What Is Cross-Site Scripting (XSS) and How to Prevent It

Cross-site scripting, commonly known as XSS, is one of those web security terms you may have heard often but rarely see explained in plain language. It’s a vulnerability that allows attackers to inject malicious code into trusted websites, which then runs in a user’s browser without them realizing it.

In this guide, we’ll break down what cross-site scripting is, how XSS attacks work, and why they remain a common security issue today. You’ll also learn about the different types of XSS, real-world examples of how it’s been exploited, and the practical steps developers and businesses can take to prevent it.

What is cross-site scripting (XSS)?

Cross-site scripting (XSS) is a web security vulnerability that lets attackers run their own code inside someone else’s browser. It happens when a website accepts user input and sends it back to the page without properly checking or cleaning it first. Since the code appears to come from a trusted site, the browser treats it as safe and runs it.

• Also Read: What is Bridge Mode on Modem & Router?

The problem isn’t the browser itself, but the trust it places in websites. When that trust is abused, an attacker can use XSS to do things like access session data, perform actions on a user’s behalf, or redirect them to malicious pages. In simple terms, XSS breaks the boundary between a website and its users by letting untrusted code act as if it belongs there.

How does an XSS attack work?

At its core, XSS works by slipping malicious code into a place where a website expects normal user input. The attack doesn’t rely on breaking into a server or exploiting the browser itself but rather takes advantage of how websites handle and display data. 

Here’s what a typical XSS attack looks like:

1. An attacker submits input: The attacker places harmful code into a field or link that the website accepts, such as a search box, a form, or a URL parameter.

2. The website outputs it unsafely: If the site doesn’t properly handle that input before displaying it, the code can be included in the page as if it were normal content.

3. A user loads the page: When someone opens the affected page or clicks the link, their browser loads it from a site they trust.

4. The code runs in the browser: As it appears to be part of a trusted page, the browser executes it, allowing the script to interact with the page in unintended ways.

Types of cross-site scripting 

Not all XSS attacks work the same way. The key difference comes down to where the malicious code lives and how it reaches the user’s browser. The three most common types are outlined below:

Reflected XSS

Reflected XSS occurs when malicious input is sent to a website and immediately returned in the response. This often happens through links, search queries, or form submissions. When a user clicks a specially crafted link or submits certain input, the site reflects it back without handling it safely, causing the browser to run the injected code.

• Read: What is a Brute Force Attack and how to prevent?

Stored XSS

Stored XSS occurs when malicious code is saved by a website, often in places like comment sections, user profiles, or message boards. Every time someone views the affected page, their browser loads and runs that code, making stored XSS especially dangerous because it can impact many users over time.

DOM-based XSS

DOM-based XSS occurs entirely in the browser. Instead of unsafe data coming from the server’s response, the issue arises when a website’s own JavaScript takes user-controlled data and inserts it into the page without proper handling. The server may never see the malicious input, but the browser still ends up running it.

Real-world examples of XSS attacks

XSS vulnerabilities have appeared on real platforms in the past, showing how easily malicious code can spread when input isn’t handled safely:

MySpace (2005)

One of the most well-known stored XSS incidents occurred on MySpace in 2005. An XSS vulnerability in profile pages allowed a user to store malicious JavaScript that automatically added itself to other users’ profiles when they viewed it. Because the code was saved on the site and executed every time a profile was loaded, it spread rapidly across the platform, affecting millions.

eBay (2014)

Security researchers identified reflected XSS vulnerabilities on eBay where malicious JavaScript was injected through specially crafted URLs. When users clicked these links, the injected code was reflected in the page response and executed in their browser. The issue was commonly abused to redirect users to phishing pages without storing any malicious code on eBay’s servers.

How to prevent cross-site scripting

Preventing XSS comes down to how a website handles user-controlled data at every stage. There isn’t a single switch that fixes everything, but a set of practices that work together to stop untrusted input from becoming executable code:

Avoid allowing raw HTML in user input

An effective way to limit XSS risk is to prevent users from submitting raw HTML through form inputs. Where rich content is needed, alternatives like markdown or carefully configured WYSIWYG editors can be used to control what markup is allowed. This reduces the chances of malicious scripts being embedded and saved.

Validate user input

Input validation ensures that data submitted by users matches what the application expects. For example, a field meant for a last name should only accept appropriate characters. Validation rules can also reject input containing unexpected tags or characters commonly used in script injection, helping stop malicious data early.

Sanitize data before displaying it

Sanitization removes or neutralizes unsafe elements from user input before it is shown to others. Even if data passes validation, sanitizing it before output adds another layer of protection by filtering out scripts or markup that could be interpreted as executable code.

Encode output properly

Output encoding is one of the most important defenses against XSS. Instead of trusting stored or processed data, applications should encode user input at the point where it is displayed. That way, special characters are treated as text rather than code, preventing scripts from executing in the browser.

Use Content Security Policy (CSP)

A Content Security Policy helps limit the damage XSS can cause by restricting where scripts are allowed to load from and whether inline scripts can run. Even if an XSS vulnerability exists, CSP can prevent injected scripts from executing or accessing sensitive resources.

Apply secure cookie settings

Cookies should be configured with security attributes that reduce their exposure to XSS. Using the HttpOnly flag prevents JavaScript from accessing cookies, while the Secure flag ensures cookies are only sent over HTTPS. The SameSite attribute can further limit how cookies are shared across requests.

Handle DOM updates safely

To reduce the risk of DOM-based XSS, client-side JavaScript should avoid inserting user-controlled data directly into the page as HTML. Using safer APIs and ensuring data is treated as text rather than markup helps prevent scripts from executing entirely within the browser.

Testing and detecting XSS

Testing for XSS focuses on identifying places where user-controlled data is handled in an unsafe way. The goal isn’t just to catch obvious issues, but to spot patterns that could allow malicious scripts to run in a browser:

Review how input is handled and displayed

One of the most effective ways to detect XSS risks is through code reviews that focus on how user input flows through an application. Any place where data is accepted from users and later displayed back in a page should be examined to ensure proper validation, sanitization, and output encoding are in place.

Use automated security testing

Automated security testing tools can help scan applications for common XSS patterns, such as unsafe input handling or missing output encoding. These tools are useful for catching known issues early, especially in large or frequently updated codebases, but they work best when combined with manual review.

Test during development and updates

XSS vulnerabilities more often than not appear when new features are added or existing ones are changed. Testing for XSS as part of regular development and release cycles helps in identifying and fixing problems before they reach production, reducing the risk of issues being exposed to users.

Frequently asked questions