Why Is It Critical to Check Security During API Testing?

Last updated: October 2026

Quick answer

API security testing is critical because APIs expose your application’s data and business logic directly, and functional tests only prove that the API works for valid requests. Security tests check what happens with invalid, malicious or unauthorized requests: who can read which record, how the API handles bad input, and whether it can be abused at scale. A good starting point is the OWASP API Security Top 10 (2023 edition), which lists the most common API risks.

Why APIs are a prime target

Modern applications are built from many services that talk to each other over APIs, and web and mobile front ends call those same APIs. That has three consequences for testers:

A large attack surface. Every endpoint, parameter and method is something an attacker can try directly, without ever opening your user interface.
Direct access to data. An API returns data in a structured form. If an authorization check is missing, a script can collect it quickly.
Shared impact. One weak API can affect every application that depends on it.

The UI also hides mistakes. A button may be hidden from a user, but if the API behind it does not check permissions, anyone who can call the endpoint can use it. That is why UI testing alone cannot prove an application is secure.

What functional API tests miss

Functional API test asks	Security API test asks
Does a valid request return the right data?	Does a user get a record that belongs to someone else when they change the ID?
Does login succeed with correct credentials?	Can login be brute-forced, or do tokens stay valid when they should not?
Is the response format correct?	Does the response expose fields the caller should never see?
Does the endpoint handle expected input?	How does it behave with oversized, malformed or unexpected input?
Is response time acceptable for one user?	Can one client exhaust resources with a flood of requests?

The OWASP API Security Top 10 as a test checklist

OWASP published its first API list in 2019 and updated it in 2023. The 2023 categories, in order, are:

Broken object level authorization. Try accessing another user’s objects by changing IDs in the request.
Broken authentication. Test login, token handling, password reset and session expiry.
Broken object property level authorization. Check that responses do not expose sensitive fields, and that clients cannot update fields they should not control.
Unrestricted resource consumption. Look for missing rate limits, size limits and timeouts.
Broken function level authorization. Call admin-only endpoints with an ordinary user’s token.
Unrestricted access to sensitive business flows. Check whether flows such as purchases or sign-ups can be abused by automation.
Server-side request forgery (SSRF). If the API fetches a URL you supply, test whether it can be pointed at internal systems.
Security misconfiguration. Review headers, CORS settings, error messages and default settings.
Improper inventory management. Find forgotten, old or undocumented API versions and endpoints that are still live.
Unsafe consumption of APIs. Check how your service treats data it receives from third-party APIs.

Injection used to be a separate category in the 2019 list. In 2023 it is covered under unsafe consumption of APIs, but validating input is still essential.

A practical API security testing checklist

Start at design time

Treat security requirements like performance and functional requirements. Write down who may call each endpoint, which fields are sensitive, and what rate limits apply. Testing is far cheaper before the API is in production.

Test authentication and authorization separately

Call every endpoint with no token, an expired token, a token for another user and a token with lower privileges.
Change object IDs in paths, query strings and request bodies.
Confirm that every protected method (GET, PUT, PATCH, DELETE) enforces the check, not just the one the UI uses.

Test input handling

Send values outside the expected range, wrong types, very long strings, special characters and missing required fields.
Confirm the API rejects them with a clear, non-revealing error instead of a stack trace.

Test limits and abuse

Check rate limiting, payload size limits and pagination limits.
Repeat sensitive flows (login, password reset, checkout) quickly to see whether protections trigger.

Keep an inventory

Maintain a list of every API version and environment. Retire old versions deliberately, and make sure non-production environments do not use real customer data.

Automate and repeat

Run these checks in your pipeline on every release, and add a regression test whenever you fix a security bug. Add a manual penetration test or specialist review for high-risk systems such as payments.

How CloudQA helps

CloudQA is a codeless test automation platform for web applications, and it includes API testing steps that you can combine with browser steps in the same test. That makes it practical to automate the repeatable checks above, such as calling an endpoint with a missing or invalid token and asserting on the response, and to run them on a schedule or from your CI/CD pipeline through the CloudQA APIs. Runs are deterministic: recorded steps run exactly as written. CloudQA does not replace a dedicated security scanner or a penetration test, but it covers the regression side: once you know a check matters, you can keep running it. Read more about API testing with CloudQA, or use the free Mock API tool to stand up test endpoints. You can also start free.

This article was originally written in collaboration with Cigniti Technologies and has been fully updated.