Web Application Vs. API testing: Understand The Difference

Web Application API testing

In the world of software development and testing, two distinct types of testing that are often confused are web application testing and API testing.

While both play a crucial role in ensuring the quality and security of applications, they have different focuses and methodologies. In this communication, we’ll delve into the key differences between web application testing and API testing.


Web application testing involves evaluating the functionality and security of a web-based application. It focuses on testing the application from both a user and administrator’s perspective by interacting with the applications user interface and back end, whilst validating its behaviour and responses.

During web application testing, the penetration tester would perform a series of tests in order to identify vulnerabilities. These tests may include, but are not limited to, identifying application logic flaws; attempting to exploit injection vulnerabilities (such as Cross-Site Scripting (XSS) or SQL Injection (SQLi)), as well as authentication and authorisation testing.

The primary goal of web application testing is to identify and evaluate security vulnerabilities and weaknesses from an attacker’s perspective. Whether this be unauthenticated (simulating that of a zero knowledge attacker) or authenticated (simulating an insider attack from a user perspective).


API (Application Programming Interface) penetration testing, on the other hand, primarily focuses on evaluating the security functionality of APIs. APIs act as a bridge, allowing different software components to communicate and exchange data. Penetration testing against this technology analyses their inputs, outputs, and behaviours.

Typically, during API testing, the penetration tester investigates the security of data transmission, error handling, authorisation, and authentication mechanisms. These, among many other tests, ensure that APIs are as secure as possible, and are not exposing themselves to injection vulnerabilities or authentication issues.

API penetration testing aims to uncover security flaws that could allow unauthorised access to sensitive data, API abuse, injection attacks, or improper handling of responses.

The main differences between web application and API penetration testing are:

Client vs. Server-Side:
Web application testing will always include elements of assessing client-side code, due to the nature of the test. API testing is always more focused on the server-side functions.
Attack Surface:
Whilst web applications generally have a full functionality UI, APIs in general do not. This ultimately changes the methods in how each application element is tested.
Common Vulnerabilities:
Whilst not mutually exclusive of each other, generally, web applications can suffer more with issues such as XSS or Clickjacking, whilst APIs generally suffer with more XML/JSON injection issues.

Web application testing and API testing are distinct but complementary testing approaches. Depending on the web application, some elements may be integrated with an API, resulting in both being tested as part of a web application test. However, a dedicated API, used for machine to machine communication, would require direct testing on its own.

In conclusion, understanding the differences between Web Application Testing and API Testing is crucial for build- ing a robust cybersecurity strategy. While both assessments share the common goal of identifying vulnerabilities, they approach security from distinct angles. Web Application Testing delves into user interfaces, client-side functionalities, and user interactions, targeting potential risks faced by end-users. On the other hand, API Testing focuses on server-side operations, data exchange, and securing communication between servers, as APIs facilitate seamless integration between various applications and systems.

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