API Penetration Testing — Protect the Backbone of Your Application
APIs power your mobile apps, integrations, microservices, and partner ecosystems. A single broken authorization check can expose your entire database. CyberGuards' API security specialists in San Francisco manually test every endpoint, parameter, and authentication flow to find what scanners cannot.
APIs Are the Most Targeted Attack Surface
API-First Means API-First Risk
Modern applications are built on APIs. Your mobile app, single-page frontend, third-party integrations, and internal microservices all communicate through API endpoints. Each endpoint is a potential entry point that handles sensitive data, executes business logic, and enforces access controls. When an API fails, the blast radius extends across every client that depends on it.
Traditional Security Tools Miss API Flaws
Web application firewalls and generic vulnerability scanners were built for HTML-based applications, not API-first architectures. They struggle with JSON payloads, GraphQL queries, custom authentication headers, and the stateless nature of REST APIs. The result is a false sense of security while critical authorization and business logic flaws go undetected.
Complete OWASP API Security Top 10 Coverage
Every API penetration test includes systematic testing against all ten categories of the OWASP API Security Top 10 2023 — the definitive standard for API security risks.
Broken Object Level Authorization
We test every API endpoint for IDOR vulnerabilities where manipulating object IDs in requests allows access to other users' data. This is the most common and impactful API vulnerability, enabling attackers to read, modify, or delete resources belonging to other users by simply changing an identifier.
Broken Authentication
Testing authentication mechanisms for weaknesses: token generation flaws, JWT implementation errors (algorithm confusion, missing signature validation), password reset vulnerabilities, credential stuffing susceptibility, and missing authentication on sensitive endpoints.
Broken Object Property Level Authorization
We verify that APIs properly restrict which object properties users can read or modify. Testing for mass assignment where extra fields in requests update unauthorized properties, and excessive data exposure where API responses return more data than the client needs.
Unrestricted Resource Consumption
Testing for missing or insufficient rate limiting, pagination controls, query complexity limits, file upload size restrictions, and batch operation limits that could allow denial of service or resource exhaustion attacks against your API infrastructure.
Broken Function Level Authorization
Can a regular user access admin API endpoints? We test for vertical privilege escalation by attempting to invoke administrative functions, management endpoints, and privileged operations from lower-privileged API tokens and user contexts.
Unrestricted Access to Sensitive Business Flows
Testing for abuse of legitimate business functionality: automated purchasing, content scraping, mass account creation, referral fraud, and other scenarios where API access enables business logic abuse at scale without proper anti-automation controls.
Server Side Request Forgery
We test for SSRF vulnerabilities where API endpoints that accept URLs or make server-side requests can be manipulated to access internal services, cloud metadata endpoints (169.254.169.254), and resources behind your network perimeter.
Security Misconfiguration
Assessment of API infrastructure configuration: CORS policies, security headers, TLS settings, error handling verbosity, default credentials, unnecessary HTTP methods enabled, and cloud service misconfigurations affecting your API deployment.
Improper Inventory Management
Discovery of undocumented API endpoints, deprecated but still active versions, shadow APIs, debug endpoints left in production, and internal APIs exposed to external networks. Proper API inventory is the foundation of API security.
Unsafe Consumption of APIs
Testing how your APIs consume third-party APIs and external data sources. We assess whether your application properly validates, sanitizes, and restricts data received from external APIs, preventing injection and trust boundary violations through third-party integrations.
We Test Every API Architecture
Different API architectures present different security challenges. Our testing methodology adapts to the specific patterns and vulnerabilities of each type.
REST APIs
The most common API architecture. We test HTTP methods, resource endpoints, query parameters, request bodies, authentication headers, pagination, filtering, and the complete CRUD lifecycle for authorization and injection vulnerabilities.
GraphQL APIs
Introspection analysis, query depth and complexity attacks, batching abuse, field-level authorization, mutation testing, subscription security, alias-based bypasses, and injection through variables. GraphQL's flexibility creates unique attack surfaces.
gRPC Services
Protocol buffer analysis, service method enumeration, authentication and authorization testing for unary and streaming RPCs, metadata manipulation, and reflection API exposure. gRPC's binary protocol requires specialized tooling and expertise.
SOAP / XML Services
WSDL analysis, XML injection, XXE (XML External Entity) attacks, WS-Security implementation testing, SOAP action spoofing, and schema validation bypass. Legacy SOAP services often have weaker security controls.
WebSocket APIs
Connection handshake security, message authentication, authorization per message type, injection through WebSocket frames, cross-site WebSocket hijacking, and origin validation testing for real-time communication channels.
Webhook Endpoints
Signature validation bypass, replay attacks, SSRF through callback URLs, payload injection, and authentication of incoming webhook requests from third-party services like payment processors and SaaS integrations.
Our API Testing Approach
Discovery & Reconnaissance
We begin by mapping your complete API surface: documented endpoints, undocumented routes, deprecated versions, and shadow APIs. We analyze OpenAPI/Swagger specs, Postman collections, client-side code, and network traffic to build a comprehensive endpoint inventory.
Authentication Analysis
We test your authentication implementation end-to-end: token generation, validation, refresh flows, session management, OAuth scopes, JWT claims, API key rotation, and multi-factor enforcement. Every authentication weakness is a potential full compromise.
Authorization Testing
Systematic testing of every endpoint with multiple user roles and permission levels. We verify object-level, property-level, and function-level authorization by cross-testing tokens, manipulating identifiers, and attempting privilege escalation across your entire API.
Input Validation & Injection
Every parameter, header, and payload field is tested for injection vulnerabilities: SQL, NoSQL, command, LDAP, GraphQL, and template injection. We also test input validation logic, type coercion, encoding bypass, and boundary conditions.
What You Receive
API Security Report
Comprehensive report with executive summary, technical findings rated by CVSS v4.0, proof-of-concept requests and responses, affected endpoints, and remediation guidance specific to your API framework and architecture.
OWASP API Top 10 Mapping
Every finding mapped to the relevant OWASP API Security Top 10 2023 category and CWE identifier. Provides standardized classification for your vulnerability management and compliance reporting workflows.
Free Retest
Complimentary retest within 90 days to validate your API security fixes. We retest each finding and provide an updated report confirming successful remediation.
Organizations That Need API Security Testing
API-First Companies
San Francisco and Bay Area technology companies building API-first products need to ensure their core business logic and customer data are protected at the API layer, where traditional web security tools provide limited coverage.
Platform & Marketplace Businesses
Multi-tenant platforms, marketplaces, and SaaS providers with public or partner APIs must prevent cross-tenant data access, privilege escalation, and API abuse that could compromise their entire user base.
Mobile-Backend Teams
APIs powering mobile applications are frequently targeted because mobile clients can be reverse-engineered. Testing the backend API independently validates security beyond what client-side controls provide.
Microservices Architectures
Organizations running microservices need to validate service-to-service authentication, inter-service authorization, and ensure that internal APIs are not inadvertently exposed to external networks.
API Penetration Testing FAQ
What is API penetration testing?
API penetration testing is a targeted security assessment of your application programming interfaces. Our engineers manually test your API endpoints for authentication bypass, authorization flaws, data exposure, injection vulnerabilities, and business logic issues. Unlike automated API scanning, manual testing uncovers complex vulnerabilities in how your APIs handle authentication, process data, and enforce access controls.
Which API types do you test?
We test all major API architectures: REST APIs, GraphQL APIs (including introspection, query depth attacks, and batching abuse), gRPC services, SOAP/XML web services, and WebSocket APIs. Each architecture has unique security considerations, and our testing methodology adapts to the specific patterns and vulnerabilities of each type.
What is the OWASP API Security Top 10?
The OWASP API Security Top 10 (2023 edition) is the industry-standard list of the most critical API security risks. It covers broken object-level authorization, broken authentication, broken object property-level authorization, unrestricted resource consumption, broken function-level authorization, unrestricted access to sensitive business flows, server-side request forgery, security misconfiguration, improper inventory management, and unsafe consumption of APIs.
How do you test GraphQL APIs specifically?
GraphQL testing includes: introspection query analysis, query depth and complexity attacks, batching abuse for brute force, field-level authorization testing, mutation access control verification, subscription security, alias-based rate limit bypass, and injection via GraphQL variables. We also test for information disclosure through error messages and schema exposure.
Do you need API documentation to perform testing?
API documentation (OpenAPI/Swagger specs, Postman collections, or GraphQL schemas) accelerates testing and ensures comprehensive coverage. However, we can also perform black-box API testing without documentation by reverse-engineering API behavior through traffic analysis, endpoint discovery, and parameter fuzzing. We recommend providing documentation when available for the most thorough assessment.
How long does an API penetration test take?
Duration depends on the number of endpoints, API complexity, and authentication models. A focused API with 20 to 50 endpoints typically takes 1 to 2 weeks. Large APIs with complex business logic, multiple authentication methods, and extensive CRUD operations may require 2 to 4 weeks. We provide a timeline after reviewing your API scope during the scoping call.
Will API testing affect our production environment?
We recommend testing against a staging or pre-production environment when possible. For production API testing, we use non-destructive techniques, respect rate limits, avoid data modification unless authorized, and coordinate with your team on any potentially impactful tests. We have tested hundreds of production APIs without incident.
How do you handle API authentication during testing?
We test both authenticated and unauthenticated API access. For authenticated testing, we work with multiple authentication methods: API keys, OAuth 2.0 tokens, JWT tokens, session cookies, mTLS certificates, and custom authentication schemes. We test the authentication mechanism itself for weaknesses, then test authorized endpoints for authorization flaws, privilege escalation, and data leakage.
Ready to Secure Your APIs?
Our San Francisco API security specialists will test your REST, GraphQL, and gRPC endpoints against the OWASP API Security Top 10. Get a free scoping call.
Book a Discovery Call