API Security Testing

What is API Security Testing?

An Application Programming Interface (API) allows software applications to interact with each other. It is a fundamental part of modern software patterns, such as **microservices architectures**.

API security is the process of protecting APIs from attacks. Because APIs are very commonly used, and because they enable access to sensitive software functions and data, they are becoming a primary target for attackers.

API security is a key component of modern web application security. APIs may have vulnerabilities like broken authentication and authorization, lack of rate limiting, and code injection. Organizations must regularly test APIs to identify vulnerabilities and address these vulnerabilities using security best practices. We explain several methods and tools for API security testing, and a range of best practices that can help you secure your APIs.

Why Is API Security Important?

API security involves securing data transferred through APIs, typically between clients and servers connected over public networks.

Businesses use APIs to connect services and transfer data. A compromised, exposed, or hacked API can expose personal data, financial information, or other sensitive data. Therefore, security is a critical consideration when designing and developing RESTful and other APIs.

APIs are vulnerable to security weaknesses in backend systems. If attackers compromise the API provider, they can potentially compromise all API data and functionality. APIs can also be exploited via malicious requests, if the API is not properly coded and protected.

For example, a denial of service (DoS) attack can take an API endpoint online or significantly degrade performance. Attackers can abuse APIs by scraping data or exceeding usage limits. More sophisticated attackers can inject malicious code to perform unauthorized operations or compromise the backend.

With the popularity of microservices and serverless architectures, almost every enterprise application depends on APIs for its basic functionality. This makes API security a core part of modern information security.

Methods Of API Security Testing

We use the following methods to manually test your APIs for security vulnerabilities.

Test for Parameter Tampering

In most cases, parameters sent through API requests can be easily tampered with. For example, by manipulating parameters, attackers can change the amount of a purchase and receive products for free, or trick an API into providing sensitive data that is not authorized for the user’s account.

Parameter tampering is often performed using hidden form fields. You can test for the presence of hidden fields using the browser element inspector. If you find a hidden field, experiment with different values and see how your API reacts.

Test for Command Injection

To test if your API is vulnerable to command injection attacks, try injecting operating system commands in API inputs. Use operating system commands appropriate to the operating system running your API server. It is recommended to use a harmless operating system command that you can observe on the server—for example, a reboot command.

Test for API Input Fuzzing

Fuzzing means providing random data to the API until you discover a functional or security problem. You should look for indications that the API returned an error, processed inputs incorrectly, or crashed.

For example, if your API accepts numerical inputs, you can try very large numbers, negative numbers, or zero. If it accepts strings, you can try random SQL queries, system commands, or arbitrary non-text characters.

Test for Unhandled HTTP Methods

Web applications that communicate using APIs may use various HTTP methods. These HTTP methods are used to store, delete, or retrieve data. If the server doesn’t support the HTTP method, you will usually get an error. However, this is not always the case. If the HTTP method is unsupported on the server side, this creates a security vulnerability.

It is easy to test if HTTP methods are supported on the server side, by making a HEAD request to an API endpoint that requires authentication. Try all the common HTTP methods—POST, GET, PUT, PATCH, DELETE, etc.

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