What DOM-Based XSS Is and How to Prevent It

JavaScript powers modern web applications, making them dynamic and interactive. But it also opens the door to vulnerabilities. DOM cross-site scripting (XSS) occurs entirely within the browser, allowing malicious scripts to execute without ever hitting the server.

Because traditional security tools frequently overlook these threats, developers must understand how they work and what it takes to stop them. Here’s a guide to DOM-based XSS, including real-world examples and the best ways to prevent it.

What Is DOM-Based XSS?

DOM-based XSS cyberattacks happen in a webpage’s Document Object Model (DOM) rather than through server responses. This means the vulnerability occurs when JavaScript processes and modifies the page’s DOM based on user input, allowing attackers to inject and execute malicious scripts directly in the browser.

Unlike reflected XSS, which involves the server sending back manipulated content, DOM-based XSS does not require a server interaction once the page loads. The vulnerability typically stems from insecure handling of dynamic content.

For example, if a website includes JavaScript that reads URL parameters and writes them into the page without validation, an attacker can craft a malicious link that executes arbitrary JavaScript when clicked. Since the script executes entirely on the client side, tools like web application firewalls (WAFs) may not detect it. This makes DOM-based XSS a serious web application security vulnerability that requires proactive mitigation measures.

DOM-based XSS has been used in phishing campaigns, account takeovers, and drive-by malware downloads. Attackers often leverage iframes and third-party scripts to enhance their payloads, making them more effective and difficult to detect.

How Do DOM-Based XSS Attacks Work?

A DOM-based XSS attack follows a source-to-sink flow. An attacker injects malicious input into a source, which is then processed by JavaScript and sent to an execution sink without proper validation.

A source is where user-controlled data enters the application, while a sink is where that data is used or executed. If input moves from a source to a sink without validation, it can trigger malicious code. Since the attack occurs entirely on the client side, it can bypass existing security measures focusing on server-side threats.

Here’s how the attack works:

1. User interaction: The attacker tricks a user into clicking a malicious link that contains an injected payload in the query string or fragment identifier.
2. JavaScript processing: The web application’s client-side script extracts data from an attacker-controlled source and writes it into the DOM.
3. Execution at the sink: The malicious input reaches a sink, allowing the injected script to run in the victim’s browser.
4. Impact: Once executed, the attacker can steal session cookies, perform actions on behalf of the user, or modify page content to facilitate phishing attempts.

Example of DOM-Based XSS

A DOM-based cross-site scripting example appears in applications that dynamically modify web page content based on URL parameters without proper sanitization. Consider a website that customizes its greeting message based on a URL parameter. The site’s JavaScript reads the username parameter from the URL and displays it dynamically on the page.

At first glance, this seems harmless. However, because the application directly inserts user input into the DOM without sanitization, an attacker can craft a malicious URL. When an unsuspecting user clicks the link, their browser executes the injected script, displaying an alert or potentially stealing sensitive data.

Attackers may also use eval(), a dangerous JavaScript function that executes arbitrary code, to further exploit vulnerabilities. Instead of using eval(), you should use safer alternatives like JSON.parse() to process dynamic content securely.

While an alert box is a common example of XSS vulnerabilities, real-world attackers take this further. A malicious script can steal authentication cookies, log keystrokes, or inject fake login forms to trick users into entering their credentials.

How to Detect and Test For DOM-Based XSS

Since DOM-based XSS happens entirely in the browser, traditional server-side security tools often miss it. Detection requires analyzing how JavaScript interacts with user input and the DOM.

Here are some ways you can detect and test for DOM-based XSS vulnerabilities:

Browser Developer Tools

Most modern browsers, including Chrome, Firefox, and Edge, offer developer tools that allow you to inspect JavaScript execution. In Chrome and Edge, use the Developer Console in Developer Tools to analyze JavaScript behavior, while in Firefox, use the Web Console and Debugger under Web Developer Tools to track how input is processed.

Manual Testing for HTML Sinks

Insert a random string into a source (like location.search) and inspect the href attributes within the DOM to see if user input is improperly reflected.

Testing JavaScript Execution Sinks

Unlike HTML sinks, execution sinks don’t visibly reflect input in the DOM. Use the browser’s JavaScript Debugger to trace how data flows from sources to execution points to trace how data flows from sources to execution points.

Automated Scanners

Tools like Burp Suite’s DOM Invader and OWASP ZAP identify XSS vulnerabilities by injecting payloads into JavaScript-exposed sources.

DAST and Dynamic IAST Scanners

Many application security testing tools focus on server-side scanning and miss DOM-based XSS. Dynamic application security testing (DAST) and dynamic interactive application security testing (IAST) tools with embedded browser engines effectively detect DOM-based XSS.

Penetration Testing by a Pentester

A pentester manually analyzes how JavaScript processes user input, looking for unsanitized data flows from sources to sinks. Using developer tools, debuggers, scanners, and real-world attack scenarios, they attempt to inject various payloads to see if execution is possible.

Unlike automated scanners, a pentester can adapt their approach based on how the application responds and find complex DOM-based XSS vulnerabilities that automated tools might overlook.

DOM-Based XSS Attack Prevention Techniques

Protecting against DOM-based XSS requires a systematic approach to securing client-side scripts. Below are key techniques that minimize the risk of exploitation and strengthen overall application security:

• Implement a Content Security Policy (CSP): A CSP helps mitigate DOM-based XSS by restricting which scripts can execute within a web page. Configuring a strong CSP prevents inline JavaScript execution and blocks unauthorized sources from injecting malicious scripts.
• Audit and monitor web assets: Regularly auditing JavaScript code and third-party libraries reduces the risk of hidden vulnerabilities. Continuous monitoring helps detect changes that may introduce vulnerabilities.
• Automate DOM-based vulnerability detection: Using automated security tools like DAST and IAST scanners identify and remediate DOM-based XSS vulnerabilities before attackers can exploit them.
• Perform an application security risk assessment: Conducting an application security risk assessment identifies weaknesses in your JavaScript code and mitigates security issues.
• Sanitize client-side code: Always sanitize user input before interacting with the DOM. Use libraries such as DOMPurify to filter out malicious scripts and avoid assigning raw user input to properties that can execute injected code.

Use Legit Security to Avoid DOM-Based XSS Attacks

Legit Security combats DOM-based XSS by embedding security into every stage of the software development lifecycle (SDLC). Through automated scanning, real-time monitoring, and developer-friendly security controls, Legit Security identifies and mitigates vulnerabilities before they can be exploited.

Book a demo today to see how Legit Security strengthens your defenses.