A Complete Guide to API Security Testing

Timothy Joseph
Timothy Joseph | March 15, 2023

A Complete Guide to API Security Testing

With the rise of cloud computing, APIs have become an increasingly popular way of sharing data and services between applications. However, the increased use of APIs also means a greater risk of malicious activity. According to a report by Salt Security, API attack traffic reached an average of 26.46M calls in 2022.

Unfortunately, many organizations still lack the necessary knowledge and resources to properly test their APIs. This leaves them vulnerable to a variety of security threats, such as malicious code injection, data leakage, and unauthorized access. That is where API security testing comes into the picture. API security testing aids in the detection and prevention of vulnerabilities and the potential organizational risk they pose.

What Is API Security Testing?

In this type of testing, we verify the API for any potential security vulnerabilities that could allow attackers to gain access to sensitive data or disrupt the functionality of the API. This type of testing ensures that the API is secure and cannot be exploited by unauthorized individuals. Also, API security testing validates that no one can acquire their information.


Importance of API Security

Various breaches can happen due to the exposure to insecure APIs as they can easily be exploited and give hackers access to sensitive and personal data.

Below are some points that will define why API security is important:

  • API security focuses on mitigating the vulnerabilities and security risks at the API layer of applications, hence making them more secure which decreases the probability of attacks and data breaches.

  • With help of API security, we secure data transferred between clients and servers connected over the networks, so that attackers can not perform any man-in-the-middle attack and can not misuse the application API.

  • Compromised API can give an attacker unauthorized access which can be used to perform malicious activities. API configured with an improper authentication mechanism enables hackers to hijack the identity of the user and access controls of an API. API security testing eliminates this possibility.
  • API injection attacks can happen if we do not carefully limit the inputs to anticipated types. With this loophole, hackers send the script to the application server through an API request to gain access to the software. API security testing ensures that there is no such loophole.

Top Five API Security Recommended Methods

Below are a few methods that describe, how to do API Security testing:

  1. Test For Authentication

    APIs can have different authentication mechanisms, standard mechanism is basic authentication with a username and password, API keys passed as header, and OAuth as Bearer Access Tokens. Test for accessing API endpoints that require a credential with no credentials or an invalid one. If your server returns a response anything other than 401 response code “Unauthorized”, make sure to fix that. Tests without authentication are very important, an API should authenticate every single request.

  2. Fuzz Testing

    It is one of the simple and common ways to test, simply send some unexpected value to the API and check if it breaks. It is similar to a black box testing technique which includes finding bugs using malformed data injection. If your API expects numbers in the input, try to send invalid values which should not be accepted, such as -1, 0, and large digit numbers. A badly coded application will accept any format, so in this way, you can check bugs in your application.

    Also, try sending SQL queries in a criterion where the API is expecting some innocuous value. An API will not run any SQL sent in a request.

  3. Command Injection

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

    If your API displays content via a URL, you can append an OS command to the end of the URL just to check if the command is executed on the server or not.

  4. Authorized Endpoints And Methods

    Authorization is also very important and we must ensure that an API should authorize every single request before processing it, because if the API reveals any sensitive data, it will allow the bad actors to make damaging actions.

  5. Input Validation

    APIs should define restrictions for inputs such as data types and ranges for example, a particular parameter could only be valid as an integer between 1 and 100. Always test outside the limitations and let your test confirm that they result in a 400 Bad Request error for any malicious input or incorrect input.

  6. API Security Best Practices

    In the application development phase, a lot of APIs are not tested to meet the security criteria, which means the API used in the application might not be secure.

    So, we have to ensure that your applications are functioning as expected with less risk potential for your data. So it is really important to test and ensure that your API is safe.

    Some most efficient API security practices following which you can strengthen your API security:

    • Use tokens: Tokens are unique identities, use trusted identities and then control access to services and resources.

    • Use encryption: Encrypt your data to ensure that the right users are decrypting and modifying your data, and no one else.

    • Identify vulnerabilities: Know how OS, network, and API components work together and identify weak spots that could be used to break into your APIs.

    • Use quotas and throttling: Make rules for throttling which helps in protecting your APIs from spikes and “Denial of Service” attacks. As more no of calls on an API may indicate that it is being abused.

    • Use an API gateway: Gateway acts as the major point of enforcement for API traffic. An effective gateway will allow you to authenticate traffic as well as control and analyze how your APIs are used.

    • Adopt a Zero-trust Philosophy: Generally, networks had a perimeter, and elements “inside” were trusted, while elements “outside” were not.

      A zero trust policy helps you ensure that APIs always authenticate users and applications whether inside or outside the perimeter, provide the least privileges they actually need to perform their roles, and closely monitor for anomalous behavior.

  7. Input Validation: For a given input an API should provide the expected output. User inputs should appear within a range and values crossing the range must be rejected. Any null input should be rejected when it is unacceptable. Incorrectly sized input must be rejected.



API security testing is very essential to secure important data from bad actors, API security testing ensures that the API used in the application is safe. In QASource we have a team of experts who are well-versed in security API testing techniques and our mission is to enable organizations to ship secure applications along with APIs. Visit QASource now to implement top-of-line software testing services for your software business.


This publication is for informational purposes only, and nothing contained in it should be considered legal advice. We expressly disclaim any warranty or responsibility for damages arising out of this information and encourage you to consult with legal counsel regarding your specific needs. We do not undertake any duty to update previously posted materials.