What Is Input Validation and Why Is It Important?

user input login details

Websites and apps often require user input in order to fulfill their full purpose. If a website allows users to subscribe, the users need to be able to provide their email address. And online shopping obviously requires the input of payment details.

The problem with user input, however, is that it also allows a hacker to enter something malicious in an attempt to trick the website or app into misbehaving. In order to protect against this, any system that offers user input needs to have input validation.

So what is input validation and how does it work?

What Is Input Validation?

Input validation is the process of analyzing inputs and disallowing those which are considered unsuitable. The idea behind input validation is that by only allowing inputs that meet specific criteria, it becomes impossible for an attacker to enter an input designed to cause harm to a system.

Input validation should be used on any website or application that allows user inputs. Even if a website or app doesn’t store any confidential information, allowing invalid inputs can also cause user experience issues.

Why Is Input Validation Important?

Man holding laptop that says you've been hacked

Input validation is important for two reasons, namely user experience and security.

User Experience

Users often enter invalid inputs, not because they are trying to attack a website or app, but because they made a mistake. A user might spell a word incorrectly, or provide the wrong information, such as entering their username in the wrong box or trying an outdated password. When this happens, input validation can be used to inform the user of their error, allowing them to rectify it quickly.

Input validation also prevents scenarios where sign-ups and sales are lost because a user is not informed and so believes that correct input has been provided when it has not.


Input validation prevents a wide range of attacks that can be performed against a website or application. These cyberattacks can cause the theft of personal information, allow unauthorized access to other components, and/or prevent a website/application from functioning.

The omission of input validation is also easy to detect. Attackers can use automated programs to enter invalid inputs on websites in bulk and determine how the websites react. They can then launch manual attacks on any websites that aren’t protected.

This means that a lack of input validation is not only an important vulnerability; it’s a vulnerability that will often be found and so can often cause hacks.

What Are Input Validation Attacks?

Image of a hacker's mask

An input validation attack is any attack that involves adding malicious input into a user input field. There are many different types of input validation attacks that attempt to do different things.

Buffer Overflow

A buffer overflow occurs when too much information is added to a system. If input validation is not used, there’s nothing stopping an attacker from adding as much information as they want. This is called a buffer overflow attack. It can cause a system to stop functioning and/or delete information that is currently stored.

SQL Injection

SQL injection is the process of adding SQL queries to input fields. It might take the form of adding an SQL query to a web form or appending an SQL query to a URL. The goal is to trick the system into executing the query. SQL injection can be used to access secure data and to modify or delete data. This means that input validation is particularly vital for any website or app which stores important information.

Cross-Site Scripting

Cross-site scripting involves adding code to user input fields. It is often carried out by adding code onto the end of a URL belonging to a reputable website. The URL can be shared through forums or social media and the code is then executed when a victim clicks on it. This creates a malicious webpage that appears to be hosted on the reputable website.

The idea is that if a victim trusts the target website, they should also trust a malicious webpage that appears to belong to it. The malicious webpage can be designed to steal keystrokes, redirect to other pages, and/or begin automatic downloads.

How to Implement Input Validation

Input validation is not difficult to implement. You simply need to figure out what rules are required to prevent invalid input and then add them to the system.

Write All Data Inputs

Create a list of all possible user inputs. This will require you to look at all user forms and consider other types of input such as URL parameters.

Create Rules

Once you have a list of all data inputs, you should create rules that dictate what inputs are acceptable. Here are a few common rules to implement.

  • Whitelisting: Only allow specific characters to be input.
  • Blacklisting: Prevent specific characters from being input.
  • Format: Only allow inputs which adhere to a particular format, i.e. only allow email addresses.
  • Length: Only allow inputs of up to a certain length.

Implement Rules

In order to implement the rules, you will have to add code to the website or application that rejects any input that doesn’t follow them. Due to the threat posed by invalid inputs, the system should be tested prior to going live.

Create Responses

Assuming that you want input validation to also help users, you should add messages that explain both why an input is incorrect and what should be added instead.

Input Validation Is a Requirement for Most Systems

Input validation is an important requirement for any website or app that allows user input. Without controls over what input is added to a system, an attacker has a range of techniques that can be used for hacking purposes.

These techniques can crash a system, alter it, and/or allow private information to be accessed. Systems without input validation become popular targets for hackers and the internet is constantly being searched for them.

While input validation is primarily used for security purposes, it also plays an important role in telling users when they add something incorrectly.

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