API Penetration Testing

Your APIs Are Your Biggest Attack Surface

APIs power your product – and they’re often the weakest link. Broken authentication, missing rate limits, insecure direct object references, and mass assignment vulnerabilities are common findings in our API assessments. We test REST, GraphQL, SOAP, and gRPC endpoints manually, going well beyond what automated tools catch.

Our consultants map every endpoint, test every authentication flow, and probe every access control boundary. We look for the vulnerabilities that matter – the ones that let attackers access other users’ data, bypass payment flows, or escalate privileges.

What We Test

We cover the full API attack surface:

• REST API Penetration Testing: We test authentication mechanisms (OAuth 2.0, API keys, JWT), authorisation logic across all endpoints, input validation, rate limiting, and data exposure. We use Burp Suite Professional to intercept and manipulate requests, testing every parameter for injection, IDOR, and mass assignment vulnerabilities.

• GraphQL Penetration Testing: We test for introspection disclosure, query depth and complexity abuse, batching attacks, authorisation bypass through nested queries, and field-level access control gaps. GraphQL APIs often expose more data than intended – we find and prove it.

• SOAP and Web Services Testing: We test WSDL exposure, XML External Entity (XXE) injection, SOAP action spoofing, and WS-Security implementation flaws.

• Microservices Penetration Testing: We examine inter-service communication, service mesh configurations, container escape vectors, and trust boundaries between microservices – where a compromise in one service can cascade to others.

Our Testing Methodology

Our API testing methodology is aligned with the OWASP API Security Top 10 and goes significantly beyond it:

• Endpoint Discovery & Documentation Analysis: We start by mapping the full API surface – reviewing OpenAPI/Swagger specifications, intercepting mobile and web client traffic, and fuzzing for undocumented endpoints. Shadow APIs and deprecated-but-live endpoints are common attack vectors we uncover.

• Authentication & Token Testing: We test OAuth 2.0 flows for redirect URI manipulation, token leakage, and scope escalation. For JWT-based authentication, we test for algorithm confusion attacks (e.g. RS256 to HS256), weak signing keys, missing expiry validation, and token reuse after logout.

• Authorisation & Object-Level Testing: We systematically test every endpoint with different user roles and privilege levels. We look for Broken Object-Level Authorisation (BOLA), Broken Function-Level Authorisation (BFLA), and mass assignment vulnerabilities where attackers can modify fields they shouldn’t have access to.

• Rate Limiting & Resource Exhaustion: We test for missing or bypassable rate limits, unrestricted resource consumption, and denial-of-service vectors through expensive queries or bulk operations.

• Data Exposure Analysis: We review every API response for excessive data exposure – PII leakage, internal identifiers, stack traces, and verbose error messages that aid further attacks.

What We Typically Find

Representative findings from recent API assessments (anonymised):

• Mass Assignment Leading to Privilege Escalation: A SaaS platform’s user update endpoint accepted a role parameter that wasn’t documented in the API specification. By adding "role": "admin" to a profile update request, any authenticated user could elevate their own privileges to administrator.

• GraphQL Introspection Exposing Internal Schema: A fintech API had GraphQL introspection enabled in production, revealing internal mutation types including transferFunds and overrideKYC that were accessible to standard user accounts – bypassing the intended UI-level restrictions entirely.

• JWT Algorithm Confusion to Authentication Bypass: An e-commerce API used RS256-signed JWTs but accepted HS256 tokens when the algorithm header was changed. Using the public key as an HMAC secret, we forged valid admin tokens without any credentials.

Why Choose FORTBRIDGE for API Pentesting?

• Senior-Only Engagements: Every assessment is led by consultants with 10-20 years of offensive experience. No juniors, no padding – you get senior expertise from day one.

• Trusted Across Industries: Trusted by organisations across banking, fintech, e-commerce, and SaaS – our clients include security-conscious teams who come back to us engagement after engagement.

• Developer-Focused Reporting: Every finding includes a clear proof-of-concept (reproducible cURL commands or Burp request/response pairs), impact analysis, and specific remediation guidance your developers can act on immediately.

• Certified Professionals: Our team holds OSCP, OSWE, and CREST certifications, ensuring the highest standards of security testing.

Partner with FORTBRIDGE for API Security

With FORTBRIDGE, your APIs are in expert hands. Our API pentesting services ensure that your digital infrastructure remains secure and resilient against cyber threats.

Book a Discovery Call to discuss your requirements and get a scoping estimate.