APIs power almost every modern application. Mobile apps, single-page web interfaces, third-party integrations, and microservice architectures all communicate through APIs. When security teams focus exclusively on front-end web applications, they overlook the API layer that handles the heavy lifting behind the scenes.
Attackers know this. They skip the polished user interface entirely and interact directly with API endpoints. Broken authentication, excessive data exposure, and missing rate limiting become trivially exploitable when someone sends crafted HTTP requests to an API that was never designed with hostile input in mind.
Common API Weaknesses
Broken Object Level Authorisation (BOLA) remains the most prevalent API vulnerability. It occurs when an API fails to verify that the requesting user has permission to access a specific resource. Changing an ID parameter in a request from one account number to another should not return someone else’s data, yet it frequently does.
Mass assignment vulnerabilities let attackers modify fields they should not have access to by including extra parameters in requests. An API that accepts a JSON body might allow a user to set their own role to administrator simply by adding a role field to their profile update request.
William Fieldhouse, Director of Aardwolf Security Ltd, comments: “Developers often build APIs under time pressure and focus on making them functional before making them secure. We routinely find APIs that expose internal data structures, accept unexpected input fields, or return far more information than the client application needs. Thorough testing catches these issues before someone exploits them in production.”
Testing APIs Properly
Automated scanners catch some API issues, but they struggle with business logic flaws. A scanner cannot determine whether a specific user should have access to a particular record. Manual web application penetration testing that includes API coverage examines authentication flows, authorisation checks, input validation, and business logic in ways that tools cannot replicate.
Provide your testers with API documentation, Swagger files, or Postman collections. Testing without documentation wastes time on endpoint discovery that could be spent finding genuine vulnerabilities.
Securing Your APIs
Implement proper authentication using OAuth 2.0 or API keys with scoped permissions. Apply rate limiting to prevent abuse and brute-force attacks. Validate every input field against an allow-list of expected values. Return only the data each client needs, nothing more.
Excessive data exposure is equally common and frequently overlooked. APIs that return complete database records when the client only needs a name and email address leak internal identifiers, timestamps, and related records that help attackers map the application’s entire data model and plan further exploitation.
Provide your testers with up-to-date API documentation, Swagger files, or Postman collections wherever possible. Testing without documentation wastes valuable time on endpoint discovery that could otherwise be spent finding genuine vulnerabilities posing real business risk to your organisation.
If your organisation depends on APIs and has not tested them recently, now is the time. Request a penetration test quote that specifically covers API endpoints alongside your web applications. The API layer is too critical to leave untested.
