API Penetration Testing: Find Real Security Risks

APIs can pass automated security checks, require valid credentials and sit behind an API gateway, yet still expose serious vulnerabilities. That’s because some of the most damaging API flaws aren’t caused by obviously malicious requests. They happen when a perfectly valid request is made by the wrong user, against the wrong object or in a way the application’s developers didn’t anticipate. API penetration testing is designed to uncover those weaknesses.

OWASP identifies authorisation as the biggest challenge in API security, with three of the first five risks in its API Security Top 10 2023 relating directly to access control. Modern API environments can also contain huge numbers of endpoints and parameters, making it increasingly difficult to predict how permissions will behave across every user, object and function.

For us, effective API security testing therefore isn’t simply about scanning a documented list of endpoints. We need to understand the real attack surface, who should be able to perform particular actions, how sensitive information moves between systems and whether apparently legitimate functionality can be abused.

What Does API Penetration Testing Actually Test?

An API penetration test simulates how an attacker could interact with application programming interfaces APIs to access data or functionality they shouldn’t have. Depending on scope, this can include REST APIs, GraphQL APIs, SOAP services and APIs supporting web applications or mobile platforms.

It’s useful to distinguish penetration testing from other forms of API vulnerability assessment.

Automated scanners are very good at repeating tests across large numbers of API endpoints and spotting certain recognisable weaknesses. Functional testing, meanwhile, checks that an API behaves as intended. A penetration test asks a different question: can somebody make this API do something they aren’t supposed to be able to do?

That distinction becomes especially important around authentication and access control.

Authentication mechanisms establish who a user is. Authorisation determines what the authenticated user can actually see or do. A user could have a completely legitimate session and valid token but still access another customer’s records if the API doesn’t verify ownership correctly.

Testing methodologies can vary. A black-box assessment gives us little internal information, while white-box testing may provide source code and architectural detail. Grey-box testing often sits somewhere particularly useful for API penetration testing because separate test accounts can represent customers, administrators and different user roles.

Having those identities available lets us test the boundaries between them rather than simply confirming that an endpoint requires a login.

How We Find Real-World API Security Risks

A good assessment starts long before anybody tests for SQL injection or server side request forgery SSRF. First, we need to understand what’s actually exposed.

Mapping the real API attack surface

Documentation such as Swagger or OpenAPI specifications gives us an excellent starting point, but it shouldn’t automatically be treated as the complete inventory.

Older API versions can remain online. Development endpoints may have become externally accessible. An application might call functionality that’s absent from the current documentation. OWASP includes Improper Inventory Management in its API Security Top 10 specifically because APIs can expose numerous endpoints and versions that need to be tracked properly.

Keeping track of that attack surface is a security issue in its own right. OWASP warns that organisations need a clear understanding of deployed hosts, API versions, integrated services and data flows so that outdated or forgotten functionality doesn’t remain exposed unnecessarily.

We therefore look at the API from more than one perspective. Documentation, application behaviour, endpoint structure, available HTTP methods and different versions can all reveal additional attack surface.

A useful practical approach, also reflected in security practitioner discussions, is to import an available OpenAPI or Swagger specification into tools such as Postman or an intercepting proxy. This removes some of the repetitive work involved in reconstructing legitimate requests and leaves more time for testing authorisation and business logic.

Testing users, objects and permissions

Once the API is mapped, we need to understand how different identities interact with it.

Imagine two customers can access their invoices through:

/api/invoices/3817

and

/api/invoices/3818

Simply requiring a valid token isn’t enough. The application also needs to verify that the authenticated user is entitled to access the requested invoice.

If Customer A can retrieve Customer B’s invoice by changing an identifier, that’s broken object-level authorisation, commonly associated with IDOR-style weaknesses. OWASP ranks BOLA as API1 in its current API security list and recommends checking object-level authorisation whenever an endpoint uses a client-supplied identifier to access a record.

In practice, testing with multiple accounts is far more informative than randomly changing IDs. Knowing which data belongs to which account gives us a reliable baseline for identifying a real access-control failure.

We also examine vertical privilege boundaries. A normal user might never see an administrator control in the interface, but what happens if their authenticated session communicates directly with the underlying admin API endpoint? OWASP describes broken function-level authorisation as a situation where legitimate API calls reach functions the caller shouldn’t be permitted to use.

Looking beyond obviously malicious requests

Some API security vulnerabilities are conventional technical flaws such as an injection attack, security misconfiguration or SSRF. Others are much more dependent on context.

A profile endpoint might legitimately allow a user to change their name and email address, for example. If the backend also accepts an undocumented role or accountType field without appropriate property-level authorisation, the same legitimate update operation could potentially change something far more sensitive.

Business processes require similar scrutiny.

Could an API be used to reserve scarce stock repeatedly? Can one account trigger excessive password-reset messages? Could a promotion be claimed more times than intended through direct API calls?

OWASP introduced Unrestricted Access to Sensitive Business Flows to its 2023 list specifically to address functionality that can harm a business when used excessively or through automation, even where there isn’t a traditional software bug.

Rate limiting may reduce some of these risks, but it’s not a substitute for understanding how a workflow could be abused.

Which API Vulnerabilities Should We Prioritise?

There isn’t one payload or checklist that captures every meaningful API weakness. The best approach is to test the assumptions that the application is relying on.

Can authenticated users access only their own resources?

This is where we look for BOLA, IDOR-style issues and failures in tenant separation. Using predictable IDs isn’t itself the vulnerability. The problem is whether the server performs the correct authorisation check when an identifier is supplied.

Can users only perform actions permitted by their role?

Here we’re testing function-level access control. Hidden buttons and front-end restrictions don’t provide security if the backend API still accepts the request from a lower-privileged account.

Can users modify only the properties they’re allowed to change?

APIs frequently exchange complex objects. If the server automatically accepts fields it shouldn’t; sensitive attributes may be exposed or modified. OWASP now groups issues historically described as excessive data exposure and mass assignment under Broken Object Property Level Authorisation.

Do tokens continue to provide access when they shouldn’t?

JWTs, OAuth implementations and other authentication mechanisms deserve detailed testing rather than a simple check that authentication exists.

Other areas can include injection vulnerabilities, unrestricted resource consumption, server side request forgery SSRF, outdated API versions and unsafe consumption of third-party APIs. What deserves the greatest attention depends on what the API actually does and the sensitive data or business functions it exposes.

API Penetration Testing vs Automated Security Testing

Automation is useful. We’d be making API security harder than necessary if we ignored it.

Security tools can enumerate endpoints, fuzz inputs, identify some configuration problems and repeatedly check a large attack surface. They can also integrate security testing into development workflows, allowing certain weaknesses to be identified earlier.

Where automated testing becomes less complete is when a finding depends heavily on context.

A scanner may know that /api/orders/582 returned HTTP 200. Understanding whether the caller should have been allowed to retrieve order 582 requires knowledge about the user, the resource and the application’s permission model.

That's why we don't treat API penetration testing and automated API security testing as opposing approaches. Our web application penetration testing combines automated techniques with manual investigation to uncover weaknesses that depend on context, permissions and business logic. Automation gives us repeatability and breadth. Manual investigation lets us spend more time on authorisation boundaries, unusual workflows and combinations of behaviour that require an understanding of how the application is supposed to work.

The same principle applies when prioritising endpoints. A simple health-check endpoint rarely deserves the same manual attention as an API that deletes records, processes payments or controls user permissions. Routine checks can be automated, while manual effort is better spent on high-value functionality and business logic.

What Should You Expect From an API Penetration Test?

Before testing begins, the scope needs to reflect the system we’re actually trying to protect. That can mean confirming which environments and API versions are included, identifying user roles and agreeing how sensitive or potentially disruptive functions will be handled.

Good reporting should then go further than presenting a list of scanner alerts.

For each significant weakness, you should be able to understand the affected endpoint, the conditions required to reproduce it, the security impact and what needs to change. Where appropriate, evidence should demonstrate the issue without unnecessarily exposing, modifying or deleting sensitive data.

Remediation also needs to consider the underlying cause. Fixing one vulnerable endpoint doesn’t necessarily solve the problem if the same flawed access-control approach appears elsewhere in the application.

Retesting is therefore an important part of the process. Once developers have applied a fix, we can verify whether the original attack path has actually been removed rather than assuming a code change solved it.

How often testing should happen depends on the API. An annual assessment may still play a role, but a frequently changing service can justify testing following major releases, new authentication architecture, changes to permissions or the introduction of new business-critical functionality.

API inventories aren’t static. OWASP specifically highlights outdated API versions, undocumented hosts and missing visibility over deployed services as security risks, so the testing schedule should reflect how quickly the environment itself changes.

Find the API Risks Attackers Could Actually Use

API penetration testing is most valuable when it moves beyond asking whether an endpoint looks secure and starts asking whether somebody can make it behave in a way the business never intended.

An API can use encryption, accept a valid token and correctly parse every request while still exposing another customer’s sensitive information because of one missing authorisation check. Equally, every individual request in a workflow could be technically valid while automation turns the overall process into an exploitable business risk.

Our approach to API security testing is therefore based on context as much as vulnerability detection. We map the real attack surface, test permissions across identities, examine sensitive functions and establish the practical impact of any weaknesses we uncover.

If you’re responsible for APIs that expose business-critical functionality or sensitive data, speak to our penetration testing team. We can assess how your APIs respond to realistic attack behaviour and help you identify vulnerabilities before somebody else gets the opportunity to exploit them.