Aviral Srivastava

Posted on Sep 30

DOM-based XSS in-depth

#frontend #javascript #security

DOM-Based XSS: A Deep Dive

Introduction

Cross-Site Scripting (XSS) is a pervasive web security vulnerability that allows attackers to inject malicious scripts into web pages viewed by other users. While traditional XSS vulnerabilities rely on the server-side application to inject malicious code into the HTML response, DOM-Based XSS takes a different route. It exploits vulnerabilities within the Document Object Model (DOM) manipulation happening client-side, in the user's browser. This means the malicious payload doesn't even have to reach the server to execute, making it often trickier to detect and prevent than server-side XSS.

In essence, DOM-Based XSS occurs when a website's JavaScript code takes data from an untrusted source (e.g., URL parameters, user input within the page, cookies) and passes it directly to a DOM API in an unsafe manner, leading to the execution of arbitrary JavaScript code in the user's browser. This article will delve into the intricacies of DOM-Based XSS, covering its underlying mechanisms, attack vectors, mitigation strategies, and practical examples.

Prerequisites

To fully grasp DOM-Based XSS, a foundational understanding of the following concepts is essential:

HTML, CSS, and JavaScript: Familiarity with web development technologies is crucial. You need to understand how websites are structured, how styling is applied, and how JavaScript manipulates the DOM.
The Document Object Model (DOM): The DOM represents the structure of an HTML document as a tree-like structure. JavaScript uses the DOM to dynamically access and modify the content, structure, and style of a web page.
URL Parameters (Query Strings): Understanding how data is passed to a web page via the URL (e.g., example.com/page.html?param1=value1&param2=value2).
Cookies: How cookies store small pieces of data in the user's browser, accessible by the website.
JavaScript Functions: Knowledge of common JavaScript functions like document.write(), eval(), innerHTML, outerHTML, and location.hash.
Basic Security Principles: A general understanding of web security concepts, including the importance of input validation and output encoding.

How DOM-Based XSS Works

The core mechanism of DOM-Based XSS revolves around two key elements:

Source: The untrusted data source. This is where the malicious payload originates. Common sources include:
- document.URL: The entire URL of the current page.
- document.URLUnencoded: Similar to document.URL, but may not be encoded correctly.
- document.location.href: The full URL, including the protocol and hash.
- document.location.hash: The part of the URL following the # symbol (e.g., example.com/#section1).
- document.location.search: The query string portion of the URL (e.g., ?param1=value1).
- document.cookie: The cookies associated with the website.
- window.name: The name of the browser window.
- localStorage and sessionStorage: Browser-based storage mechanisms.
Sink: The DOM API that executes the malicious payload. This is where the vulnerability lies. Common sinks include:
- eval(): Executes a string as JavaScript code. This is almost always a dangerous sink.
- document.write(): Writes HTML code to the document.
- document.innerHTML: Sets the HTML content of an element.
- document.outerHTML: Sets the HTML content of an element, including the element itself.
- element.setAttribute(): Sets an attribute of an HTML element. Using this to set attributes like href, src, or event handlers like onclick can be dangerous.
- element.src: Sets the source URL of an <img> or <script> element.
- element.href: Sets the URL of an <a> element.
- location: Assigning a value to location or location.href can redirect the user to a malicious page.

The vulnerability exists when the data from the source is passed to the sink without proper sanitization or encoding.

Example Code and Explanation

Let's consider a simple example where a website displays a welcome message based on a URL parameter:

<!DOCTYPE html>
<html>
<head>
  <title>DOM-Based XSS Example</title>
</head>
<body>
  <h1>Welcome Message</h1>
  <div id="welcome"></div>

  <script>
    const urlParams = new URLSearchParams(window.location.search);
    const name = urlParams.get('name');
    document.getElementById('welcome').innerHTML = "Welcome, " + name + "!";
  </script>
</body>
</html>

In this code:

Source: window.location.search (specifically, the 'name' parameter).
Sink: document.getElementById('welcome').innerHTML.

An attacker could craft a URL like this:

example.com/index.html?name=<img src=x onerror=alert('XSS')>

When the browser renders this page, the JavaScript code extracts the malicious string and inserts it directly into the innerHTML of the welcome div. The <img> tag is rendered, but since src=x is an invalid image source, the onerror event handler is triggered, executing the alert('XSS') JavaScript code.

Categorizing DOM-Based XSS

DOM-Based XSS can be further categorized based on the location of the vulnerability:

Reflected DOM-Based XSS: The malicious payload is injected into the DOM through the URL. This is the most common type. (As shown in the example above)
Stored DOM-Based XSS: The malicious payload is stored somewhere (e.g., in localStorage, a database) and later retrieved and injected into the DOM.
Blind DOM-Based XSS: The attacker doesn't directly see the outcome of the attack. The payload is injected, and the attacker relies on external evidence (e.g., server logs, reports) to confirm success.

Advantages and Disadvantages of DOM-Based XSS

Advantages (from an Attacker's Perspective):

Difficult to Detect: Because the server isn't directly involved in injecting the malicious code, traditional server-side XSS filters are often ineffective.
Client-Side Attack: The attack happens entirely in the client's browser, potentially bypassing server-side security measures.
Stealthy: Attackers can often craft URLs that appear benign, making it harder to identify malicious activity.

Disadvantages (from an Attacker's Perspective):

User Interaction Required: Typically, the attacker needs to trick a user into clicking a malicious link or visiting a page with a crafted URL.
Browser-Specific: Some DOM-Based XSS vulnerabilities might be browser-specific, limiting the attack's effectiveness.

Mitigation Strategies

Preventing DOM-Based XSS requires a combination of techniques:

Input Validation: Strictly validate and sanitize all data received from untrusted sources (URL parameters, cookies, etc.). Use whitelists to allow only expected characters and formats.
Output Encoding: Encode data before inserting it into the DOM. Use context-aware encoding:
- HTML Encoding: For text that will be rendered as HTML (e.g., using innerHTML). Encode characters like <, >, &, " and '.
- JavaScript Encoding: For text that will be used within JavaScript code (e.g., when setting attribute values).
- URL Encoding: For data that will be used in URLs.
Avoid Dangerous Sinks: Minimize the use of dangerous sinks like eval(), document.write(), innerHTML, and outerHTML. If you must use them, ensure the data is thoroughly sanitized and encoded. Consider using safer alternatives like textContent or innerText when you only need to display plain text.
Content Security Policy (CSP): CSP is a powerful HTTP header that allows you to control the resources that the browser is allowed to load. Using CSP can help prevent the execution of malicious scripts injected into the page.
Trusted Types: A W3C specification that aims to prevent DOM-based XSS by only allowing trusted data to be used with DOM APIs. When Trusted Types is enabled, the browser rejects any attempt to assign a string to a DOM API without first converting it to a Trusted Type object.
Regular Security Audits: Conduct regular security audits of your web applications to identify and fix potential DOM-Based XSS vulnerabilities.
Use a Web Application Firewall (WAF): A WAF can help to detect and block malicious requests before they reach your web application.

Code Example with Sanitization (Mitigation)

Here's how you could modify the previous example to mitigate the DOM-Based XSS vulnerability:

<!DOCTYPE html>
<html>
<head>
  <title>DOM-Based XSS Example (Mitigated)</title>
</head>
<body>
  <h1>Welcome Message</h1>
  <div id="welcome"></div>

  <script>
    function escapeHTML(str) {
      return str.replace(/[&<>"']/g, function(m) {
        switch (m) {
          case '&':
            return '&amp;';
          case '<':
            return '&lt;';
          case '>':
            return '&gt;';
          case '"':
            return '&quot;';
          case "'":
            return '&#039;';
          default:
            return m;
        }
      });
    }

    const urlParams = new URLSearchParams(window.location.search);
    const name = urlParams.get('name');
    const sanitizedName = escapeHTML(name || ''); // Sanitize the name
    document.getElementById('welcome').innerHTML = "Welcome, " + sanitizedName + "!";
  </script>
</body>
</html>

In this improved code, the escapeHTML function encodes special characters, preventing the injection of malicious HTML tags. By calling escapeHTML before inserting the name into the innerHTML, we neutralize the XSS risk.

Conclusion

DOM-Based XSS is a subtle but dangerous web security vulnerability that arises from insecure handling of client-side data. Understanding the sources and sinks involved, along with implementing proper input validation, output encoding, and adopting security measures like CSP and Trusted Types are crucial steps in preventing DOM-Based XSS and protecting users from malicious attacks. Continuous vigilance and regular security audits are essential to ensure the ongoing security of web applications.

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DOM-based XSS in-depth

DOM-Based XSS: A Deep Dive

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