API Security Testing: An Enterprise Guide to Securing the Connective Tissue
APIs are how modern software talks, and they are where attackers now look first. Testing them requires understanding the contract, not just scanning the endpoints.
Every modern application is a collection of APIs. The user interface is a thin layer over them, and attackers have noticed: API vulnerabilities now account for a significant proportion of serious data breaches. The shift happened because APIs expose business logic directly — if you can call an endpoint, you can potentially access or modify data that the UI would never show you.
Why traditional web scanning misses API flaws
Web application scanners are built for forms, pages, and navigation flows. APIs do not have pages. They have contracts: request schemas, response schemas, authentication tokens, and parameters that control which objects are returned. A scanner that does not understand the contract cannot test whether changing a user ID in a request returns another user's account — which is the most common and damaging API vulnerability in existence.
The OWASP API Security Top 10
| Risk | What it means | Why it matters |
|---|---|---|
| BOLA (Broken Object Level Authorization) | Accessing other users' objects by manipulating IDs | Most common and impactful; bypasses authentication entirely |
| Broken Authentication | Weak token implementation, missing expiry, credential stuffing | Account takeover at scale |
| Excessive Data Exposure | API returns more data than the UI needs, including sensitive fields | Data the user never sees but the API hands over freely |
| Lack of Rate Limiting | No throttling on requests | Brute force, enumeration, and scraping at scale |
| Broken Function Level Authorization | Regular user can access admin functions by calling the endpoint directly | Privilege escalation through direct API calls |
How to test APIs properly
1. Understand the contract first
Get the OpenAPI specification or Postman collection. Without the contract, you are guessing at parameters and missing entire endpoint families. If no specification exists, the first finding is: there is no specification.
2. Test authentication and authorisation separately
Authentication asks 'are you who you say you are?' Authorisation asks 'are you allowed to do this?' Most API breaches are authorisation failures, not authentication failures. Test with two users of different privilege levels and attempt to access each other's objects.
3. Test for BOLA on every object-referencing endpoint
If an API endpoint takes an object ID — user_id, order_id, account_id — and returns data, test whether changing that ID returns someone else's data. This is the single highest-yield test in API security.
Testing the APIs that connect your business
API security is not a subset of web security — it is a discipline of its own, and the testing methodology must match. We test APIs against the OWASP API Top 10 with both automated and manual techniques, scoped to your actual contract.
Kinematic Digital designs and operates security programmes for mid-market and enterprise organisations. See our cybersecurity services and preemptive security programme, or book a security review.
Frequently Asked Questions
What is API security testing?
The systematic evaluation of application programming interfaces for vulnerabilities — broken authentication, authorisation flaws, injection, excessive data exposure, and rate-limiting gaps — using a combination of automated scanning, manual testing, and business-logic analysis.
Why is API security different from web application security?
APIs expose business logic and data directly, often without a user interface. Traditional web scanners miss API-specific flaws like BOLA, mass assignment, and improper rate limiting. APIs need purpose-built testing that understands the API contract.
What is BOLA and why does it matter?
Broken Object Level Authorization — accessing another user's data by changing an object ID in a request. It is the most common and impactful API vulnerability because it bypasses authentication entirely; the user is legitimate, they are just accessing objects they should not see.
How often should APIs be tested?
Continuously in CI/CD for known vulnerabilities, and at least annually with manual penetration testing for business-logic flaws. Every API change should trigger automated security checks before deployment.
What is the OWASP API Security Top 10?
The Open Worldwide Application Security Project's ranked list of the most critical API security risks, updated regularly. It is the de facto framework for API security testing and remediation prioritisation.