API Fuzzing

API fuzzing (also called fuzz testing) works by systematically generating and sending unusual inputs to API endpoints and monitoring the responses for anomalies such as server errors, unexpected status codes, stack traces, excessive response times, or data leakage. Unlike manual penetration testing, fuzzing can explore a vast number of input combinations automatically, often discovering edge cases that human testers would miss.

There are several approaches to API fuzzing. Schema-based fuzzing uses the OpenAPI specification or similar API documentation to generate inputs that test boundary conditions for each parameter. Mutation-based fuzzing takes valid requests and randomly modifies them by changing data types, exceeding length limits, injecting special characters, or omitting required fields. Generation-based fuzzing creates entirely new inputs based on grammar rules or statistical models of the input format.

API fuzzing is particularly effective at finding injection vulnerabilities (SQL, NoSQL, command injection), buffer overflows in native code behind APIs, authentication and authorization bypasses through parameter manipulation, denial-of-service conditions from unexpected input sizes or formats, and business logic flaws from invalid state transitions. Popular API fuzzing tools include RESTler from Microsoft Research, Schemathesis, OWASP ZAP with its fuzzer plugin, and Burp Suite's Intruder. For best results, fuzzing should be integrated into CI/CD pipelines with baseline profiles to detect regressions, and results should be triaged by security engineers to distinguish real vulnerabilities from false positives.