APIs power almost every modern application, from the mobile banking app on your phone to the integrations that quietly shuttle data between cloud services. They are also responsible for a large and growing share of breaches. The OWASP API Security Top 10 captures the patterns that show up most often, and learning to recognise these patterns prevents most of the incidents that make the news.
Broken Object Level Authorisation
The single most common API flaw is the easiest to describe: an endpoint that lets you access data belonging to other users by changing an identifier. The endpoint checks that you are logged in. It does not check that you are allowed to view the specific resource you have requested. Modern frameworks make this trivial to introduce and surprisingly hard to detect through casual testing. web application penetration testing that includes proper API coverage hammers this category specifically because it produces such reliably high-impact findings.
Broken Authentication
API authentication often inherits the same patterns from the web application that hosts it, but with subtler flaws. Tokens that never expire, refresh logic that issues new tokens to attackers who already have one, and authentication endpoints that accept the same credentials a thousand times a second without rate limiting all appear regularly. The mobile clients consuming these APIs frequently store tokens insecurely on device, which makes recovery from a compromise considerably harder.
Excessive Data Exposure
An API that returns the entire user object when only the username was needed leaves sensitive fields exposed in transit and in client-side storage. Filtering at the front end does not help, because anyone with developer tools can see what the server actually sent. Returning only what each endpoint genuinely needs is a small change that pays back over time, and it tends to surface during code review only when someone explicitly looks for it.
Expert Commentary
Name: William Fieldhouse
Title: Director of Aardwolf Security Ltd
Comments: The API findings that cause the worst incidents are usually layered. A small data exposure issue on its own might rate medium severity. Combine it with weak authorisation on a related endpoint and the impact jumps to critical. Treating each finding in isolation tends to underestimate the real risk.
Lack of Resources and Rate Limiting
APIs without proper rate limiting invite abuse. Brute force attacks against authentication, scraping of personal data, denial-of-service through resource exhaustion, and credential stuffing all become significantly easier when no one is watching the call rates. Implement throttling at multiple layers, alert on unusual usage patterns, and treat unexpected traffic spikes as potentially hostile until proven otherwise.
Server-Side Request Forgery and Mass Assignment
Several of the newer entries in the OWASP API list cover patterns that have grown in importance as APIs become more interconnected. Server-side request forgery via API calls that fetch external URLs on the user’s behalf can pivot into cloud metadata services and internal networks. Mass assignment, where the API trusts client-supplied properties to update database fields, lets attackers grant themselves admin rights through a property they were never meant to control. Both classes of issue remain easy to introduce and quick to exploit.
Building API Security In
Treat your API surface with the same rigour you would apply to a web application, because it almost always carries higher value with thinner defences. Document every endpoint, validate every input, authorise every request properly, and monitor everything that moves through it. Engage a best penetration testing company who has experience with the specific frameworks you use, since the subtleties of each ecosystem matter when it comes to root cause analysis and remediation guidance.