Modern web applications increasingly use Single Page Application (SPA) architectures, where the browser communicates with backend services through APIs. These APIs handle everything from retrieving customer information to processing transactions and enforcing business rules.
Yet many organisations assume that API penetration testing is unnecessary if their APIs are used only by their own web application, rather than external customers or business partners.
This assumption can leave critical security gaps undetected.
1. Internal APIs Are Not Necessarily Secure
An API’s intended use does not determine whether it is accessible to attackers.
In an SPA, the browser typically sends API requests directly to the backend. Users can inspect these requests, modify parameters and attempt to invoke API endpoints outside the application’s normal workflow.
For example, a web application may display only the logged-in user’s account details. However, if the backend API fails to verify account ownership, an attacker might change an account identifier to access another user’s information.
Even when APIs are restricted to server-to-server communication, compromised web servers, stolen credentials or other internal systems may provide a route to them.
Security controls must be enforced by the backend, not merely by the web interface.
2. API Pentesting Provides More Explicit Coverage
A web application penetration test may include API testing, but the depth of coverage depends on the agreed scope and methodology.
A dedicated API penetration test provides a structured assessment of API endpoints, HTTP methods, parameters, authentication, authorisation and business logic.
At Perennial Consultancy, we request the customer’s API documentation and Postman collection to establish the testing scope.
Comparing these materials helps identify discrepancies, such as documented endpoints missing from the supplied collection. It also helps our testers understand the intended functionality and security requirements of each API.
This documentation-driven approach improves:
- Precision: Focus testing on relevant endpoints, parameters, roles and security controls.
- Recall: Systematically assess the defined API scope to reduce the likelihood of overlooking vulnerabilities.
- Coverage: Identify discrepancies between documented APIs and supplied requests, and clarify gaps before testing.
Documentation and Postman collections may not capture every deployed endpoint, so additional verification may be necessary.
3. Test Against the OWASP API Security Top 10
API penetration testing should go beyond checking whether endpoints respond correctly. It should assess whether APIs enforce security controls and resist abuse.
Using the OWASP API Security Top 10 (2023) as a testing framework helps structure the assessment around risks such as:
- Broken Object Level Authorization (BOLA): Accessing another user’s data by manipulating object identifiers.
- Broken Authentication: Weaknesses in authentication or token validation.
- Broken Object Property Level Authorization: Exposing or allowing unauthorised modification of sensitive fields.
- Broken Function Level Authorization: Accessing functions reserved for privileged users.
- Unrestricted Access to Sensitive Business Flows: Abusing business operations through excessive or automated requests.
- Unrestricted Resource Consumption: Exploiting insufficient limits on resource-intensive API requests.
Other categories address security misconfiguration, server-side request forgery (SSRF), improper API inventory management and unsafe consumption of third-party APIs.
The specific tests should be adapted to the application’s architecture, functionality and risk profile.
4. Is a Separate API Penetration Test Always Necessary?
Not necessarily. If an existing web application penetration test already provides comprehensive manual testing of all relevant APIs, a separate engagement may duplicate some effort.
However, dedicated API penetration testing is worth considering when:
- The application relies heavily on APIs, particularly in an SPA architecture.
- APIs handle sensitive data, financial transactions or critical business functions.
- API testing is not explicitly included in the existing web pentest scope.
- The application has complex authorisation rules, numerous endpoints or multiple user roles.
- The organisation requires a structured assessment against the OWASP API Security Top 10.
The decision should depend on actual testing coverage, architecture and business risk—not simply whether APIs are intended for internal use.
Conclusion
Internal-use APIs still need security testing because intended use does not guarantee restricted access or secure backend controls.
API penetration testing helps identify weaknesses in authentication, authorisation, data exposure and business logic that may not be apparent from the web application’s user interface.
Learn more about our API penetration testing service.
