What is Cross-Site Scripting (XSS) With Examples For Beginners?

Introduction

These days, it’s far more accurate to think of websites as online applications that execute a number of functions, rather than the static pages of old. Much of this robust functionality is due to widespread use of the JavaScript programming language. While JavaScript does allow websites to do some pretty cool stuff, it also presents new and unique vulnerabilities — with cross-site scripting (XSS) being one of the most significant threats.

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What is Cross-Site Scripting (XSS)?

Cross-site scripting, commonly referred to as XSS, occurs when hackers execute malicious JavaScript within a victim’s browser.

Unlike Remote Code Execution (RCE) attacks, the code is run within a user’s browser. Upon initial injection, the site typically isn’t fully controlled by the attacker. Instead, the bad actor attaches their malicious code on top of a legitimate website, essentially tricking browsers into executing their malware whenever the site is loaded.
The Use of JavaScript in Cross-Site Scripting

The use of JavaScript in Cross-Site Scripting (XSS) attacks is a significant security threat because JavaScript is a powerful scripting language that is commonly used on the web. The following are some of the key points and examples of how JavaScript can be used in XSS attacks:

1. Stealing Sensitive Information: Attackers can use JavaScript to steal sensitive information, such as login credentials, from unsuspecting victims. For example, an attacker can inject a malicious JavaScript code into a web page that captures the user's login credentials and sends them to the attacker's server.


2. Altering the Appearance of a Web Page: Attackers can use JavaScript to alter the appearance of a web page and present false information to victims. For example, an attacker can inject a malicious JavaScript code into a web page that changes the displayed text or images to something misleading or harmful.


3. Redirecting Victims to a Malicious Site: Attackers can use JavaScript to redirect victims to a malicious site, where they can be tricked into downloading malware or revealing sensitive information. For example, an attacker can inject a malicious JavaScript code into a web page that redirects the victim to a fake login page where the attacker can capture the user's login credentials.

These are just a few examples of how JavaScript can be used in XSS attacks. It is important for web developers to understand the risks associated with using JavaScript and to implement appropriate mitigation techniques, such as input validation, encoding, escaping, and strict mode, to prevent XSS attacks.

How Do Cross-Site Scripting Attacks Work?

Cross-Site Scripting (XSS) attacks are a type of security vulnerability that allows an attacker to inject malicious code into a web page viewed by other users. This code can be used to steal sensitive information such as login credentials or other sensitive data, or to launch further attacks on the website and its users.

Here's how XSS attacks work:

• A vulnerable website: The attacker finds a website that allows user input to be displayed on the page without proper validation or sanitization.

• Injection of malicious code: The attacker then injects malicious code, often in the form of a script, into a web page. This code can be inserted into the website through a variety of means, including search fields, comments, contact forms, and other user input fields.

• Code execution: When a user visits the infected web page, the malicious code is executed in their browser, allowing the attacker to steal sensitive information or launch further attacks.

• Data theft: The attacker can steal sensitive information such as login credentials, personal information, and more, and use it for malicious purposes such as identity theft or financial fraud.

To prevent XSS attacks, it's important to validate and sanitize all user input, and to encode special characters in input fields that may be interpreted as code. Additionally, keeping software and browser plugins up-to-date can help protect against XSS attacks, as well as staying informed about the latest security threats and best practices for web security.

What Are the Types of Cross-Site Scripting Attacks?

1) Stored (Persistent) Cross-Site Scripting

(also known as Persistent Cross-Site Scripting) is a type of security vulnerability that occurs when an attacker is able to inject malicious code into a web page that is later served to unsuspecting users.

For example, consider a website that allows users to post comments on articles. If the website does not properly validate user input, an attacker could submit a comment containing malicious JavaScript code, such as the following:

(script payload.jpg)

 

If the comment is approved and posted, the malicious JavaScript code will be executed in the browser of any user who visits the page containing the comment. When the user visits the page, the browser will execute the JavaScript code and display a pop-up message that reads "You've been hacked!".

This simple example demonstrates the potential dangers of stored XSS attacks. In reality, attackers could use more sophisticated techniques to steal sensitive information, install malware, or take control of a user's machine. To prevent stored XSS attacks, it is important to properly validate user input and sanitize any data that will be displayed on a web page. Additionally, input validation and output encoding should be implemented on both the client and server sides.

2 ) Reflected (Non-Persistent) Cross-Site Scripting

Reflected (Non-Persistent) Cross-Site Scripting, also known as Non-Persistent XSS, is a type of web security vulnerability that occurs when user input is directly included into a web page without proper validation or sanitization. The term "reflected" refers to the fact that the malicious payload is delivered via a request to the website, and immediately reflected back to the user's browser.

Here's an example of a simple Reflected XSS attack:

1) The attacker creates a malicious payload, for example, a JavaScript code, and embeds it in a URL that is sent to a vulnerable website. The payload could look something like this:

Reflected XSS.jpg

2. When a user clicks on the URL, the malicious payload is sent to the website, which includes the payload in the search results page without proper validation or sanitization.


3. The user's browser then renders the malicious JavaScript code, which is executed and displays an alert pop-up with the text "XSS".

This is just a simple example, but in a real-world scenario, a malicious attacker could use XSS to steal sensitive information, such as login credentials, or to execute harmful actions, such as deleting data or defacing a website.

It's important for web developers to take XSS vulnerabilities seriously and to properly validate and sanitize all user input to prevent such attacks.

DOM-Based Cross-Site Scripting

DOM-Based Cross-Site Scripting (XSS) is a type of security vulnerability that occurs when a malicious script is executed on a web page through the Document Object Model (DOM), instead of the server. It allows attackers to inject malicious scripts into web pages viewed by other users, which can steal sensitive information or perform malicious actions.

Here is an example to demonstrate DOM-based XSS:

Consider a web page that displays a search query entered by a user in the URL after processing. For example, if the user searches for "books," the resulting URL might look like this:

If the web page does not properly sanitize the user-supplied input before using it to generate the page content, an attacker could inject a malicious script into the search query. For example, the attacker could enter the following search query:

If the web page displays the search query directly in the page without proper sanitization, the malicious script would be executed in the browser of any user who visits the resulting URL.

To prevent DOM-based XSS attacks, it is important to properly sanitize all user-supplied input before using it in the web page. This can be achieved by encoding special characters and filtering out any malicious scripts. Additionally, it is recommended to use a Content Security Policy (CSP) to restrict the types of scripts that can be executed on a web page.

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