Understanding Cross-Site Scripting (XSS)

Cross-site scripting, commonly abbreviated as XSS, represents one of the most widespread web security vulnerabilities. Despite being well-understood and having straightforward solutions, XSS continues to plague web applications because it requires constant vigilance across every point where user data meets HTML output.

What Makes XSS Possible

XSS vulnerabilities arise when applications include untrusted data in their HTML output without proper encoding. Just as SQL injection allows attackers to break out of data context and inject SQL commands, XSS allows attackers to break out of data context and inject JavaScript code that the victim's browser will execute.

Consider a search page that displays "You searched for: [user input]." If the application simply echoes back whatever the user entered without sanitization, an attacker can submit a search containing JavaScript code. When other users visit the results page, their browsers dutifully execute the injected script, not knowing it came from an attacker rather than the legitimate website.

The damage potential is significant. JavaScript running in the context of a page has access to all the data and functionality available to legitimate scripts. It can read cookies (unless they're marked HttpOnly), submit forms, make API calls, and fundamentally impersonate the user. An XSS attack can steal session tokens, capture credentials, redirect users to phishing sites, or modify page content to manipulate user behavior.

Types of XSS Attacks

Security professionals categorize XSS into three main types, each with distinct characteristics and exploitation requirements.

Reflected XSS occurs when malicious input is immediately echoed back in the response. The classic example involves a search query that appears directly in the results page. The attacker crafts a malicious URL containing their script and tricks victims into clicking it. Since the attack requires victim interaction with a specially crafted link, reflected XSS typically requires social engineering for successful exploitation.

Stored XSS is more dangerous because the malicious script becomes permanently embedded in the application's data. Comments sections, user profiles, and forum posts commonly fall victim to stored XSS. Once an attacker successfully injects their script, it executes for every user who views the affected content, potentially reaching thousands of victims without any additional attacker effort.

DOM-based XSS differs from the other types because the vulnerability exists entirely in client-side JavaScript. The malicious payload never reaches the server; instead, client-side scripts read data from the URL or other sources and unsafely insert it into the page. This makes DOM XSS harder to detect with server-side security tools but follows the same fundamental pattern of unsanitized data reaching executable context.

The Browser's Role

Understanding browser behavior is crucial for effective XSS exploitation and prevention. Browsers parse HTML documents and execute any JavaScript they encounter, whether that JavaScript was written by the site's legitimate developers or injected by an attacker.

Different contexts require different exploitation approaches. JavaScript can appear in script tags, event handlers, URLs with javascript: schemes, and various HTML attributes. Each context has its own parsing rules and requires tailored payloads. What works in a script tag might fail in an attribute context, and vice versa.

Modern browsers implement some XSS protection through features like Content Security Policy, which allows sites to restrict what scripts can execute. However, these protections require explicit implementation by developers and don't catch all attack vectors. Defense in depth remains essential.