API Security Testing: How to Find Vulnerabilities in Your APIs

API security testing is the practice of probing your APIs for the vulnerabilities attackers actually exploit — broken authorization, weak authentication, injection, misconfiguration, and data exposure — before those flaws reach production. APIs now carry the majority of web traffic and sit directly on top of your business logic and data, which makes them one of the most attacked surfaces on the internet. Testing them is no longer optional.

This guide covers what API security testing is, the OWASP API Security Top 10 that frames it, a practical testing checklist, the types of tools involved, and how to automate the parts that can be automated.

Key takeaways

• API security testing checks APIs for authorization, authentication, injection, and misconfiguration flaws — the classes an attacker targets first.
• The OWASP API Security Top 10 is the standard framework; broken authorization (BOLA) and broken authentication top the list.
• Effective programs combine manual/authenticated testing (for logic and auth flaws) with automated scanning (for injection, misconfiguration, and known issues).
• Use the checklist below to scope a test, and start automated coverage of reachable endpoints with a deep website scan.

What is API security testing?

API security testing is the process of evaluating an API — REST, GraphQL, or SOAP — for security weaknesses across its endpoints, parameters, authentication, and authorization logic. Unlike testing a traditional web page, where a human interface hides much of the logic, an API exposes operations directly: create a user, read an order, transfer funds. That directness is exactly what makes APIs powerful and what makes them dangerous when a single authorization check is missing.

The goal is to answer concrete questions: can a user read another user's data by changing an ID? Can an unauthenticated request reach an admin function? Does a parameter flow unsanitized into a database query or an internal request? Good testing turns those questions into evidence.

Why APIs are a different security problem

Web application scanning grew up around HTML pages, forms, and links a crawler can follow. APIs break those assumptions. There is no page to crawl; endpoints are documented (or worse, undocumented) in a spec, requests carry structured JSON, and authorization is enforced per-object rather than per-page. As a result, the most damaging API bugs are logic flaws — a valid, well-formed request that simply should not have been allowed — which generic scanners miss and which require understanding the API's intended behavior.

The OWASP API Security Top 10

The OWASP API Security Top 10 is the industry-standard list of the most critical API risks, and it is the backbone of any serious API security testing plan. The current edition highlights:

• Broken Object Level Authorization (BOLA) — accessing another user's objects by manipulating an ID. The single most common and impactful API flaw.
• Broken Authentication — weak tokens, missing expiry, or endpoints that skip authentication entirely.
• Broken Object Property Level Authorization — exposing or accepting object fields a user shouldn't see or set (mass assignment, excessive data exposure).
• Unrestricted Resource Consumption — no rate limiting or quotas, enabling denial of service and cost abuse.
• Broken Function Level Authorization (BFLA) — a regular user reaching admin-only functions.
• Unrestricted Access to Sensitive Business Flows — automating a flow (checkout, signup) in a way that harms the business.
• Server-Side Request Forgery (SSRF) — the API fetches a user-supplied URL, letting an attacker reach internal systems.
• Security Misconfiguration — verbose errors, missing headers, permissive CORS, exposed debug endpoints.
• Improper Inventory Management — forgotten, undocumented, or old API versions still live in production (shadow APIs).
• Unsafe Consumption of APIs — trusting data from third-party APIs without validation.

Two of these — misconfiguration and SSRF — are exactly the kind of issue an automated external scanner can catch. Others, like BOLA and BFLA, are authorization logic and need testing that understands your users and roles.

What these flaws look like in practice

The Top 10 is easier to test against once you've seen how the categories translate into real requests:

• BOLA in one request. An endpoint like GET /api/orders/1043 returns your order. Change the ID to 1044 and it returns someone else's — because the server checked that you're authenticated but never checked that the order is yours. This one-line change is the most common serious API bug.
• Excessive data exposure. A /api/users/me response includes password_hash, internal_notes, or another user's email because the API returns the whole database object and relies on the front end to hide fields.
• SSRF through a "helpful" feature. An endpoint that accepts an image_url to fetch a thumbnail will happily fetch http://169.254.169.254/ — the cloud metadata service — handing an attacker credentials.
• Shadow endpoints. /api/v1/ was deprecated but never turned off, and it lacks the authorization fixes shipped in /api/v2/. Attackers enumerate old versions precisely because they're forgotten.

Seeing the shape of each flaw is what turns a checklist into an actual test plan.

Types of API security testing

No single technique covers the whole Top 10. A complete program layers several:

• Static analysis (SAST) — reads source code for insecure patterns before deployment.
• Dynamic analysis (DAST) — tests a running API by sending crafted requests. This is where injection, SSRF, and misconfiguration surface.
• API penetration testing — a human exercises the API's business logic and authorization, chaining flaws the way an attacker would.
• Fuzzing — sends malformed and unexpected inputs to find crashes and edge-case handling bugs.

Automated tools shine at breadth and repetition; humans are irreplaceable for the authorization and business-logic flaws that dominate real API breaches.

API security testing checklist

Use this checklist to scope and run a test, mapped to the risks above:

1. Inventory every endpoint. Pull the OpenAPI/Swagger spec and confirm it matches reality — hunt for undocumented and old-version "shadow" endpoints (see external asset discovery).
2. Test object authorization (BOLA). As user A, try to read and modify user B's objects by changing IDs.
3. Test authentication. Check token expiry, signature validation, and whether any endpoint answers without a token.
4. Test function authorization (BFLA). As a low-privilege user, call admin and privileged endpoints.
5. Probe for injection. Test parameters and JSON fields for SQL, NoSQL, and command injection (see SQL injection scanners).
6. Check for SSRF. Wherever the API accepts a URL, test whether it will reach internal addresses.
7. Review data exposure. Confirm responses don't leak fields (passwords, tokens, PII) beyond what the client needs.
8. Verify rate limiting. Confirm sensitive flows and expensive endpoints are throttled.
9. Audit configuration. Check TLS, CORS, security headers, and error verbosity.

Manual vs automated API security testing

The practical question for most teams is not "manual or automated" but "which layer catches which risk".

Automated API security testing is what makes coverage continuous and repeatable — it re-runs on every release and catches the injection and misconfiguration classes cheaply, freeing human testers to focus on authorization and logic.

REST vs GraphQL: what changes when you test

Most guidance assumes REST API security testing, where each operation is a separate endpoint and testing means walking those endpoints methodically. GraphQL flips the model: a single endpoint accepts arbitrary queries, so the attack surface moves into the query itself. GraphQL adds risks a REST-oriented scanner won't think to check — introspection left enabled in production, deeply nested queries that exhaust resources, and batching that defeats naive rate limits. The OWASP API Top 10 still applies to both; what changes is where you look. Whatever the style, the authorization questions (can this token reach this object and this function?) stay the same.

When to run API security testing

API security testing pays off most when it runs early and often rather than once before launch. The mature pattern is to shift left: run static checks in the pipeline on every commit, run automated dynamic scans against a staging or production endpoint on every release, and schedule a deeper manual penetration test periodically and after any major change to authentication or authorization. Continuous, automated scanning is what keeps a fix from silently regressing three releases later — the failure mode that manual-only testing never catches in time.

How to choose API security testing tools

When you evaluate API security testing tools — sometimes searched for as an API vulnerability scanner — weigh them against a few practical criteria:

• Coverage of the OWASP API Top 10 — especially whether it can reason about authorization, not just injection.
• Spec-driven vs endpoint-driven — can it import an OpenAPI spec, or does it test the endpoints it can reach?
• Authentication support — can it carry tokens/sessions to test authenticated endpoints?
• Automation and CI — does it run on a schedule and in your pipeline?
• Actionable findings — severity, evidence, and a concrete fix per issue.
• Cost to start — can you validate it for free before committing?

Where automated scanning fits with Secably

Secably's deep website scan works as an automated API vulnerability scanner for your internet-facing endpoints: it actively tests the web and API endpoints it can reach for injection, cross-site scripting, SSRF, and misconfiguration — the automated, external half of API security testing (the injection, SSRF, and misconfiguration rows above). It is the fast, repeatable layer you run continuously against your internet-facing services.

It is not a replacement for authenticated, spec-driven testing of authorization logic — no automated tool reliably finds BOLA or BFLA on its own, because those depend on your roles and data. The effective pattern is to pair the two: use Secably to keep injection, SSRF, and configuration issues from ever shipping across all your reachable endpoints, and use focused manual testing for the authorization layer. Start with a free scan of any host you own, or add it as a monitored target for continuous coverage — see how to read an external vulnerability scan and pricing for scheduled monitoring.

Frequently asked questions

What is API security testing?

API security testing evaluates an API's endpoints, parameters, authentication, and authorization for security weaknesses — such as broken object-level authorization, injection, SSRF, and misconfiguration — so they can be fixed before attackers find them. It combines automated scanning with manual, authenticated testing.

What is the difference between API security testing and API penetration testing?

API security testing is the broad discipline, including automated scanning, static analysis, and manual review. API penetration testing is the manual, adversarial part of it: a tester deliberately chains authorization and logic flaws to demonstrate real impact, the way an attacker would.

What framework should API security testing follow?

The OWASP API Security Top 10 is the standard reference. It defines the most critical API risks — led by Broken Object Level Authorization (BOLA) and Broken Authentication — and gives testing a shared, prioritized structure.

Can API security testing be automated?

Partly. Automated tools reliably catch injection, SSRF, misconfiguration, and known-vulnerable components, and they should run continuously. Authorization and business-logic flaws (BOLA, BFLA) still require manual, authenticated testing because they depend on your specific roles and data.

How do I test my API's reachable endpoints for free?

Run a deep website scan with Secably against any host you own. It actively tests reachable web and API endpoints for injection, XSS, SSRF, and misconfiguration, with no signup required for a first scan.