API Security Penetration Testing
API penetration testing methodology — REST, GraphQL, gRPC, and WebSocket vulnerabilities including BOLA, mass assignment, injection, and authentication flaws.
2026-08-19
APIs are now the primary attack surface for most organizations. Modern applications rely on dozens of internal and external APIs, each representing a potential entry point for attackers. An API-specific penetration test goes beyond what a standard web application pentest covers.
Why API pentesting needs a separate focus
Traditional web application pentests focus on the browser-facing interface — forms, URL parameters, and cookies. APIs introduce a fundamentally different attack surface:
- Programmatic access patterns — attackers can send crafted requests at machine speed
- Authentication is not authorization — a valid API token does not mean the caller should have access to every resource
- Massive data surface — APIs often return far more data than the UI displays
- Chained endpoint exploitation — a weak endpoint can enable attacks on stronger ones
API vulnerabilities to test for
| Vulnerability | Description | Where it lives |
|---|---|---|
| Broken Object Level Authorization (BOLA) | Accessing another user's data by modifying object IDs | Every GET/PUT/DELETE endpoint |
| Broken User Authentication | Weak JWT, missing token validation, session fixation | Auth endpoints |
| Mass Assignment | API accepts unintended fields in request bodies | POST/PATCH endpoints |
| Excessive Data Exposure | API returns more fields than the client needs | Response bodies |
| Rate limiting bypass | Brute-forcing without throttling | Login and enumeration endpoints |
| Injection (SQL, NoSQL, LDAP) | Untrusted input reaches a parser | Search and filter endpoints |
Testing across API paradigms
REST APIs — test standard CRUD operations, HTTP method abuse, content-type switching, and parameter pollution.
GraphQL APIs — test introspection query abuse, batching attacks, deep query nesting for DoS, and field-level authorization gaps.
gRPC APIs — test protobuf message tampering, service reflection, and streaming endpoint abuse.
WebSocket APIs — test message injection, origin validation, and session binding.
Compliance requirements
PCI DSS and SOC 2 both require API security testing when APIs are in scope. Many auditors now specifically ask whether API endpoints were included in the pentest scope separately from the web application scope.
Start testing your APIs
Affordable Pentesting's web application and Pentest+ plans include full API testing coverage. Launch a pentest to validate your API security posture, or see pricing for plans that include up to 100 API endpoints.
Related reading: SaaS Penetration Testing Guide