Why API Testing Matters in a Penetration Test

API Testing Matters in a Penetration Test

A web application might look simple at first.

A customer logs in, clicks a button, submits a form, and receives a response.

But behind the scenes, the application usually relies on several APIs to handle authentication, customer details, payments, orders, account management, reporting, and other business tasks.

The user sees the website.

Often, it’s the API that does most of the work.

This raises important security concern for businesses:

If your penetration test doesn’t fully examine the APIs within your application, you could overlook the systems that enforce your web app’s security.

APIs have become a basic part of web application architecture. OWASP notes that APIs commonly expose application logic and sensitive data, including personally identifiable information, making them an increasingly important security target.

For businesses, API security shouldn’t be something developers address only after the app is built.

API testing should be part of your basic security assessment.

What Is an API?

An API, or an Application Programming Interface, allows different software systems to communicate with one another.

Think of an API as similar to the service window at a restaurant.

You do not walk into the kitchen and prepare your own meal. You place an order through the service window. The kitchen receives the request, processes it, and sends something back.

APIs operate in a similar way.

A web application might send an API request asking for a customer’s account information. The API processes the request, retrieves the appropriate information, and returns the response to the application.

This type of interaction is common in today’s software.

When you log into an application, view an account balance, update your profile, submit an order, upload a document, or check a transaction status, an API may handle the underlying request.

This also introduces a security challenge.

The API has to check not only whether a request is technically correct, but also whether the person making it is allowed to perform that action.

Why APIs Can Become a Security Weak Point

The front end of an application may prevent a user from performing certain actions.

But security shouldn’t rely only on what the user interface allows someone to do.

The API needs to enforce those rules on the server side.

Imagine an online banking application.

The website may show a customer only their own account information. The customer sees their checking and savings accounts and has no button that lets them view another customer’s account.

But what happens if the underlying API accepts an account ID supplied in the request?

If the API does not independently verify that the authenticated customer has permission to access that account, an attacker may be able to modify the request and access someone else’s information.

The website might appear totally secure.

But the API might tell a different story.

This type of weakness, known as Broken Object Level Authorization, ranks as the first risk in the OWASP API Security Top 10. OWASP describes the problem as occurring when an API fails to properly verify that a user has permission to access a specific object referenced in a request.

This is a major reason why API testing is important.

The API May Be the Real Door

Picture a commercial building.

The front entrance has a security guard. Visitors must check in, and the guard lets them into only the areas they are authorized to visit.

Now imagine that the building also has a service entrance around the back.

The service entrance connects directly to the warehouse.

If no one monitors who uses that entrance, the front door security won’t actually protect the warehouse.

APIs can create a similar risk.

A web application’s user interface may enforce certain restrictions, while an API provides another path to the underlying functionality.

This doesn’t mean that APIs are always insecure.

It means security experts need to test the actual interfaces that apps use to access data and perform actions.

What Does API Penetration Testing Examine?

API penetration testing evaluates the security controls surrounding API endpoints and the business functions they expose.

A tester may examine authentication, authorization, session handling, input validation, data exposure, error handling, rate limiting, business logic, API configuration, and interactions with other systems.

Testing should also check how the API responds to unexpected requests.

This is important because APIs typically use structured requests and predictable endpoints.

Attackers can often change parameters, object IDs, HTTP methods, headers, request bodies, tokens, and other parts of an API request.

So, the question is:

What happens when someone changes something they were never supposed to change?

Testing Authentication Is Only the Beginning

Authentication answers an important question:

“Who are you?”

But API security also requires answering another question:

“What are you allowed to do?”

A user might log in successfully but still access information or features they shouldn’t.

This distinction matters most when an application has multiple user roles.

A regular employee may be allowed to view their own records.

A manager may be allowed to view records for their department.

An administrator may have access to much more.

The API needs to enforce those differences.

An OWASP penetration test can check these boundaries by seeing what different user types are actually able to do.

What If a User Changes an ID?

This is one of the simplest ways to show a business executive why API testing matters.

Imagine that a customer views their invoice through an application.

The application makes an API request that includes an invoice identifier.

The customer should see Invoice 1001.

What happens if the customer changes the request to Invoice 1002?

The application should verify that the customer has permission to access Invoice 1002.

If the API simply returns the invoice because the request appears correct, the app could have an authorization weakness.

The same concept can apply to customer records, medical information, financial information, employee records, documents, orders, vehicles, tickets, projects, and other business data.

OWASP identifies this class of vulnerability as particularly important because manipulating object IDs can result in unauthorized disclosure, modification, or deletion of another user’s data.

This can be hard to spot just by looking at the website.

It’s much easier to test when a security professional reviews the underlying API requests.

APIs Can Expose More Information Than the Application Displays

Another key reason to test APIs is the information that API responses return.

A webpage might display only a customer’s name and email address.

The underlying API response could contain additional fields that the application does not display to the user.

If the API returns sensitive information that the user shouldn’t receive, it’s still a security problem, even if the website never displays it.

The risk can involve unauthorized disclosure or modification of sensitive object properties and, in some circumstances, can contribute to privilege escalation or account takeover.

This highlights an important principle in application security:

Security should protect the data being returned, not just what the user interface shows and this type of finding would be reviewed in a custom application penetration test.

APIs Can Expose Administrative Functions

APIs do not only retrieve information.

They can also perform actions.

An API may create users, change account settings, approve transactions, reset passwords, modify records, upload files, delete information, or perform administrative tasks.

This creates another area for penetration testing.

A tester can examine whether administrative functions remain protected when someone attempts to call them directly.

The website may never present an administrator function to a normal user.

But that doesn’t always mean the API will block the request.

A secure API should always check authorization on the server side, regardless of where the request comes from.

Business Logic Can Be Abused Through APIs

Some API vulnerabilities are not traditional software flaws.

The API might work exactly as the developers planned.

The problem arises when someone uses that feature in a way the business didn’t expect.

Consider an application that allows customers to purchase tickets.

The API may process a ticket purchase correctly.

But what happens if someone automates thousands of requests?

Or repeatedly request a limited resource?

Or manipulates the order in which transactions occur?

Or attempts to bypass a business restriction by interacting directly with the API?

OWASP’s API Security Top 10 includes Unrestricted Access to Sensitive Business Flows, recognizing that some APIs expose business processes attackers can abuse even when the underlying implementation contains no traditional software vulnerability.

This is another reason for manual API testing matters and is the role of web app pen testing.

A scanner can find technical weaknesses.

A security professional can check if the API lets someone manipulate the company’s business process.

API Rate Limiting Can Become a Business Issue

APIs can also use a lot of resources.

Every request requires processing power, bandwidth, memory, storage, or other resources. Some API requests also trigger costs from third-party services.

For example, an API might trigger an email, text message, identity verification service, cloud workload, file-processing service, or other paid functions.

If the API doesn’t limit requests properly, an attacker could use too many resources or cause unexpected costs.

OWASP identifies Unrestricted Resource Consumption as an API security risk and notes that exploitation can cause denial of service and increase infrastructure or third-party service costs.

For a business, API security isn’t always just about preventing data theft.

Sometimes, the risk is about keeping services running or avoiding surprise expenses.

Third-Party APIs Create Another Layer of Risk

Many applications rely on other APIs.

Your application might communicate with a payment processor, shipping provider, cloud service, identity provider, marketing platform, artificial intelligence service, or another third-party system.

This adds another layer for security testing to be considered.

Your application may trust the information returned by another service.

But what happens if that information is manipulated?

OWASP specifically identifies Unsafe Consumption of APIs as an API security risk because applications may trust data from third-party APIs more than direct user input.

So, API security doesn’t always stop at your own server.

It can also extend to the systems your API depends on.

API Inventory Matters More Than Many Companies Realize

One challenge is simply knowing which APIs exist.

Development teams may create new endpoints during application development and leave older versions running after a newer version launches.

Testing environments can also expose endpoints that were never intended for public access.

OWASP identifies Improper Inventory Management as an API security risk because companies may have more API endpoints, hosts, versions, or undocumented functionality than they realize.

You can’t secure an API if you don’t even know it exists. That’s why keeping an inventory is an important part of a penetration test.

Why Automated API Scanning Is Not Enough

Automated API testing can provide significant value.

It can identify patterns, test large numbers of requests, discover weaknesses, and help security professionals work efficiently.

But automation has its limits.

An automated tool may recognize that an API accepts an account ID.

A human tester can ask whether the account belongs to the authenticated user.

An automated tool may identify an administrative endpoint.

A human tester can determine whether a regular user can invoke it.

An automated tool may identify an endpoint that allows many requests.

A human tester can determine whether that functionality could be abused in a way that creates a meaningful business impact.

That’s why Tanner Security uses both automated tools and manual testing.

The goal isn’t just to find API weaknesses.

It’s about understanding what an attacker could do by combining them.

APIs Should Be Tested as Part of the Application

API testing shouldn’t always happen separately.

For many applications, the API and the web application are really two sides of the same system.

The website provides an interface.

The API provides the underlying functionality.

If you only test one side, you could miss important issues.

A web application penetration test should consider how the application communicates with its APIs and how those APIs handle authentication, authorization, data, business logic, and interactions with other systems.

If APIs make up a large part of your app’s attack surface, dedicated API penetration testing can go even deeper.

What Tanner Security Looks for When Testing APIs

Tanner Security approaches API testing by focusing on what an attacker could do.

That starts with understanding the application’s architecture and identifying the APIs that support it.

The assessment can then examine authenticated and unauthenticated functionality, user roles, authorization boundaries, API requests and responses, business logic, data exposure, and interactions with connected systems.

Manual testing is especially important for finding out if one vulnerability can be combined with another.

For example, an API may expose a small amount of information. By itself, the finding may not create significant risk.

But if that information helps an attacker find another API endpoint, change an object ID, and access someone else’s account, the risk becomes much greater.

That’s the kind of review a penetration test should provide.

Learn more about Tanner Security’s Web Application Penetration Testing Services

When Should Your Company Test Its APIs?

If your web application uses APIs to handle private information or important business functions, API security should be part of your penetration testing strategy.

This becomes especially important when an application handles customer accounts, financial transactions, healthcare information, employee records, confidential documents, payments, administrative functions, or other sensitive business processes.

You should also focus on API testing when launching a major new app, changing authentication or authorization, adding new APIs, moving to the cloud, or making significant updates to an existing app. Read more about what events should trigger a penetration test.

The point isn’t to test APIs just because they’re common.

The real reason is that APIs can give direct access to the data and business functions your company needs to keep safe.

API Testing Should Be Part of Your Penetration Test 

Applications depend more on APIs.

They connect websites to databases, mobile applications to back-end systems, internal applications to cloud services, and businesses to third-party providers.

This connectivity brings significant value.

It also creates more ways for attackers to get in.

If a penetration test ignores those paths, it might not give you a complete picture of your app’s security.

At Tanner Security, we look beyond what users can click. We check how the app communicates, what its APIs reveal, how authentication and authorization work, and what an attacker could do by taking advantage of those connections.

Because the real question is not whether your API works.

The real question is whether it stays secure when someone tries to make it do something it was never meant to do.

Talk with Tanner Security about API and Web Application Penetration Testing

API Testing Matters in a Penetration Test FAQ’s

What is API penetration testing?

API penetration testing is an authorized security assessment that examines APIs for hidden vulnerabilities, authentication and authorization weaknesses, data exposure, business logic flaws, configuration problems, and other security issues an attacker could exploit.

Why should APIs be included in a penetration test?

APIs often provide the underlying functionality and data access for web and mobile applications. Testing the visible application without adequately testing the APIs behind it can leave important security controls and attack paths unexamined. Read more about a web application penetration test case study.

What is the difference between API testing and web application penetration testing?

Web application penetration testing reviews the application’s overall security. API penetration testing focuses specifically on the interfaces that applications use to communicate with back-end systems and services. In many engagements, API testing forms an important part of the web application penetration test. Read more about the difference between custom application penetration testing and a standard web application penetration test.

What are the most common API vulnerabilities?

OWASP’s API Security Top 10 includes risks such as Broken Object Level Authorization, Broken Authentication, Broken Object Property Level Authorization, Unrestricted Resource Consumption, Broken Function Level Authorization, unrestricted access to sensitive business flows, SSRF, security misconfiguration, improper API inventory management, and unsafe API consumption. Read more about Tanner Security’s OWASP Penetration Testing services.

What is Broken Object Level Authorization?

Broken Object Level Authorization occurs when an API does not properly verify that the authenticated user has permission to access a specific object. For example, a customer might manipulate an account or invoice identifier and access another customer’s information.

Can API testing find authentication vulnerabilities?

Yes. API testing can examine how authentication works, including authentication tokens, session handling, authentication flows, and attempts to access protected functionality without appropriate credentials.

Can API testing find authorization problems?

Yes. Authorization testing is one of the most important components of API security testing. Testers can examine whether users can access objects, functions, or data outside their intended permissions.

Can APIs expose sensitive information even when the website does not?

Yes. An API response may contain information that the application interface does not display. Testing the API responses can help determine whether the server returns information that the requesting user should not receive.

Can API penetration testing find business logic vulnerabilities?

Yes. Manual testing can examine whether an attacker can manipulate legitimate API functionality to bypass business restrictions or abuse sensitive business processes. Read more about what makes our web penetration tests different.

Should authenticated API testing be performed?

For many applications, yes. Authenticated testing allows security professionals to evaluate what different types of legitimate users can access and whether the API properly enforces authorization boundaries.

Should APIs be tested separately from a web application?

Not always. APIs frequently form an integral part of the web application, so API testing should often occur as part of the application penetration test. Dedicated API testing may provide additional depth when APIs have a significant portion of the application’s functionality or attack surface.

Can API penetration testing test third-party APIs?

A penetration test can review how your application interacts with third-party APIs, although directly testing a third-party service generally requires appropriate authorization from its owner. The focus can include how your application validates, trusts, and processes information received from external services.

Can API testing identify old or forgotten API endpoints?

It can. API discovery and inventory review (automated scan) can help identify undocumented, deprecated, or otherwise unexpected endpoints that may increase the application’s attack surface.

Does API penetration testing replace vulnerability scanning?

No. Vulnerability scanning and penetration testing serve different purposes. Scanning can identify potential weaknesses at scale, while penetration testing validates vulnerabilities and determines what an attacker could accomplish. Read more about the difference between penetration test vs vulnerability assessment.

How does Tanner Security test APIs?

Tanner Security combines automated tools with manual security testing to examine API functionality, authentication, authorization, data exposure, business logic, and potential attack paths. The testing approach depends on the application’s architecture, functionality, and business requirements.