API Penetration Testing in 2025: The New Battleground for Cyber Offence

Introduction

In 2025, cybercriminals are no longer just targeting traditional websites. Instead, they are exploiting the invisible backbone of modern digital services: Application Programming Interfaces (APIs). These data pipelines power everything from mobile apps to banking platforms, and as their use has surged, so too have the threats targeting them.

This blog is for IT professionals, security leaders, developers, and businesses that rely on APIs to serve users. If your organisation uses or builds APIs, penetration testing is no longer optional. It is your frontline defence in an age of automated, scalable cyberattacks.

API Penetration Testing 

API penetration testing is the process of simulating real-world attacks on APIs to uncover vulnerabilities before criminals do. Think of an API like the waiter in a restaurant, taking your request, fetching what you asked for, and delivering it. But what if someone altered the request to steal the chef's recipe or access the restaurant's financial records? That is exactly how attackers manipulate APIs.

APIs are often less visible and more dynamic than websites. This makes them harder to secure using traditional tools. Attackers target APIs to exploit logic flaws, insecure authentication, or poor coding practices. These flaws can lead to large-scale breaches, reputational damage, and non-compliance with data protection laws.

The API Threat Landscape

Why APIs Are the New Favourite Target

The shift to cloud-first infrastructure, microservices, and mobile-first products has created an API explosion. APIs now represent over 80 percent of all internet traffic (Akamai, 2024). That makes them an irresistible target for attackers who look for:



• Insecure authentication methods

• Lack of rate limiting

• Exposure of sensitive data through weak endpoints

• Poorly implemented object-level permissions

The OWASP API Security Top 10 highlights how APIs differ from traditional applications. Threats like broken object level authorisation, mass assignment, and excessive data exposure are specific to APIs and have already been exploited in real-world breaches.

One striking example occurred in 2023, when a fintech app exposed thousands of customer account details due to an unprotected API endpoint. Attackers were able to alter an ID in a request and access data for other users without authentication. This incident not only led to regulatory fines but also significant user attrition.

How API Pen Testing Differs from Web App Testing

API penetration testing is not just a bolt-on to standard web application testing. It requires a tailored approach. Unlike web apps, APIs lack a traditional user interface. Testers must interact directly with API endpoints using tools like Postman, Burp Suite, or custom scripts.

Attackers use API documentation, reverse engineering, or exposed Swagger files to map out the system. They manipulate requests, headers, tokens, and methods like GET, POST, PUT, and DELETE to find vulnerabilities. A good API pen tester needs both technical fluency and an understanding of business logic flaws.

Whereas web app testing focuses heavily on front-end forms and session handling, API testing examines authentication, parameter handling, and data leakage through backend functions. For example, APIs must be tested for over-permissive roles, misconfigured CORS policies, and unvalidated input fields that bypass server-side checks.

Real-World Exploits: Lessons from Recent API Breaches

Recent breaches reveal the urgent need for proactive API testing. Here are some notable examples:

In 2024, a global healthcare provider suffered a breach due to insecure direct object references (IDOR) in its API. A simple change to a numerical ID allowed attackers to download thousands of patient reports.

Another case involved a ride-sharing app, where unauthenticated API access gave users the ability to view and modify other passengers’ trip history. The lack of access control at the object level meant attackers did not need admin rights, just a bit of creativity.

Mass assignment bugs are also common. These occur when APIs accept too many parameters in user inputs and unintentionally update sensitive fields. For instance, a vulnerability in an HR platform allowed users to change their own roles by passing unauthorised data in a POST request.

Each of these cases could have been prevented by structured API pen testing.

What to Expect in a Professional API Pen Test Report

An effective API pen test report should not be a generic checklist. It should include:

• A detailed methodology outlining how tests were conducted

• A list of discovered vulnerabilities, each ranked by severity

• Proof-of-concept requests showing exploitability

• Business impact analysis

• Clear, actionable remediation guidance

Reports should be structured so that both technical teams and senior decision-makers can understand the findings. Technical summaries should include raw request/response data and screenshots. Management summaries should explain how risks align with business outcomes.

Reports must also align with frameworks like OWASP and standards such as ISO 27001 or NIS2. CREST-certified providers are particularly trusted, as their methods and testers are rigorously assessed.

Regulatory Pressures and Compliance Links

Why API Security Has Become a Regulatory Priority

In today’s digital landscape, regulatory bodies across the UK and EU have recognised that APIs represent a growing attack surface. As a result, they are placing increasing emphasis on the security of APIs and associated data interfaces. Compliance is no longer satisfied by protecting web applications alone. Modern regulations require evidence of robust API security practices, including penetration testing and risk assessments.

APIs process and transmit vast amounts of sensitive data, often handling authentication, financial transactions, and user interactions. This makes them subject to the same (or higher) scrutiny as traditional web platforms. When poorly secured, APIs can expose systems to unauthorised access, data leakage, and logic abuse, triggering regulatory concerns.

ISO 27001 and the Role of Continuous Testing

ISO 27001, the globally recognised standard for information security management, explicitly requires organisations to conduct ongoing risk assessments and implement appropriate technical controls. This includes penetration testing of digital assets—APIs among them. In a 2022 update, guidance was clarified to include “interfaces and third-party integrations” as part of the security perimeter.

For ISO-certified businesses, failing to include APIs in their testing regime can result in non-conformance. Auditors now expect to see penetration test reports, evidence of remediation, and documentation of how API vulnerabilities are being identified and resolved.

NIS2 and Sector-Specific Expectations

The EU’s revised NIS2 Directive expands the list of essential and important entities required to adopt advanced cybersecurity practices. This includes sectors like healthcare, transport, and digital infrastructure—many of which rely heavily on APIs. Under NIS2, organisations must demonstrate proactive risk management, including threat detection and mitigation through penetration testing.



The UK’s implementation of NIS2 places similar demands on regulated sectors. Failure to comply can lead to significant regulatory enforcement actions, including fines and reputational damage.

DORA: A Game Changer for Financial and Tech Providers

Under the EU’s Digital Operational Resilience Act (DORA), financial entities and technology providers must meet strict standards for digital resilience. This includes technical testing of all information and communication technology (ICT) components. DORA specifically mandates vulnerability assessments and penetration testing of APIs that support critical functions.

Regulators expect to see not only evidence of testing, but also active monitoring and mitigation efforts. Inaction is viewed as negligence, especially in the event of an incident linked to an API exploit.

Compliance Is Now Lifecycle-Driven

Modern compliance is no longer a one-time audit. It is a continuous cycle of discovery, testing, remediation, and review. Encryption and firewalls are necessary but insufficient. Regulators want to see full-lifecycle API security, discovery, testing, reporting, and proof of remediation as standard business practice.

Building a Resilient API Security Testing Process

Begin with API Discovery and Documentation

Before any testing can begin, organisations must know what they are protecting. That means starting with thorough API discovery. It is not uncommon for businesses to have dozens, sometimes hundreds, of undocumented or poorly documented APIs. These so-called “shadow APIs” emerge when developers push updates or prototype integrations without involving the security team. Unfortunately, these endpoints can become low-hanging fruit for attackers.

API discovery involves auditing all public-facing and internal endpoints, including those in development, production, and staging environments. Once located, each API should be properly catalogued with details such as its function, user access levels, data handled, and integration points.

Tools like API gateways, traffic sniffers, and source code analysis platforms can assist in creating a complete inventory.

Documentation is not just a compliance box to tick. It helps security teams understand the context of each API and plan effective tests. It also enables developers to maintain a consistent security posture as APIs evolve or new ones are introduced.

Evaluate Authentication and Access Control

Once all APIs are mapped and documented, the next step is reviewing how they handle authentication and authorisation. APIs should never rely solely on usernames and passwords. Secure implementations often use OAuth2, token-based authentication, or mutual TLS to verify the identity of users and services.

Testing teams should simulate scenarios where access tokens are reused, stolen, or manipulated. They should also validate whether tokens expire appropriately and if refresh tokens are used securely. Role-based access must be strictly enforced, ensuring that users can only access the data and functions appropriate to their role.

In many past breaches, attackers were able to manipulate IDs or URLs to gain access to other users’ data due to broken object-level authorisation. These issues must be addressed by embedding access control logic within the application itself—not just relying on external firewalls or proxies.

Conduct Frequent and Varied Testing

Security testing must be regular, thorough, and part of the development lifecycle. Many organisations still test APIs annually or only before a major release. This is insufficient in 2025, where API deployments can occur weekly or even daily in agile environments.

Manual testing brings human insight into logical and contextual flaws. For example, a tester may notice that certain endpoints are revealing sensitive metadata or allowing privilege escalation due to inconsistent permissions. Automated tools such as Burp Suite Pro, OWASP ZAP, and Postman are invaluable for running large-scale fuzzing, vulnerability scans, and regression tests.

Combining both approaches provides the most comprehensive view. A tester may use automation to identify a suspicious behaviour, then use manual techniques to confirm exploitation and assess impact.

Integrate Testing into CI/CD Pipelines

To ensure consistency, API testing should be integrated directly into the Continuous Integration and Continuous Deployment (CI/CD) pipelines. This means that every new API version or feature undergoes automated scans and validation before going live. Security controls become part of the quality assurance process rather than an afterthought.

APIs that are automatically tested as part of the build process will reveal issues early, when fixes are cheaper and easier to implement. Vulnerability thresholds can be set to block a release if critical risks are detected, helping teams adopt a “secure by design” mindset.

In addition, version control and audit logs must be maintained to track all API changes, including third-party updates. APIs evolve constantly, and without structured testing and tracking, even secure systems can become vulnerable over time.

Prioritise Token Management and Third-Party APIs

Secure token management is central to API security. Tokens must be unique, encrypted, and time-limited. Poor token practices, such as reusing tokens or storing them insecurely in browser storage, create easy targets for attackers.

Every API should also include input validation at both the client and server levels. Never trust data from users or external applications. All parameters must be checked for type, length, format, and content before processing.

Finally, third-party APIs require just as much scrutiny as internal ones. Integrations with payment providers, CRMs, or analytics tools may expose sensitive data or create dependency-based vulnerabilities. Regular security reviews and contractual security requirements are essential when using third-party services.

The Cybergen Approach to API Penetration Testing

Cybergen provides tailored API penetration testing services that go beyond generic assessments. Their approach is grounded in deep knowledge of the OWASP API Top 10, CREST methodologies, and the compliance needs of UK-based businesses.

They begin with a discovery and scoping process to identify API endpoints, evaluate documentation, and assess risk context. Cybergen testers then simulate a variety of attacks, from injection and IDOR to broken authentication and mass assignment.

Each engagement ends with a comprehensive report that includes clear technical findings and prioritised action plans. Follow-up retesting is also offered to validate that issues have been resolved. For organisations operating under NIS2, ISO 27001, or DORA requirements, Cybergen helps align testing with your broader risk management strategy.

Summary

API penetration testing is no longer optional. In a digital landscape shaped by microservices, SaaS, and mobile platforms, APIs are often the softest target and the most valuable asset. The risks of ignoring API security are growing, and the regulatory stakes are rising.

Whether you are a fintech startup, an NHS partner, or a SaaS provider, proactive testing is essential. Cybergen offers the tools, talent, and tailored approach to keep your APIs secure in 2025 and beyond.

Take control of your API security posture now. Visit https://www.cybergensecurity.co.uk/penetration-testing to speak with Cybergen’s team today.

Ready to strengthen your security posture? Contact us today for more information on protecting your business.