The Critical Role of API Penetration Testing in Your Web App Security Strategy

Application Programming Interfaces (APIs) are a crucial part of your environment, enabling seamless communication between applications, systems, and devices. One study found that 71% of all internet traffic in 2023 consisted of API calls, with an average enterprise processing 1.5 billion API calls annually. With organizations having an average 614 API endpoints in production, hackers are increasingly targeting APIs to access sensitive data and services. Unfortunately, our team finds that one of the most frequently overlooked areas in security testing is API penetration testing for your web applications. We sat down with our Penetration Testing Team Manager, Tom Pohl, to dive into why API penetration testing is crucial and get some tips for what to include in an effective testing program. So, without further ado, let’s dive in!

Why You Should Add API Penetration Testing to Your Web App Security Plan

1. APIs Are Prime Targets. One study found that API-related security incidents cost global businesses up to $75 billion annually. With the number of API endpoints growing and the massive traffic volume expanding your attack surface, web application APIs are becoming increasingly attractive targets for cybercriminals. Common vulnerabilities include improper authentication, excessive data exposure, and broken access controls.

How are attackers targeting web application APIs? “We frequently conduct web application API penetration testing, particularly for APIs built with modern frameworks,” stated Tom Pohl. “During API testing, we focus on identifying vulnerabilities within the APIs themselves, and we find the number one vulnerability is API injection attacks. This includes SQL injection, XML, and other query language manipulations such as JSON payload tampering.” Tom continued, “We also frequently see Insecure Direct Object References (IDOR) attacks, where attackers can see data they shouldn’t be able to access because of three main causes: a query that is not limited properly, an API that allows a call function that is shouldn’t, or no/low authorization requirement for high-value functions.”

For example, during a recent web app and API penetration test, our team discovered an API that allowed unrestricted access to sensitive data due to weak authentication controls. A bank had forgotten to put an authorizer on an API endpoint, so our team was able to find and pull sensitive information including account balances, names, account numbers, and more.

2. API Complexity is Increasing. With the proliferation of microservices, APIs have become more complex. Modern APIs often integrate multiple third-party services, which increases the risk of introducing vulnerabilities. API penetration testing is one of the best ways to uncover these vulnerabilities.

In one API penetration test, our team discovered a major bank had an API issue related to a vulnerability in their Spring framework, which exposed multiple default endpoints. “Frameworks frequently have additional code users often don’t notice, and it can cause security gaps due to unanticipated behavior of the API framework code,” Pohl stated. In this case, it led to one of these endpoints providing a memory dump revealing clear text credentials, including usernames and passwords. It enabled our team to gain full access to applications. By identifying these issues in an API penetration test, the bank was able to fix its code to eliminate the use of cleartext credentials and address the security gap.

3. Web APIs Face Specific Threats. APIs face unique threats that require specific testing, such as:

• • Injection Attacks: Exploiting unvalidated input to manipulate backend systems.
  • Insecure Endpoints: Exposing functionality intended only for internal use.
  • Rate Limiting Bypasses: Allowing attackers to overwhelm servers through brute force or DoS attacks.

Now that we’ve covered some of today’s threats, let’s review what you should look for in API penetration testing.

5 Things an Effective API Penetration Test Should Include

1. The Proper Approach. Most organizations use automated testing suites in the API development process, but this approach does not assess APIs as an attacker does. Tom shared, “Often API developers focus on USE cases when they should focus on ABUSE cases. They test for success when they should be testing for anticipated failures like invalid data and inappropriate access—this is crucial for good API penetration testing.”
2. Test Authentication and Authorization. Your API penetration test should look for authentication flaws as they are a critical API vulnerability. “Testers should add and test every endpoint,” said Pohl. “You may have 200 endpoints, and 199 are secure but 1 is not. Testing teams must be thorough.” They should look for:
   1. Proper implementation of OAuth2 or API key mechanisms.
   2. Sensitive endpoints requireing valid tokens.
   3. Role-based access controls (RBAC) to prevent unauthorized access and more.
3. Validate Input and Output. Your API penetration testing team should conduct robust validation of all inputs to prevent injection attacks like SQL injection and XML External Entity (XXE) injection. Similarly, they should inspect responses to ensure sensitive data such as stack traces or keys are not exposed.
4. Check for all OWASP API Security Top 10 Vulnerabilities. API Penetration testing should focus on common vulnerabilities like:
   • Broken Object-Level Authorization (BOLA): Test if users can access data or functionality intended for others.
   • Excessive Data Exposure: Analyze API responses for unnecessary or sensitive data. This is a HUGE issue in API penetration tests. Often a query is only looking for one or two fields and instead sends back multiple data fields that can include sensitive data and cause a breach. Data is hazardous material—carefully check for exposure and lock your data down.
   • Improper Inventory Management: Failing to maintain an accurate inventory of APIs, including shadow or deprecated endpoints, increases the risk of security gaps and unauthorized access.
   • Unsafe Consumption of APIs: Consuming third-party or external APIs without proper validation and security checks can expose systems to vulnerabilities like data tampering or injection attacks.
5. Test Rate Limiting and Throttling. Your testing team should use automated tools to simulate high-traffic volumes and verify throttling mechanisms are implemented effectively to prevent brute-force attacks. If there is a gap here, you should address it at the infrastructure level.

The Role of API Penetration Testing in Your Security Strategy

The cost of proactive penetration testing is always lower than the cost of remediation. Adding API penetration testing to your web app penetration testing is not a one-time activity. It must be integrated into your organization’s broader security strategy. In a world where APIs drive everything from mobile apps to IoT devices, regular API penetration testing is essential for uncovering and mitigating risks that traditional testing methods may overlook, and it will help your organization stay ahead of threats. We also suggest reading our blogs on API security best practices and how to reduce web app and cloud app security risks.

We hope you have found this information helpful! Please contact us if you need help with web application and API penetration testing. Our expert team is ready to help!