple injection contexts

Phase 4: Business Logic Testing

Business logic flaws are unique to each API and require understanding the application’s intended behaviour.

Common scenarios:

  • Price manipulation in e-commerce APIs
  • Referral or reward system abuse
  • Race conditions in financial transactions
  • Workflow bypasses (skipping payment steps)
  • Coupon or discount code issues

Testing approach: 1. Understand the intended business workflows 2. Test for missing validation steps 3. Attempt out-of-sequence operations 4. Test race conditions with concurrent requests 5. Look for client-side validations that aren’t enforced server-side

Phase 5: Data Exposure and Privacy

Test for:

  • Sensitive data in API responses (passwords, keys, tokens)
  • PII exposure beyond what’s necessary
  • Error messages revealing system information
  • Debug endpoints or admin functions exposed
  • GraphQL introspection in production

Testing approach: 1. Review all API responses for sensitive data 2. Test error handling with malformed requests 3. Check for verbose error messages 4. Look for debug headers or parameters 5. Test GraphQL introspection queries

Essential Tools for API Security Testing

Burp Suite Professional

The industry standard for web application and API testing. Essential features for API testing:

  • Repeater: Modify and replay requests
  • Intruder: Automated fuzzing and parameter manipulation
  • Proxy: Intercept and analyse API traffic
  • Extensions: OpenAPI parser, JWT tools, GraphQL security

When to use: All phases of API testing, especially manual testing and authorisation checks.

OWASP ZAP

Free and open-source alternative to Burp Suite with strong API testing capabilities.

Key features:

  • API scanning from OpenAPI definitions
  • Fuzzing capabilities
  • Automation scripts for CI/CD integration

When to use: Budget-conscious projects, automated scanning, CI/CD integration.

Postman

While primarily used by developers, Postman is valuable for API security testing.

Security testing uses:

  • Collection Runner for automated test sequences
  • Pre-request scripts for token management
  • Environment variables for multi-user testing
  • Test scripts for response validation

When to use: Organising test cases, automated regression testing, team collaboration.

curl and Custom Scripts

For complex scenarios or automation, command-line tools provide flexibility.

Example – Testing BOLA:

#!/bin/bash
# Test access to 1000 user accounts with User A's token
TOKEN="user_a_token_here"
for i in {1..1000}; do
  curl -H "Authorization: Bearer $TOKEN" \
       "https://api.example.com/users/$i/profile" \
       -o "user_$i.json"
done

When to use: Large-scale testing, custom workflows, specific attack scenarios.

Specialized API Security Tools

Nuclei: Template-based vulnerability scanner with API-specific templates **GraphQL Cop**: GraphQL-specific security testing **Kiterunner**: API endpoint discovery and fuzzing **JWT_Tool**: JWT manipulation and testing

Common Pitfalls in API Security Testing

1. Only Testing the Happy Path

Testing valid requests with valid data won’t find security issues. You must test invalid inputs, unexpected sequences, and edge cases.

2. Ignoring Rate Limiting

Many testers assume rate limiting is in place. Always verify with actual testing, especially on authentication endpoints.

3. Testing with Only One User Account

Authorisation vulnerabilities require testing with multiple user contexts. Create test accounts at different privilege levels.

4. Relying Only on Automated Scanning

Automated tools miss business logic flaws, complex authorisation issues, and context-specific vulnerabilities. Manual testing is essential.

5. Not Testing Mobile APIs Directly

Mobile apps often communicate with APIs differently than web applications. Test the actual mobile traffic, not just the web version.

Securing Your APIs: Developer Checklist

If you’re building APIs, implement these security controls:

Authentication & Authorisation:

  • [ ] Use industry-standard authentication (OAuth 2.0, OpenID Connect)
  • [ ] Implement proper authorisation checks on every endpoint
  • [ ] Validate user permissions for each object accessed
  • [ ] Use strong, randomly generated tokens
  • [ ] Implement token expiration and refresh

Input Validation:

  • [ ] Validate all inputs (path, query, headers, body)
  • [ ] Use allow-lists rather than deny-lists
  • [ ] Validate data types and formats
  • [ ] Implement request size limits
  • [ ] Sanitise output to prevent injection

Rate Limiting & Resource Control:

  • [ ] Implement rate limiting on all endpoints
  • [ ] Use stricter limits on authentication endpoints
  • [ ] Limit query complexity (especially GraphQL)
  • [ ] Restrict batch operations
  • [ ] Monitor for abuse patterns

Data Exposure:

  • [ ] Return only necessary data in responses
  • [ ] Remove debug information in production
  • [ ] Disable introspection in production (GraphQL)
  • [ ] Implement proper error handling (don’t leak system info)
  • [ ] Use HTTPS for all API communication

Monitoring & Logging:

  • [ ] Log authentication attempts (successes and failures)
  • [ ] Log authorisation failures
  • [ ] Monitor for unusual access patterns
  • [ ] Alert on potential abuse (rate limit hits, enumeration)
  • [ ] Maintain audit logs for sensitive operations

When to Conduct API Security Testing

During development: Integrate security testing into your CI/CD pipeline with automated API scans.

Before launch: Conduct thorough penetration testing covering all endpoints and user scenarios.

Regular intervals: Quarterly or bi-annual testing for production APIs, especially after significant changes.

After incidents: If a security issue is found, test the entire API surface, not just the affected endpoint.

Compliance requirements: Many standards (PCI DSS, SOC 2, ISO 27001) require regular security assessments.

For organisations serious about API security, professional penetration testing services provide comprehensive assessment by experienced security professionals who understand both common vulnerabilities and business-specific risks.

Conclusion

API security testing is no longer optional in 2026. With APIs forming the foundation of modern applications, organisations must prioritise testing to protect customer data, business logic, and system integrity.

Effective API security testing requires:

  • Understanding different API architectures and their unique challenges
  • Following a structured methodology from discovery through business logic testing
  • Using the right tools for the job
  • Testing with multiple user contexts and privilege levels
  • Thinking like an attacker to find what automated tools miss

Whether you’re conducting in-house security testing or engaging professional services, the investment in API security testing pays dividends by preventing breaches, protecting reputation, and maintaining customer trust.

At BSG, our application security testing services include comprehensive API security assessments covering REST, GraphQL, SOAP, and modern protocols. Our experienced penetration testers combine automated tools with manual testing to identify vulnerabilities that matter most to your business.

Frequently Asked Questions

What is API security testing?

API security testing is the process of evaluating an application’s APIs to identify vulnerabilities, misconfigurations, and design flaws that could be exploited by attackers. It includes testing authentication, authorisation, input validation, business logic, and data exposure across all API endpoints.

How long does API security testing take?

API security testing duration depends on the API’s complexity and scope. A basic REST API with 10-20 endpoints might take 2-3 days, whilst a complex microservices architecture with dozens of APIs and multiple authentication mechanisms could require 2-3 weeks. Factors include the number of endpoints, authentication complexity, user roles, and business logic complexity.

What’s the difference between API testing and API penetration testing?

API testing (functional testing) verifies that APIs work as intended, returning correct data and handling requests properly. API penetration testing (security testing) actively attempts to exploit vulnerabilities, bypass security controls, and access unauthorised data. Penetration testing simulates real-world attacks to find security weaknesses before malicious actors do.

Can automated tools find all API vulnerabilities?

No. Automated tools excel at finding common issues like missing authentication, SQL injection, and excessive data exposure. However, they miss business logic flaws, complex authorisation issues, and context-specific vulnerabilities. Effective API security testing requires both automated scanning and manual testing by experienced security professionals.

How often should we test our APIs for security issues?

Test APIs during initial development, before production launch, and regularly thereafter. Minimum recommendation: annual penetration testing for stable APIs, quarterly testing for frequently updated APIs or those handling sensitive data. Also test after major changes, security incidents, or when compliance requirements dictate. Many organisations integrate automated API security scanning into CI/CD pipelines for continuous testing.

What’s BOLA and why is it so common in APIs?

BOLA (Broken Object Level Authorisation), also called IDOR (Insecure Direct Object Reference), occurs when an API doesn’t verify that an authenticated user should access a specific object. For example, if /api/users/123/orders returns data for user 123 without checking if the requesting user owns that account, any authenticated user can access any other user’s orders. It’s common because developers often implement authentication (checking who you are) but forget authorisation (checking what you can access).

Should we test our internal APIs?

Yes. Internal APIs are critical attack targets. If an attacker gains access to your network (via phishing, compromised employee account, or other means), insecure internal APIs provide pathways to sensitive data and systems. Treat internal APIs with the same security rigour as public-facing ones.

What credentials do pentesters need for API testing?

Effective API security testing requires test accounts at different privilege levels: standard user, power user/premium account, administrator, and ideally multiple accounts of each type to test horizontal authorisation. Providing API documentation, OpenAPI specifications, and access to staging/test environments significantly improves testing quality. Never provide production credentials that could affect live data.