The Role of Functional Testing in Application Security

Mayhem Team
January 26, 2022
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Application Security Testing (AST) is a vital component of the software development process. It ensures that applications are built to specification and can be used reliably in production environments. This article explores one type of software testing called functional testing. Functional Testing is an important part of application security as it verifies that features work as intended without exposing sensitive information or attack vectors to hackers. 

This article will discuss the role of functional testing in Application Security and provide guidance for executing this type of test efficiently.

What is Functional Testing?

Functional testing is a software testing technique that verifies the features of a software program. The purpose of functional testing is to ensure that the features of the software work as intended. This type of testing ensures that the software functions properly and meets the requirements specified by the user. Functional testing is primarily black box and as such is not concerned with the source code of the application. 

Importance of Functional Testing in AppSec

Functional testing is one of the most important aspects of Application Security. functional testing verifies that the features of the software work as intended, which is essential for ensuring the security of an application. Functional testing is an important part of the software development life cycle and should be carried out early in the development cycle to ensure that the application is secure. 

Benefits of a Successful Functional Test

functional testing in code

The benefits of a successful functional test are as follows:

  • The software functions as intended and meets the requirements specified by the user.
  • The software is free of functional errors and vulnerabilities.
  • The software is stable and reliable which ensures optimum functional performance.
  • Compliance with security policies and a high level of rigor in Application Security Testing.

Functional Testing vs. Fuzzing

Functional testing differs from fuzzing in that functional testing primarily verifies that the functional requirements of the software are met while fuzzing checks for functional errors in the software. Fuzzing is an exploration engine that continuously tries out different combinations of input to trigger different behaviors and errors can be raised if these behaviors fall outside the specification of the application. The errors do not need to be just security vulnerabilities, they can also be violations of the specification.

A benefit of using an advanced fuzzer such as Mayhem is that it extends forms of functional testing by parameterizing the test within an array of bytes and then searching for strings of input bytes that trigger bugs. Better yet, developers can execute a fuzz test case in less time than is typically required to write individual functional unit tests. Additionally, fuzzing tools like Mayhem can easily be set up for automation testing, easing the load for developers and supporting continuous integration. 

Functional Testing vs. Non functional Testing

Functional testing differs from non-functional testing in that functional testing focuses on verifying the functional requirements of software while non-functional testing serves for performance testing and checks for performance problems, usability issues, and other issues that are not directly related to functional use cases.

Types of Functional Tests

types of functional tests as doors

There are several types of functional testing that can be used to test the features of a software application. These include:

Unit Testing

Unit Testing is used to test individual units or components of a software program. This type of testing is important because it helps to identify and fix errors in the code before they can be exploited. A unit test is usually carried out by the developers who wrote the code and helps to ensure the quality of the code.

The main objective of unit testing is to verify that individual units of code are working as intended. This type of testing is done by verifying the output of each unit against the expected output. Unit tests should be easy to write and run, and should be reliable and fast.

Component Testing

Component testing, also known as module testing, is a type of software testing that is used to test individual components or modules of a software program. Component testing is usually carried out by the developers and typically takes place after unit testing.

The main objective of module testing is to verify that individual units of code are working as intended. This type of testing is done by verifying the output of each unit against the expected output. 

Smoke Testing

Smoke testing is designed to identify most of a software's common functional and integration issues with minimal development effort. The name comes from electronics where it is used to catch gross failures of components before they are installed into a system for a final functional check. In software development, it is the process of ensuring new code performs correctly before it is released to production. A failure of the smoke test case will result in the code being sent back to the initial development stage.

Integration Testing

Integration testing also referred to as functional interface testing, is a type of software testing that involves functional testing of integrated parts of an application. This type of testing is usually done after unit testing and system testing.

The goal of integration testing is to find errors that occur when different parts of an application are combined. These errors can be caused by incorrect data, mismatched functions, or incorrect sequencing of events. An example of functional integration tests would be the functional tests performed after a new module is integrated with an existing application.

Regression Testing

Regression testing re-runs functional and non-functional tests to ensure that previously developed and tested software still performs after new code commits are submitted.

In the field of application security, functional testing is an important method for validating that newly implemented functions do not introduce new vulnerabilities to existing code that had been previously secured. Failing a regression test would mean a feature added to the software has inadvertently resulted in a new bug or vulnerability.

Sanity Testing

Sanity Testing is a non-exhaustive test that confirms whether the functionalities of an application are working as expected in some basic scenarios, without going into details of validation and egress functionalities. It follows the logic that if a basic function fails, more advanced functions that follow will also fail. Sanity testing falls under the category of regression testing. 

Negative Testing

Negative testing is a black-box testing technique that is used to identify errors by simulating invalid or unexpected inputs to the system. It can be used to test both functional and non-functional requirements. In contrast to positive testing, which focuses on verifying that the system works as expected, negative testing tries to find ways to break the system.

API Testing

API Testing is a functional testing approach where the Application Programming Interface(API) is tested by making requests using a computer program to check for functional defects. It is often used in application security and functional tests to verify that a software or web product performs according to its specifications.

User Acceptance Testing 

User acceptance testing is another functional testing method that is meant to be done on the application after the previous functional tests have been completed and before it is handed off for release. 

The goal of functional testing is to answer the question "are we building the right thing?" while UAT's goal is to answer the question "are we building this right?"

Executing a Successful Functional Test

successful functional test shown as a bullseye

When it comes to functional testing, there are a few key things that you need to keep in mind in order to ensure successful execution. 

Understand Requirements

First, it's important to have a good understanding of the business requirements and what is expected from the application. This will help you to focus your testing efforts on the right areas. 

Cover All Scenarios

Second, you need to have a good set of test cases that cover all of the scenarios that could be possible. This will help you to ensure that all aspects of the application are tested. 

Secure Resources

Third, you need to make sure that you have adequate resources to complete the testing. This includes both people and tools. 

Controlled Environment

Fourth, you need to make sure that test case and environment are set up properly and that functional testing can be performed in a controlled environment. 

Test Automation

Lastly, you need to make sure that functional tests are repeatable and can be executed as automated tests in order for them to run on a continuous basis.


Functional testing is a crucial step in the software development life cycle. It helps ensure that newly implemented functionalities do not introduce new functional defects and vulnerabilities to existing code and serve as a quality assurance measure for software developement.

When executing functional tests, it's important to understand requirements, cover all scenarios, secure resources, establish a controlled environment and automate functional testing as much as possible so it can run on a continuous basis. By doing this you will be able to execute successful functional tests and ensure that newly developed applications do not introduce new functional or application security defects. 

You can learn more about Application Security Testing and fuzzing by exploring our blog and resources pages.

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