Cross-Site Scripting (XSS) is a security flaw in web applications that allows attackers to interfere with the way users interact with a website. Essentially, it enables attackers to bypass the “same-origin policy,” which is meant to isolate websites from each other. When a website is vulnerable to XSS, attackers can impersonate users, perform actions on their behalf, and access sensitive data. If the compromised user has administrative privileges, attackers could potentially take full control over the website.
How XSS Works
XSS attacks occur when a website improperly handles user-supplied data and returns it to the browser without proper validation or sanitization. The malicious code (usually JavaScript) runs in the victim’s browser, allowing the attacker to manipulate the user’s session and actions on the website.
Demonstrating XSS
A common way to test for XSS is by inserting a small JavaScript snippet, such as an alert, into a vulnerable field. For example:
alert('XSS Test');
This displays a popup in the browser when executed. In modern browsers like Chrome (version 92 and above), some contexts such as cross-origin iframes block alert(), so alternatives like print() may be used in testing.
Types of XSS Attacks
XSS vulnerabilities generally fall into three categories:
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Reflected XSS – The malicious code comes from the current HTTP request.
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Stored XSS – The attacker’s input is saved on the server and served to other users later.
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DOM-based XSS – The vulnerability exists in client-side code that processes untrusted data unsafely.
Reflected XSS
Reflected XSS occurs when user input from an HTTP request is returned immediately in a webpage without proper sanitization.
Example:
https://example.com/status?message=Hello
<p>Status: Hello</p>
An attacker could manipulate this to inject malicious scripts:
https://example.com/status?message=<script>/* Malicious code */</script>
<p>Status: <script>/* Malicious code */</script></p>
When a user clicks this URL, the injected script runs in their browser.
Stored XSS
Stored XSS, also called persistent XSS, happens when unsafe input is saved on the server and later displayed to other users. Common sources include:
- Comments on blogs
- User profiles
- Chat messages
<p><script>/* Malicious code */</script></p>
If a website displays this message without filtering, every visitor could execute the attacker’s script.
DOM-based XSS
DOM-based XSS arises when client-side JavaScript manipulates untrusted data and injects it back into the page’s DOM.
Example:
var search = document.getElementById('search').value;
document.getElementById('results').innerHTML = 'You searched for: ' + search;
An attacker could input:
<img src="1" onerror="/* Malicious code */">
This runs the malicious code in the user’s browser.
What Can Attackers Do With XSS?
XSS attacks can allow an attacker to:
- Impersonate other users
- Perform actions on their behalf
- Access private data
- Steal login credentials
- Deface websites virtually
- Inject harmful scripts
- The actual impact depends on the application and the user’s privileges.
Detecting XSS Vulnerabilities
Tools like Burp Suite can scan for most XSS issues automatically. Manual testing involves:
- Injecting unique strings in input fields
- Observing where the input appears in responses
- Testing execution of JavaScript to confirm vulnerabilities
- For DOM-based XSS, inspecting client-side code is often necessary to identify unsafe data handling.
Preventing XSS
Effective XSS prevention usually combines several approaches:
- Input Filtering – Validate and sanitize user inputs strictly.
- Output Encoding – Encode data before rendering to prevent execution.
- Secure Headers – Use Content-Type and X-Content-Type-Options headers.
- Content Security Policy (CSP) – Limits the types of scripts a page can execute.
Language-specific examples:
- PHP: Use whitelists for inputs and htmlentities() for output.
- Java: Whitelist input and encode output using libraries like Google Guava.
Common Questions About XSS
How common are XSS vulnerabilities?
Very common; they are among the most frequently found web security issues.
How does XSS differ from CSRF?
XSS injects malicious scripts into a website; CSRF tricks users into performing unintended actions.
How does XSS differ from SQL injection?
XSS targets users via the browser; SQL injection targets the server and database.
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