API Security Testing: Tools, Risks, and Strategies

API attacks have gone up by 400% since 2020. According to Gartner, they're now the most common way hackers break into enterprise web apps. The average company has over 15,000 APIs - and every single one is a door that could be left unlocked. This guide covers what API security testing is, the risks you need to know about, the best tools available, and how to build a testing process that actually works.

What Is API Security Testing?

API security testing is the process of checking your APIs for weaknesses, misconfigured settings, and design flaws that could let attackers in, steal data, or disrupt how your system works.

APIs expose your business logic directly to callers, skipping the frontend entirely. And they sit at a layer that most standard security tools can't fully see. Regular scanners often miss a large chunk of your API endpoints - especially when your frontend doesn't actually call every path that exists.

API security testing protects things like:

• Who can see your data (and who can't)
• How your login and access controls work
• Whether request limits are actually in place
• Whether data is encrypted while it moves
• Whether your error messages accidentally give away too much

The three types of APIs teams test most often are: REST (about 76% of all web APIs), GraphQL (growing fast but with its own issues, like letting anyone query your full schema), and SOAP (still used a lot in older enterprise systems).

Attackers don’t try to break your UI. They're looking for exposed IDs, weak tokens, no rate limits, and admin pages accessible to regular users.

Key API Security Risks: The OWASP API Security Top 10

The OWASP API Security Top 10 (2023 edition) is the go-to reference for API security testing across the industry:

API1: Broken Object Level Authorization (BOLA)

BOLA is the most common API problem out there. It's behind roughly 40% of API attacks. It happens when an API doesn't check whether the person asking for data should actually be allowed to see it.

Here's a simple example: someone logs in normally, then changes another user's order ID in the request - from /orders/1234 to /orders/1235 - and gets back that other person's order details. That's BOLA.

What testers check: Whether swapping IDs in requests gives back someone else's data. You should always do this with two different accounts. Check that the API confirms both that the user is logged in AND that they own the thing they're asking for - not just that they have a valid login. That two-account test is the most reliable way to catch BOLA, which many teams skip.

API2: Broken Authentication

After BOLA, login problems are the most exploited issue. This covers weak passwords, tokens that never expire, JWT tampering, token replay attacks, and login pages with no protection against brute-force attacks.

What testers check: 

• Whether expired or changed JWTs are let through
• Whether login pages have request limits
• Whether tokens are killed when a user logs out
• Whether API keys are sent safely - in headers only, never in the URL, where anyone can see them.

API3: Broken Object Property Level Authorization

This happens in two ways: either the API shows more data than it should, or it lets users change fields they're not supposed to by slipping extra data into a request that the API processes without checking.

What testers check: 

• Whether adding extra fields to a POST or PUT request changes things behind the scenes. 
• Whether responses include fields that the user shouldn't be able to see.

API4 - Unrestricted Resource Consumption

No rate limits means attackers can try thousands of passwords, scrape all your data, or flood your server until it falls over. This also includes no limits on request size, the number of records returned, or file upload size.

What testers check: 

• Whether sending a ton of requests triggers any slowdown or blockage. 
• Whether huge or deeply nested data gets rejected. 
• Whether page limits are actually being enforced.

API5: Broken Function Level Authorization

This is like BOLA, but for entire endpoints rather than individual objects. It happens when admin or sensitive pages are accessible to regular users because the API doesn't check who's allowed to access them.

What testers check: 

• Whether low-level accounts can reach admin pages like /admin/users or /internal/config. 
• Whether HTTP methods not documented - like DELETE or PUT on read-only endpoints - are accidentally working.

API6 - API10: Additional Risks

API6 - Unrestricted Access to Sensitive Business Flows: Attackers automate actions such as bulk buying or creating many fake accounts at a rate the API was never built to handle.

API7 - Server-Side Request Forgery (SSRF): The API fetches a URL provided by an attacker, potentially exposing internal systems that were never meant to be public.

API8 - Security Misconfiguration: Detailed error messages, overly permissive CORS settings, exposed debug pages, and weak TLS all fall under this. Small setup mistakes can create real problems.

API9 - Improper Inventory Management: Forgotten APIs and old versions with weaker security than your current API are some of the most-exploited entry points - and usually the least watched.

API10 - Unsafe Consumption of APIs: When you connect to third-party APIs without verifying what they return, you inherit their security issues.

API Security Testing Methods

A good API security program uses a mix of approaches - each one aimed at a different stage of building and a different type of risk.

Static Application Security Testing (SAST)

SAST analyzes your API source code, settings, and OpenAPI/Swagger specs for issues before the app even runs. It finds issues such as hardcoded passwords, weak encryption, and unsafe code patterns. It works best when you plug it into pull request reviews - catching problems before they ever go live.

The catch: SAST can't see how your API behaves while running. It only sees what's written in the code.

Dynamic Application Security Testing (DAST)

DAST tests a running API by sending bad requests and watching what comes back - the same way an attacker would. It finds OWASP Top 10 issues, injection flaws, broken login, and setup problems that only show up at runtime. It works best when you give it your OpenAPI/Swagger spec to guide the scan.

The catch: Standard DAST struggles with API flows that require requests to occur in a specific order. It also can't find business logic problems on its own. This is where API security testing tools that support logged-in, step-by-step scanning really shine.

API Penetration Testing

This is a manual test where a real person simulates an attack. It finds complex multi-step exploits, business logic flaws, and new vulnerabilities that automated tools simply can't spot. It's usually done on a schedule - every quarter or before a big release - rather than all the time.

Think of it this way: automated DAST gives you width (runs on every build). Manual pen testing gives you depth (targeted, high-value, scheduled).

Fuzz Testing

Fuzz testing throws random, broken, or unexpected inputs at API endpoints to see what breaks. It's great for finding crashes, unhandled errors, memory leaks, and gaps in input validation - especially in endpoints that take file uploads, complex JSON, or user-controlled data.

API Discovery and Inventory Testing

Before you test what you know about, you need to find what you don't. Shadow APIs - undocumented endpoints that got deployed without any oversight, and zombie APIs - old versions that were never switched off - are some of the most-exploited entry points in large companies.

The average company has 30–40% of its APIs untracked or undocumented. Inventory testing uses traffic logs, API gateway data, and code scanning to build a full map of your attack surface.

Best API Security Testing Tools

The right tool depends on your team's skills, what you're trying to test, and where you are in the build process. Most solid security programs use a mix - one for automated DAST in CI/CD, one for deep manual testing, and one for spec-level checks.

OWASP ZAP (Zed Attack Proxy)

OWASP ZAP is the most widely used free DAST tool, now looked after by Checkmarx. It scans REST and GraphQL APIs for OWASP Top 10 issues. It's very flexible and has a big community.

Best for: Teams with security know-how who want full control and no licensing costs. It's the top pick among free API security testing tools for teams just getting started.

Limitation: It requires significant setup and tuning. It flags more false positives than paid tools and isn't easy to plug into CI/CD without someone who knows what they're doing.

Burp Suite Professional (PortSwigger)

Burp Suite is the standard tool for manual API pen testing. Its proxy, intruder, repeater, and scanner let testers change requests, tokens, headers, and parameters with precision. Nothing else comes close to finding complex, multi-step vulnerabilities and business logic flaws.

Best for: Security engineers and pen testers doing deep, scheduled tests. Less suited to fully automated CI/CD scanning without an expert driving it.

StackHawk

StackHawk is a DAST platform built specifically for developer teams and CI/CD pipelines. It uses simple YAML settings, works natively with GitHub Actions and CircleCI, and adds clear notes directly on pull requests. It supports REST, GraphQL, and SOAP. Scans finish in 3–10 minutes - way faster than running Burp Suite.

Best for: Dev and DevSecOps teams who want automated security testing on every build without the hassle of a traditional DAST tool. It's consistently listed among the best API security testing tools for teams without a full-time security person.

42Crunch

42Crunch focuses on spec-first API security. It checks your OpenAPI specs for security issues before any code is written, catching issues at the design stage. It connects to IDEs and CI/CD pipelines to apply security rules right in the API contract itself.

Best for: Teams that design APIs using OpenAPI and want to catch security issues before a single line of code is written. One of the top API security testing solutions for teams serious about catching problems early.

Postman (Security Testing Features)

Postman is primarily known as an API testing and collaboration tool, but it also supports security-focused test scripts, login flow checks, and collection-based security tests. It's useful if you want to add basic security checks to your existing tests without bringing in a whole new tool.

Best for: Teams already using Postman who want to layer simple security checks into their existing workflow.

Other Tools Worth Knowing

Several other leading API security testing solutions are worth keeping on your radar:

• APIsec: Ongoing automated security testing with business logic coverage
• Salt Security: Live traffic analysis and API protection in production
• Traceable AI: AI-based detection of unusual behavior in live API traffic
• Nuclei: Free, template-based vulnerability scanner
• Pynt: Security testing built for CI/CD workflows
• Snyk API: Developer-focused security scanning for APIs and code dependencies

Why Functionize Is the Best Choice for API Security Testing Automation

Functionize brings continuous API security testing into automated workflows - making it a regular part of every release, not a one-time task:

• AI-Driven Security Test Creation: Functionize uses AI to automatically build API security test cases from your specs and real usage data. It covers OWASP Top 10 vulnerability types without needing a security engineer to write every test by hand.
• Self-Healing Security Automation: APIs change all the time - new endpoints get added, parameters shift, login flows get updated. Functionize's self-healing engine automatically updates existing security tests to keep them up to date, so your test suite doesn't fall behind.
• Seamless CI/CD Integration: Functionize plugs API security testing directly into your CI/CD pipeline. Every build is automatically scanned before it goes out, closing the gap that opens up when security testing only happens at release time.
• Scalability for Enterprise API Portfolios: Functionize's cloud setup supports running security tests in parallel across hundreds of API endpoints and multiple environments. It's built for large teams managing complex, spread-out API setups.
• Clear Security Reporting and Compliance Diagnostics: Functionize produces detailed, audit-ready reports that link results to OWASP Top 10 findings and compliance needs. Teams can share results right away, track what's been fixed, and pull together docs for security reviews.
• Data-Driven Security Test Coverage: Functionize runs security tests across multiple user roles, login states, and input types, ensuring access controls are tested in every scenario, not just the easy cases.

How to Implement API Security Testing: Key Steps

Here's a practical, step-by-step process for building a real API security testing program - whether you're starting from zero or trying to bring some order to what you already have.

Step 1: Build a complete API list

Find every API endpoint in your setup - including forgotten APIs and old deprecated versions. Use traffic logs, API gateway data, and code scanning through GitHub or GitLab to make sure nothing gets missed. 

Step 2: Check your API specs

Make sure your OpenAPI/Swagger docs are up to date, accurate, and complete. If they don't exist yet, create them for your most important endpoints before you start testing. Good specs make scanners much more accurate - and most modern DAST tools need them.

Step 3: Figure out what's most at risk

Work out which API endpoints handle the most sensitive data or key business actions - things like payments, login, and admin functions. Start your security testing there, not with the low-risk stuff.

Step 4: Pick and set up your tools

Choose tools that fit your team and goals: OWASP ZAP or StackHawk for automated DAST testing, Burp Suite for scheduled manual penetration testing, and 42Crunch for spec checks. Set up authentication properly - API keys, OAuth flows, JWT tokens - so your tools can test logged-in endpoints, not just public ones.

Step 5: Run your first scan and review the results

Run an initial scan, review the results, and separate real problems from false alarms. Set up a list of known-safe patterns to keep future scans cleaner. Sort findings by how serious they are and how easy they'd be to exploit.

Step 6: Connect it to CI/CD for ongoing testing

Set up automated security scans to run on every pull request and before every release. Define severity levels that stop a build when serious issues are found. Send findings to places developers actually look - Jira, Slack, pull request comments - so they get fixed quickly.

Step 7: Schedule regular manual pen tests

Don't rely only on automated tools. Bring in manual pen testing at least quarterly and before major releases. Use it to dig into multi-step exploits, business logic problems, and complex access issues that automated tools can't find on their own.

API Security Testing Best Practices

• Test logged-in endpoints, not just public ones: Most BOLA and broken function-level access problems are only reachable after login. Set up your tools with working credentials for every user type - admin, regular user, guest - so you're actually testing access controls, not just public pages.
• Use two separate accounts when testing access controls: For BOLA testing, always try to reach Account B's data using Account A's login. Testing with just one account will miss every horizontal access problem.
• Test old API versions just as carefully as new ones. Older versions often skip login checks that were added later. Find every version path - /v1/, /v2/, /api/legacy/ - and check that they all apply the same security rules.
• Never use harmful test payloads on live systems: Run automated DAST and fuzz tests against a staging environment that looks like production. Injection and fuzz payloads can corrupt real data or trigger real transactions if you're not careful.
• Look at what your error messages say: Error responses should give back the right status codes without revealing stack traces, internal paths, database names, or framework versions. Data leakage in error messages is one of the most overlooked problems.
• Automate all the time, test manually on a schedule: Automated tools find known problems fast and at scale. Manual pen testing finds what automated tools structurally can't - business logic flaws, chained attacks, and new ways to exploit things. You need both.
• Keep API keys and credentials in a secrets manager, not in scripts: Hardcoded credentials in test files, YAML configs, or code repos are a common cause of credential leaks. Always use environment variables and secrets managers for testing credentials.
• Link your findings to compliance requirements from the start: OWASP Top 10 findings connect directly to PCI DSS 6.2/6.4, HIPAA security requirements, and SOC 2 controls. Structuring your reports around compliance from day one saves a lot of extra work when audit time comes.

Future Trends in API Security Testing

• Security testing is moving earlier in the process: API security checks are shifting into IDEs, pull request reviews, and pre-commit hooks. As AI coding tools build full API setups in minutes, security has to move just as fast - or the gaps will grow faster than teams can close them.
• Teams are pulling their tools together: Instead of running separate tools for discovery, DAST, spec checks, WAF rules, and live monitoring, companies are moving toward all-in-one platforms. With 30-40% of APIs untracked at the average company, unified platforms are becoming the practical answer.
• AI is starting to power attack simulation: Security tools are starting to use AI to run realistic multi-step attack patterns - not just scan for known issues. This approach finds chained vulnerabilities and business logic flaws that standard DAST tools can't detect.
• Regulators are paying more attention: As APIs become the primary gateway to personal and financial data, regulators are stepping in. Open Banking, healthcare APIs, and data protection rules are moving toward clear API security standards - making reliable API security testing a real business need, not just a nice-to-have.
• Live protection is becoming standard: Testing before release isn't enough on its own anymore. Production APIs face new attacks and zero-day exploits that only show up at runtime. Platforms like Salt Security and Traceable AI that watch live API traffic are being used alongside DAST to cover the full API lifecycle.

FAQs on API Security Testing

What is API security testing?

API security testing is the process of evaluating your APIs for vulnerabilities, authentication flaws, access bypasses, injection risks, and data exposure. It combines automated scanning, manual pen testing, and ongoing checks to ensure your APIs withstand real attacks. 

What are the most common API security vulnerabilities?

The OWASP API Security Top 10 (2023 edition) is the standard reference. The top three are BOLA (Broken Object Level Authorization), Broken Authentication, and Broken Object Property Level Authorization. BOLA alone accounts for roughly 40% of real-world API attacks and is the most consistently underestimated issue for teams new to API security testing.

What is the difference between DAST and penetration testing for APIs?

DAST is automated - it sends crafted requests and checks responses for known issues at scale, making it a great fit for CI/CD pipelines. Pen testing is manual - a real person simulates attacks to find chained exploits, business logic flaws, and new vulnerabilities that automated tools can't surface. A solid program uses both: DAST for width on every build, pen testing for depth on a set schedule.

Can API security testing be integrated into CI/CD pipelines?

Yes - and in 2026, it's the standard way to do it. Tools like StackHawk, 42Crunch, OWASP ZAP (via Docker/CLI), and Nuclei all support CI/CD integration. The goal is to run automated security scans on every pull request and block releases when serious issues are found. 

What is the best tool for API security testing?

There's no single best tool - the right pick depends on your team, your testing stage, and the level of risk you're comfortable with. OWASP ZAP is the best free starting point. Burp Suite Professional is the gold standard for manual pen testing. StackHawk leads for CI/CD-native automated DAST. 42Crunch is best for spec-first security. Enterprise teams often use several tools together, and a platform like Functionize to bring test automation, reporting, and pipeline integration into one place.