API penetration testing focuses on the security of application programming interfaces, which form the backbone of modern web and mobile applications. Testers systematically evaluate REST APIs, GraphQL endpoints, SOAP services, and WebSocket connections for vulnerabilities including broken authentication, excessive data exposure, injection attacks, improper rate limiting, mass assignment, and broken object-level authorisation (BOLA).
Testing follows the OWASP API Security Top 10 and examines how APIs handle authentication tokens, enforce authorisation between users, validate input data, and protect against abuse.
API pen testing is increasingly important as organisations adopt microservices architectures, expose APIs to partners and third parties, and build API-first products. Insecure APIs are a leading cause of data breaches, making thorough security testing essential. API pen testing helps organisations that must comply with PCI DSS, HIPAA, SOC 2, and GDPR by identifying vulnerabilities that could lead to unauthorised data access or manipulation through API endpoints.
UK offensive security consultancy delivering CREST-accredited penetration testing, adversary simulation and CBEST, TIBER-EU and TBEST threat-led testing, mainly for financial services and other large organisations.
Penetration testing firm trusted by nine of the top ten US banks, with the Resolve platform for continuous attack surface management.
Global enterprise cybersecurity consultancy founded in 1999 in Oxford, holding rare CREST STAR-FS accreditation and delivering penetration testing, red and purple teaming, and PCI QSA services across five international offices.
US penetration testing firm founded in 2011, running 600+ engagements a year with a fully remote, US-based team. Manual, expert-led testing across web, network, cloud, mobile, wireless, OT/ICS, and AI/LLM. Gartner Sample Vendor for PTaaS.
Global Big Four professional services firm delivering CREST, CHECK, and CBEST-accredited penetration testing and red teaming services from London, serving the UK's largest enterprises and regulated organisations.
CREST and CHECK-accredited UK penetration testing and managed security provider formed in 2026 from Pentest People, Bulletproof, and Target Defense, with CREST STAR/TLPT-FS capability and the GuardNest vulnerability management platform.
Full-service London-based cybersecurity consultancy with CREST, CHECK, and NCSC accreditations delivering offensive testing, managed detection, and strategic advisory services.
Global managed security provider with the elite SpiderLabs penetration testing team and deep PCI DSS compliance expertise.
CREST and CHECK-accredited UK penetration testing consultancy with CBEST approval, specialising in infrastructure, application, and simulated attack assessments across the public and private sectors.
CHECK and CREST-accredited Birmingham-based cyber security consultancy with over 15 years of experience delivering penetration testing, red teaming, and OT security assessments for government and private sector clients.
Fast-growing CREST and CHECK-accredited UK cybersecurity consultancy with deep expertise in critical national infrastructure sectors.
World-renowned cybersecurity firm now part of Google Cloud, delivering threat intelligence-led penetration testing and red teaming informed by front-line incident response experience.
REST, GraphQL, SOAP, gRPC, WebSocket, and any custom API protocols can be tested. Testers need API documentation or access to the application that consumes the API.
API documentation (Swagger/OpenAPI specs, Postman collections) helps testers work more efficiently but is not strictly required. Testers can discover and map API endpoints through application analysis.
API pen testing focuses specifically on the API layer - authentication, authorisation, data validation, and business logic at the API level. Web app testing covers the full application including the user interface.