APIs are the connective tissue of modern software. They power mobile apps, link microservices, open business logic to partners and customers, and let third parties plug into your platform. All that connectivity carries real security risk. As organisations go API-first, the attack surface grows, often faster than security teams can keep up.

The Postman 2024 State of the API report found that 74% of organisations now call themselves API-first, and 62% earn revenue directly from their APIs. AI-driven API traffic has jumped 73%, adding yet more complexity. For security managers and technical leads, knowing how to test and protect these interfaces is no longer a nice-to-have. It is a core part of any credible security programme.

Why APIs Are Increasingly Targeted

Microservices, mobile backends, and cloud-native development mean most organisations now expose dozens or hundreds of API endpoints. Each one is a potential way in for an attacker.

A few things make APIs especially attractive targets. They are designed to be consumed programmatically, so they are easy to discover and probe at scale. Unlike a web application where the UI acts as a filter, APIs often sit closer to the data layer and may return sensitive information directly. Connecting with external services introduces trust boundaries that are frequently misconfigured. And many organisations apply strong security to their web front ends but treat APIs as an afterthought, particularly internal or partner-facing ones.

The UK Government Digital Service (GDS) publishes API technical and data standards for public sector bodies, reflecting growing recognition that APIs need deliberate governance and security attention.

Common API Vulnerabilities

The OWASP API Security Top 10 (2023 edition) is the most widely referenced taxonomy of API-specific risks. The highlights:

Broken Object Level Authorisation (BOLA/IDOR) is the single most prevalent API vulnerability. Attackers manipulate object identifiers (user IDs, order numbers) to access other users' data. It is often trivial to exploit and hard to catch with automated tools alone.

Broken Authentication covers weaknesses in how APIs verify identity: poor token validation, missing expiry checks, insecure credential storage.

Broken Object Property Level Authorisation encompasses excessive data exposure (APIs returning more fields than the client needs) and mass assignment (clients updating sensitive fields they should not have access to).

Unrestricted Resource Consumption is the absence of rate limiting, opening the door to brute-force attacks, credential stuffing, or denial-of-service through resource exhaustion.

Security Misconfiguration includes overly permissive CORS policies, verbose error messages, unnecessary HTTP methods left enabled, and default credentials still in place.

Injection remains relevant wherever input validation is inadequate. SQL injection, NoSQL injection, and command injection all appear in APIs that accept complex query parameters or request bodies.

These are not theoretical. They show up regularly in breach reports and bug bounty disclosures.

How API Security Testing Differs from Web App Testing

API penetration testing shares principles with web application testing, but the day-to-day work looks different.

Stateless authentication. APIs typically use token-based mechanisms like OAuth 2.0 or JSON Web Tokens (JWTs) rather than server-side sessions and cookies. Testers need to understand token lifecycle, scope, and revocation to assess authentication properly.

No user interface. Without a browser-based front end, testers interact directly with endpoints using tools like Burp Suite, Postman, or custom scripts. Removing the UI layer often reveals logic that the front end was quietly masking.

Business logic focus. Many critical API vulnerabilities, particularly authorisation flaws, are rooted in business logic rather than technical weaknesses. Automated scanners struggle here because they do not understand what each endpoint should allow.

Data serialisation formats. APIs may use JSON, XML, Protocol Buffers, or other formats, each with its own parsing behaviour and attack vectors. XML External Entity (XXE) attacks against SOAP services are a classic example.

API Testing Methodologies

Effective API security testing combines automated and manual approaches. Neither is sufficient alone.

Automated Scanning

DAST tools and API-specific scanners catch common issues: missing authentication, schema mismatches, known vulnerability patterns. They are good for broad coverage and fit neatly into CI/CD pipelines for ongoing checks. But they produce false positives and miss logic-based flaws.

Manual Penetration Testing

A skilled tester is still essential for identifying authorisation bypasses, business logic abuse, and chained vulnerabilities that need contextual understanding. A thorough manual assessment covers authentication testing (token handling, session management, credential policies), authorisation testing (horizontal and vertical privilege escalation, BOLA checks across roles), input validation (injection, boundary analysis, unexpected data types), and business logic testing (workflow abuse, state manipulation, rate limiting enforcement).

The best engagements use automated tools for reconnaissance and coverage, then concentrate manual effort where human judgement matters most.

Types of API and How Testing Differs

Not all APIs are built the same, and testing has to adapt.

REST APIs are the most common. Testing usually starts from an OpenAPI (Swagger) specification that defines endpoints, parameters, and expected responses. This spec-driven approach lets testers systematically cover the full surface.

GraphQL introduces unique problems. Its flexible query language means clients can request arbitrary combinations of data, so testers must check schema introspection exposure, query depth and complexity limits, and whether authorisation is enforced at the field level, not just the query level.

SOAP APIs use XML and WSDL. Testing centres on XML parsing vulnerabilities (including XXE), WSDL exposure, and WS-Security implementation.

gRPC uses binary serialisation via Protocol Buffers, which requires specialised tooling to generate and manipulate test messages. Standard HTTP-based tools may not work without adaptation.

When scoping an engagement, identify which API types are in play so the testing team can bring the right tools and expertise.

What a Good API Security Assessment Covers

A credible assessment should address the following:

Scope definition. All endpoints in scope need to be documented, including internal, partner-facing, and public APIs. Third-party integrations that cross trust boundaries deserve extra attention.

Authentication. Testing should examine token generation, validation, expiry, revocation, and the handling of different authentication flows.

Authorisation. Systematic testing of object-level and function-level access controls across all relevant user roles.

Rate limiting and abuse prevention. Verification that endpoints enforce sensible limits against brute-force attacks and resource exhaustion.

Data exposure. Checking that responses return only the data the consuming application needs, with no leakage of internal fields or PII.

Error handling. Ensuring error messages do not reveal stack traces, internal paths, or other details useful to an attacker.

When Your Business Needs API Testing

API security testing should be part of your development and risk management lifecycle, not a one-off exercise. Common trigger points:

Before launching a new API or product. Testing before go-live reduces the risk of exposing vulnerabilities to production traffic.

After significant changes. New endpoints, authentication changes, or major refactoring can introduce regressions.

As part of compliance programmes. ISO 27001, UK GDPR, and sector-specific standards like PCI DSS may require evidence of regular security testing that includes APIs.

On a regular cycle. Annual or six-monthly testing is a common baseline, supplemented by automated scanning in the pipeline for continuous assurance.

Preparing for an API Test

Good preparation makes a real difference to the quality and efficiency of an assessment. Before the engagement starts, try to provide:

API documentation. OpenAPI/Swagger specs, Postman collections, or equivalent descriptions of endpoints, parameters, and expected behaviour. Even draft or incomplete docs help.

A suitable test environment. Ideally a staging or sandbox environment that mirrors production but uses synthetic data. Testing against production carries risk and needs careful management.

Test credentials. Authenticated accounts for each role the API supports (standard user, administrator, read-only, etc.). This enables proper authorisation testing without time lost on account setup.

Context on business logic. A brief overview of key workflows and what the API is designed to do helps testers zero in on the most security-relevant areas.

Acting on Results

An API security assessment produces a report with findings categorised by severity, typically based on exploitability and potential impact. To act on those findings effectively:

Prioritise by risk. Critical and high-severity findings, particularly data exposure or authorisation bypass, should be addressed urgently. Medium and low findings can be scheduled into upcoming sprints.

Apply short-term mitigations where needed. While code-level fixes are in progress, interim controls like WAF rules, rate limiting adjustments, or disabling vulnerable endpoints can reduce immediate risk.

Retest after remediation. Confirm that fixes work and have not introduced new issues. Many providers offer a focused retest as part of the engagement.

Feed findings back into development. Recurring vulnerability patterns may point to gaps in secure development training, code review, or API design standards.

API security testing is a practical, necessary discipline for any organisation that relies on APIs, which increasingly means all of them. If you are looking for specialist support, pentestingcompanies.co.uk can help you find firms with proven API security testing expertise and get a thorough assessment tailored to your technology stack and risk profile.