APIs power nearly everything now. Your mobile app, your third-party integrations, your internal microservices architecture. They’ve become the connective tissue of modern software, and they’re also one of the most consistently vulnerable attack surfaces in any organisation.
What makes API security particularly tricky is that the same development speed that makes APIs so useful also encourages shortcuts. Move fast, ship features, worry about security later. Except later never comes, and the same mistakes keep showing up in assessment after assessment.
Broken Object-Level Authorisation
This is the big one. An API endpoint that returns user data based on an ID parameter without checking whether the requesting user has permission to access that specific record. Change the ID in the request, and you get someone else’s data.
William Fieldhouse, Director of Aardwolf Security Ltd, comments: “Broken object-level authorisation is the single most common API vulnerability we discover during testing. Developers assume that because the API sits behind a frontend application, users won’t access endpoints directly. That assumption gets blown apart within the first hour of every API assessment we run.”
The fix isn’t complicated. Every API endpoint that returns data tied to a specific resource needs to verify that the authenticated user has permission to access that resource. But when development teams manage hundreds of endpoints across multiple services, consistency becomes the challenge.
Excessive Data Exposure
APIs frequently return more data than the frontend actually needs. A mobile app might display a user’s name and profile picture, but the underlying API response includes their email address, phone number, internal account ID, and account creation date.
Developers build the API to return a complete object and rely on the frontend to display only what’s relevant. An attacker doesn’t use the frontend. They read the raw API response and harvest every piece of data the endpoint exposes.
Authentication and Rate Limiting Failures
API keys hardcoded in mobile applications, JWT tokens that never expire, and endpoints with no rate limiting at all. These aren’t edge cases. They’re standard findings during web application penetration testing.
Rate limiting deserves particular attention. Without it, an attacker can enumerate every user account, brute-force authentication tokens, or scrape your entire dataset through the API. Implementing sensible rate limits per API key and per IP address is straightforward and dramatically reduces your risk.
Getting Ahead of API Vulnerabilities
Document your APIs properly. Maintain an up-to-date inventory of every endpoint, what data it exposes, and what authentication it requires. Shadow APIs that nobody remembers building are a favourite target for attackers.
Build security testing into your development pipeline. Automated tools catch the low-hanging fruit, but manual testing by experienced professionals finds the logic flaws that scanners miss entirely. If you’re not sure where your API security stands, getting a penetration test quote is a practical first step. The cost of testing pales in comparison to the cost of a data breach through an API endpoint you forgot about.
