API Security Testing: A Complete Guide for Developers

In August 2025, a security researcher discovered that Intel’s internal APIs exposed the personal data of 270,000 employees—valid credentials not required. One portal’s API allowed anyone to download employee information simply by bypassing a login page. 

The vulnerabilities? Broken authentication and a security misconfiguration, both of which rank in the OWASP API Security Top 10. These are exactly the kind of issues that automated API security testing catches during development. 

In this guide, we’ll cover:

• What API security testing is and how it works
• Types of API security testing and when to use them
• How to get started and integrate testing into your CI/CD pipeline
• Common vulnerabilities that testing catches
• What to look for in an API security testing solution

API Security Testing TL;DR

• 99% of organizations experienced API security issues in the past 12 months, with over half forced to delay application releases due to security concerns
• API security testing finds vulnerabilities before attackers do by systematically testing endpoints, authentication, and authorization during development—not after deployment
• Dynamic Application Security Testing (DAST) is most effective for APIs because it tests running applications to find exploitable vulnerabilities
• The top 3 API vulnerabilities are Broken Object Level Authorization (accessing other users’ data), Broken Authentication (compromised tokens), and Broken Object Property Level Authorization (excessive data exposure)
• Fixing vulnerabilities in production costs 100x more than catching them during development, which makes continuous testing essential for AI development velocity

What Is API Security Testing?

API security testing is the process of finding security vulnerabilities in your APIs before attackers do. It works by systematically testing your API endpoints, authentication mechanisms, and data handling to identify weaknesses that could be exploited.

The approach to API security testing has fundamentally changed over the past few years. It used to be something a security team did once or twice a year through penetration testing. Now? It’s automated testing that runs in your CI/CD pipeline on every pull request. When you push code that touches an API endpoint, security tests should run automatically and give you feedback while the code is still fresh in your mind.

This shift makes sense when you think about how we build software now. APIs are the foundation of modern applications, and API-specific security testing is required. Every mobile app, single-page web application, and microservice architecture runs on APIs. A vulnerability in one API endpoint can compromise your entire application ecosystem.

Why API Security Testing Matters Now

APIs have become the primary attack vector for modern applications, and recent research confirms the severity of the challenge. 99% of organizations have encountered API security issues within the past 12 months, with more than half (55%) forced to slow application rollouts due to API security concerns.

The reason APIs present such significant risk is straightforward: APIs expose structured endpoints that directly connect to your business logic and sensitive data. There’s no user interface providing an extra layer of validation or obscurity. When an API endpoint is vulnerable, attackers get direct access to whatever’s behind it. 

Organizations need continuous, automated API security testing that catches vulnerabilities before deployment, not reactive approaches that leave APIs exposed for months while security teams struggle to find and fix issues in production for 100x the cost.

How API Security Testing Works

At its core, API security testing simulates how attackers interact with your APIs. It systematically probes for weaknesses in authentication, authorization, input validation, and data handling.

Here’s how it actually works in practice.

API discovery is first. You need to map out what endpoints exist and how they’re structured. Modern testing tools read your API specifications (OpenAPI documents, GraphQL schemas, gRPC service definitions) to understand exactly what endpoints are available, what parameters they accept, and what authentication they require.

Once you know what you’re testing, the tool generates targeted requests designed to expose specific vulnerability types. Looking for broken object-level authorization? The tool authenticates as one user, then systematically tries to access resources belonging to other users by manipulating ID parameters. Testing for injection vulnerabilities? It sends specially crafted payloads with SQL commands or scripting code to see if your API executes them.

Vulnerability detection happens by analyzing how your API responds. When an API returns a 500 error with a stack trace showing database details, that’s information disclosure. When manipulating an ID parameter lets you access data you shouldn’t, that’s broken authorization. When the API accepts DELETE requests on an endpoint that should only support GET and POST, that’s a security misconfiguration.

Let’s walk through a concrete example. Say you have an API endpoint /api/user/profile that should only return the authenticated user’s profile data. A comprehensive security test would:

1. Send an authenticated request and capture what comes back
2. Try requests with missing, expired, or invalid tokens
3. Attempt to access other users’ profiles by manipulating user IDs
4. Check if the endpoint leaks information through error messages
5. Verify that rate limiting prevents brute force attacks

If any of these tests succeed inappropriately—say, an expired token still grants access, or you can grab another user’s profile by changing an ID—the tool flags it as a vulnerability, captures the exact request that triggered it, and gives you everything you need to reproduce and fix the issue.

What makes this approach powerful is that it tests your running application. Unlike static code analysis that looks for vulnerability patterns in source code, dynamic API security testing actually executes attacks against your APIs. This catches vulnerabilities that only appear when code runs, when different components interact, or when specific data conditions exist.

Types of API Security Testing

You need multiple types of security testing to comprehensively protect your APIs. Here’s what each one does and when to use it.

Dynamic Application Security Testing (DAST)

Dynamic Application Security Testing tests your APIs by actually interacting with them at runtime—sending requests, analyzing responses, looking for vulnerabilities. This simulates how attackers engage with your APIs: from the outside, focusing on what’s actually exploitable.

DAST is particularly good at catching authentication and authorization flaws, injection vulnerabilities, security misconfigurations, and business logic errors. Modern DAST tools built specifically for APIs understand OpenAPI specifications, GraphQL schemas, and gRPC service definitions, so they can test comprehensively based on your actual API structure rather than guessing at endpoints.

Best for: 

• Finding exploitable vulnerabilities in deployed applications
• Testing authentication and authorization logic
• Catching configuration issues
• Proving that vulnerabilities are actually fixed.

Static Application Security Testing (SAST)

SAST tools analyze your API’s source code without executing it, scanning for coding patterns that typically indicate security vulnerabilities. For APIs, SAST looks for things like hard-coded credentials, SQL injection opportunities, insufficient input validation, and insecure data handling.

The advantage is early detection. You can run static analysis on code that isn’t even deployed yet. The downside? SAST generates more false positives and struggles with business logic flaws or vulnerabilities that only emerge when components interact.

Best for: 

• Catching common coding errors early in development
• Enforcing secure coding standards
• Complementing DAST findings with source-level context.

Software Composition Analysis (SCA)

Software Composition Analysis examines your API’s dependencies (i.e. all those open-source libraries and frameworks your code uses) and matches them against databases of known vulnerabilities. The challenge with SCA is figuring out exploitability. Just because your application includes a vulnerable library doesn’t automatically mean the vulnerability affects you; you might not even use the vulnerable part of that library.

Best for: 

• Staying aware of open-source vulnerabilities
• Meeting compliance requirements
• Prioritizing dependency updates.

Penetration Testing

Manual penetration testing brings in security experts who attempt to exploit your APIs using the same techniques as real attackers. Unlike automated tools that test for known vulnerability patterns, pen testers apply creative thinking to find novel attack vectors and business logic flaws.

It’s expensive and time-consuming, and it gives you point-in-time findings rather than continuous validation. But human experts find things automated tools miss.

Best for: 

• In-depth security assessments before major releases
• Validating automated testing findings
• Meeting compliance requirements.

Getting Started with API Security Testing

Moving from understanding to actually implementing API security testing requires a few practical steps. Here’s how to get started.

Step 1: Inventory Your APIs

You can’t secure what you don’t know exists. Create a comprehensive inventory of all your API endpoints: REST APIs, GraphQL endpoints, gRPC services, SOAP APIs, internal microservice communications, and third-party APIs your applications consume.

For each API, document what protocol it uses, where it’s deployed, what authentication mechanisms protect it, whether you have specifications (OpenAPI documents, GraphQL schemas, etc.), and what level of data sensitivity it handles. Many teams discover “shadow APIs” during this process, i.e. endpoints that got deployed without security review or that everyone forgot existed.

Step 2: Establish API Specifications

API security testing works much better when you have accurate specifications. If you already maintain OpenAPI documents or GraphQL schemas, validate that they’re current and complete. If specifications don’t exist, creating them before implementing security testing provides value beyond just testing. Specifications improve documentation and clarify API contracts between teams.

Don’t let perfect specifications block progress, though. Start with basic specs covering your critical endpoints and expand iteratively. Even partial specifications enable meaningful security testing.

Step 3: Configure Authentication

API security testing requires the ability to authenticate with your APIs. Prepare test credentials with appropriate permissions and document any special authentication requirements (API keys, OAuth flows, JWT tokens, etc.).

Consider creating dedicated testing accounts rather than using production credentials. Testing accounts should have realistic but non-production privileges, enabling comprehensive testing without risking actual customer data.

Step 4: Set Up Your First Scan

Start with a focused pilot rather than trying to test everything at once. Pick one representative API. Ideally it’s something that’s actively developed, has clear specifications, and handles moderately sensitive data. Avoid starting with your most critical production API until you’ve validated your testing approach.

Configure the testing tool with your API specification, provide authentication credentials, set appropriate rate limits, and identify any endpoints that should be excluded. Run an initial scan and review the results.

Expect some tuning. First scans often reveal configuration issues, produce false positives that need filtering, or identify endpoints that need special handling. This is normal and exactly why you start with a pilot.

Step 5: Integrate Into CI/CD

Once you’ve validated that testing works for your pilot API, integrate security testing into your continuous integration pipeline. Most modern API security tools provide plugins for popular CI/CD platforms (GitHub Actions, GitLab CI, Jenkins, CircleCI) or offer command-line interfaces for custom integrations.

Configure testing to run on pull requests, giving you feedback before code merges. Set failure thresholds that balance security rigor with practical development needs. For example, failing builds on high-severity vulnerabilities while allowing lower-severity findings to pass with visibility.

Step 6: Establish Remediation Workflows

Testing only provides value if findings lead to fixes. Establish clear workflows for how security findings move from discovery to remediation: who gets notified, how findings are triaged for severity and priority, what SLAs apply for different severity levels, and how fixes get validated and closed.

Integration with issue tracking systems (Jira, GitHub Issues, etc.) helps you manage security findings alongside other development work. Make sure security findings include enough context for developers to understand and reproduce issues, including the exact request that triggered the vulnerability, expected versus actual behavior, and remediation guidance.

Common API Vulnerabilities Caught by Testing

Here are the top three vulnerability types that comprehensive API security testing identifies according to the OWASP API Security Top 10 list.

Broken Object Level Authorization

BOLA vulnerabilities happen when APIs fail to verify that authenticated users should access specific data objects they’re requesting. An attacker authenticates legitimately, then accesses other users’ data by manipulating object IDs and systematically harvesting records. Security testing catches this by authenticating as one user, then attempting to access resources belonging to others.

Broken Authentication

Authentication mechanisms implemented incorrectly allow attackers to compromise tokens or exploit flaws to assume other users’ identities. Common issues include weak password policies, insufficient token expiration, and authentication bypass through alternative endpoints. Testing identifies these by attempting invalid credentials, expired tokens, and requests without authentication.

Broken Object Property Level Authorization

APIs often return more data than needed or allow users to modify properties they shouldn’t control. Testing examines API responses for sensitive fields and attempts to modify restricted properties like isAdmin flags or account balances.

Choosing an API Security Testing Solution

When selecting an API testing tool, focus on capabilities that support effective implementation, not just feature checklists.

Speed That Enables Continuous Testing

Look for solutions that complete API scans in minutes, not hours. Fast scans let you test on every code change without bottlenecking deployments. If testing takes 30+ minutes, developers will skip it or run it less frequently, defeating the purpose of shift-left security.

Developer-First Integration

The best security testing happens where developers already work. Evaluate tools based on CI/CD platform integration (GitHub Actions, GitLab CI, Jenkins), local testing for pre-commit validation, clear remediation guidance in findings, and integration with pull request workflows. If developers need to context-switch to a separate security dashboard, adoption suffers.

Complete Protocol Coverage

Verify support for all API types in your environment: REST APIs, GraphQL endpoints, gRPC services, and SOAP APIs. Protocol-specific testing improves both accuracy and speed—REST endpoints should receive JSON requests, SOAP endpoints should receive XML.

Specification-Driven Discovery

Modern tools leverage API specifications (OpenAPI documents, GraphQL schemas, gRPC Protocol Buffers, SOAP WSDLs) for comprehensive, accurate discovery. This beats crawling applications or proxying traffic, which inevitably misses endpoints.

Accuracy Over Volume

Prioritize tools with low false positive rates. Security findings that turn out to be non-issues erode developer trust and waste engineering time. Look for technology-specific scanning that applies appropriate tests based on what you’re actually running.

Pricing That Scales With Your Team, Not Your Architecture

Traditional per-API pricing becomes prohibitively expensive for microservices architectures with hundreds of endpoints. Per-developer pricing with unlimited API scanning provides predictable costs that scale with team size rather than punishing you for modern architecture choices.

Building Security Into Development Velocity

As development velocity increases exponentially thanks to AI, API security testing integrated directly into development workflows becomes the only viable approach for maintaining both speed and security.

The API vulnerabilities making headlines are all preventable through comprehensive security testing. Organizations building secure APIs successfully treat security testing as an enabler of confident, rapid development rather than a gate that slows releases. Security becomes a competitive advantage when:

• Developers receive security feedback in minutes
• Findings include clear remediation guidance
• Testing integrates seamlessly into existing workflows

Ready to implement pre-production, runtime API security testing that embeds directly into development workflows? See StackHawk in flight.