Services Penetration Testing API Penetration Testing
+44 20 3996 1505

API Penetration Testing Services

APIs now carry the majority of business logic and sensitive data flow in modern applications, yet they often receive less security scrutiny than the UI surfaces they back. Broken object-level authorisation, mass assignment, excessive data exposure, and missing rate limiting consistently top the OWASP API Security Top 10 because they remain genuinely prevalent.

RedSecLabs delivers specialised API penetration testing across REST, GraphQL, SOAP, and gRPC interfaces. Our testers approach APIs with the depth they deserve, manual authorisation matrix testing across roles, business logic exploration, rate limiting and abuse testing, mass assignment hunting.

Every engagement combines automated tooling for surface coverage with senior manual testing on the issues automation systematically misses.

CREST Certified Pen Test Provider ISO Certified OSCP Certified Industry Certification

Free Security Quote

Just a few questions to scope your project. We respond the same business day.

✓ UK-based CREST member · ✓ QSA-aligned methodology · ✓ Same-day scoping response · ✓ Executive + technical reports · ✓ Retest included
Who this is for

This service is a fit if you’re..

1
API-first products
Companies whose API IS the product (developer tools, B2B SaaS, fintech infrastructure).
2
Microservice architectures
Engineering teams with REST/GraphQL/gRPC services that need authorisation and rate-limit validation.
3
Payment / sensitive data APIs
APIs that handle cardholder data, PHI, PII, or financial transactions where API breaks have regulatory weight.
OWASP API
Top 10 coverage
REST/GraphQL
Multi-protocol testing
CREST
Certified testers
5-10 days
Typical engagement

What is API penetration testing?

API penetration testing is the targeted, predominantly manual assessment of API endpoints for exploitable security weaknesses. Modern APIs use a range of protocols (REST, GraphQL, SOAP, gRPC) and authentication schemes (OAuth 2.0, JWT, API keys, mTLS), each with characteristic weakness patterns testers must understand to assess effectively.

The OWASP API Security Top 10 codifies the most common categories: broken object-level authorisation (BOLA, the #1 API risk), broken authentication, broken object property-level authorisation (mass assignment), unrestricted resource consumption (rate limiting / DoS), broken function-level authorisation, server-side request forgery, security misconfiguration, lack of protection from automated threats, improper inventory management, and unsafe consumption of APIs.

What API testing delivers:

OWASP API Security Top 10 coverage across all endpoints

Authorisation matrix testing across roles and objects

Mass assignment and excessive data exposure identification

Rate limiting and abuse vector testing

Authentication and token handling validation

Business logic testing for API workflows

API testing has become as important as traditional web application testing, and arguably more important for organisations whose APIs back mobile apps, B2B partners, or microservice architectures.

Why API security testing matters

APIs are now the dominant attack surface for many organisations. Mobile apps, single-page web apps, B2B integrations, and microservice architectures all expose far more API surface than traditional web pages, and developers consistently apply weaker authorisation logic to API endpoints than to UI-fronted equivalents, on the assumption that "no one will call the API directly". They will.

Major breaches in 2024 and 2025 have repeatedly traced back to API-level vulnerabilities: BOLA in mobile app backends exposing customer records, mass assignment in admin APIs enabling privilege escalation, weak rate limiting enabling credential stuffing at scale.

Common consequences of weak API security:

Customer record exposure via BOLA in mobile/SPA backends

Privilege escalation via mass assignment in admin APIs

Credential stuffing at scale due to weak rate limiting

Sensitive data exposure via verbose API responses

Account takeover via authentication weaknesses

Compliance failures across PCI DSS and SOC 2

APIs deserve at least the same testing depth as the UI surfaces they back, often more, given they typically have weaker authorisation enforcement.

Who needs API testing?

Any organisation operating APIs, internal microservices, mobile app backends, public APIs, B2B integrations, needs regular testing:

SaaS with mobile and SPA frontends

Mobile-first companies

Open banking and PSD2 APIs

Payment platforms

AI/ML platforms with API surfaces

Microservice architectures

B2B platforms with partner APIs

HealthTech with FHIR APIs

If your API powers a mobile app, a B2B integration, or a microservice architecture handling sensitive data, dedicated API testing is essential, generic web application testing rarely covers API security depth.

Our API Testing Methodology

CREST-aligned methodology combining OWASP API Security Top 10 with hands-on protocol-specific testing across REST, GraphQL, SOAP, and gRPC.

01

Scoping & API Documentation Review

We agree the in-scope API endpoints (typically working from OpenAPI/Swagger, GraphQL schema, or postman collections), user roles, authentication schemes, and test credentials.

02

Endpoint Enumeration & Mapping

Complete mapping of every endpoint, method, parameter, and response shape, building the full attack surface before testing begins.

03

Authentication & Token Testing

Detailed testing of authentication mechanisms. JWT validation, OAuth flow integrity, token refresh logic, session fixation, brute force resistance.

04

Authorisation Matrix Testing

Systematic testing of object-level and function-level authorisation across every user role pair, the highest-value work in any API engagement.

05

Mass Assignment & Data Exposure

Testing for mass assignment (unintended property writes), excessive data exposure (verbose responses leaking sensitive data), and improper input filtering.

06

Rate Limiting & Abuse Testing

Testing of rate limiting, account lockout, and abuse vectors that enable credential stuffing or data scraping at scale.

07

Business Logic & Workflow Testing

Manual exploration of business workflows for parameter tampering, race conditions, and logic flaws that automated tools cannot find.

08

Reporting & Developer Walk-Through

Detailed findings with reproduction steps via curl/Postman, code-level remediation guidance, and live walk-through with your engineering team.

Typical engagement: 5-10 days for mid-complexity APIs (under 50 endpoints), 10-15 days for larger APIs (50-150 endpoints), longer for major platforms.

What you receive

Every API testing engagement with RedSecLabs includes:

  • Scoping document with endpoint inventory
  • Executive summary for board and management consumption
  • Detailed technical findings with curl/Postman reproduction
  • CVSS plus exploitability prioritisation
  • Code-level remediation guidance with examples
  • Authorisation matrix test coverage report
  • OWASP API Security Top 10 compliance mapping
  • Remediation retest of critical and high findings

Industries We Serve

We deliver this service across these industries:

Financial Services
Healthcare
SaaS & Technology
E-commerce & Retail
Defence & Government
Cloud & Managed Services
Education
Professional Services

Why RedSecLabs for API testing

API testing demands different skills from web application testing, proficiency across protocols (REST, GraphQL, SOAP, gRPC), comfort with API tooling (Postman, Burp, custom scripts), and the patience to test every endpoint × every user role authorisation pair. Our testers approach APIs with the seniority they need, finding the BOLA and mass assignment issues automation systematically misses.

CREST-certified API security specialists
Manual authorisation matrix testing
OWASP API Top 10 systematic coverage
REST, GraphQL, SOAP, gRPC coverage
Code-level developer remediation guidance
Remediation retest included

Schedule Your API Pentest

Book a free 30-minute scoping call. Fixed-fee proposal within 48 hours, engagement starts within 1-2 weeks.

Book a Scoping Call Contact Us

Where to next

Related services and industry packages worth exploring.

Web App Pentesting

Mobile App Pentesting

Secure Code Review

Frequently Asked Questions

Yes. GraphQL testing is a specialist area we cover regularly. GraphQL has distinctive risk patterns (introspection abuse, query complexity DoS, schema field-level authorisation, batch query abuse) that require targeted testing methodology. We use specialist GraphQL tooling alongside manual investigation.

Broken Object-Level Authorisation, accessing or modifying objects you shouldn't have access to by manipulating object identifiers in API calls. It's the #1 OWASP API Security risk because it's both prevalent (most APIs have at least some BOLA exposure) and damaging (typically exposes other users' data). Manual authorisation matrix testing is the only reliable way to find BOLA at scale.

Yes, mobile app API backends are core scope for API testing. We intercept the mobile app traffic to enumerate the API surface, then test directly against the backend with appropriate authentication. This often finds more issues than testing the mobile app itself, since attackers will always interact with the backend directly.

OpenAPI/Swagger specs are ideal. GraphQL schemas work directly. Postman collections are acceptable. For undocumented APIs, we work from traffic capture (intercepting mobile or SPA traffic), this takes longer but is feasible. We discuss documentation status during scoping.

They overlap but are distinct. Web app pentests cover the UI surface; API pentests cover the backend API. For SPAs and mobile apps, API testing finds most exploitable issues because that's where business logic and authorisation live. For monolithic server-rendered apps, web app testing covers most ground. Many clients scope both.

Mid-complexity APIs (under 50 endpoints) £6,000-£15,000; larger APIs £15,000-£30,000; major platforms £30,000-£70,000+. Pricing reflects tester days. CREST premium 10-20%. Fixed-fee quote within 48 hours of scoping.
What you receive

Every engagement includes

  • ✓ Scoping call. A 30-minute call to define scope, timeline, and authorisation boundaries.
  • ✓ Test plan. Written test plan covering targets, methodology, and rules of engagement.
  • ✓ Technical report. Detailed findings with reproduction steps, evidence, and remediation guidance.
  • ✓ Executive summary. Board-ready 1-2 page summary with risk ratings and business impact.
  • ✓ Audit-ready evidence. Findings letter formatted for auditors, customers, and supervisory authorities.
  • ✓ Retest letter. Free retest of remediated findings within an agreed window. Confirmation letter included.
  • ✓ Remediation call. A call with our lead tester to walk through findings and remediation strategy.
How we deliver

Our process, end to end

  1. 1
    Scoping call & fixed-scope quote
    A 30-minute call. We define scope, targets, timeline. You get a fixed-scope quote within one working day. No surprise invoices.
  2. 2
    Test plan & authorisation
    Written test plan covering methodology, targets, and rules of engagement. Authorisation letter signed before any testing begins.
  3. 3
    CREST-aligned execution
    Senior tester runs the engagement. Critical findings flagged immediately during testing. Daily updates if you want them.
  4. 4
    Technical + executive report
    Detailed technical findings with reproduction steps. Board-ready executive summary. Delivered within agreed working days.
  5. 5
    Remediation call & retest
    Walkthrough with our lead tester. Retest of remediated findings within the agreed window. Confirmation letter for your auditors.
Engagement scope

What shapes the quote

Small scope
Single app, focused scope, smaller surface. 5-7 working days.
Medium scope
Multi-role platform, several user types, integrations. 8-12 working days.
Enterprise scope
Complex environment, multiple targets, compliance evidence. 12-25 working days.
Fixed-scope quote within 1 working day
No surprise invoices, no scope-creep. We commit to a number before you commit to us.
Sample report
See exactly what we deliver
Download a redacted RedSecLabs penetration test report. Same format, same depth, same clarity as the report your team will receive.
Download sample report
Why RedSecLabs

Grounded reasons clients choose us

⚑
UK-based team
Testers based in the UK. Data stays within UK/EU jurisdiction for sensitive engagements.
❄
CREST member company
CREST-aligned methodology. Senior testers hold CREST CRT or CCT certifications.
⚙
Manual testing, not scanner-only
Automated scanners catch the obvious. Our human testers find the issues that matter.
✎
Clear executive reporting
Reports your board can read and your developers can act on. No jargon padding.
♚
Compliance-aware delivery
PCI, SOC 2, ISO 27001, DORA, GDPR. We map findings to your compliance framework.
↺
Retest support included
Free retest of remediated findings within agreed window. Confirmation letter for auditors.
Related services

Often paired with this engagement

Web App Pentesting →
Test the web frontend.
AWS Pentesting →
Cloud infrastructure assessment.
PCI DSS Compliance →
Required for payment APIs.
SOC 2 Compliance →
For B2B SaaS APIs.
Amazon SP-API Audit →
For Amazon marketplace integrations.
📞 Call us Book a call