DOM-based XSS: exploiting weaknesses in client-side script

With the development of dynamic web applications, there has been a lot of change in the security scenario. One of the more powerful forms of cross-site scripting (XSS) is DOM-based XSS, a vulnerability that exploits weaknesses in a client-side script. Unlike stored XSS or reflected XSS, including server-side processing, DOM-based cross-site scripting is completely within the browser, making it more elusive and challenging to detect.

In this broad blog, we will find out the nature of DOM-based XSS, how the attackers exploit it, real-world landscape, ways to detect, and find out the best strategies to protect your web applications. This guide is designed to inform developers, penetrated examiners and security professionals about the importance of security of client-side code.

What is DOM-based XSS?

DOM-based XSS (Document Object Model-Site scripting) is a type of XSS vulnerability, where the dom atmosphere in the browser using client-side JavaScript is triggered by modifying the DOM atmosphere in the browser. When the web page's JavaScript takes the user input from the URL, cookies, or DOM, the vulnerability is present and processes it in an unprotected manner, leading to the execution of the attacker-controlled code.

In short, the defect arises when:

• Client-side JavaScript reads data from a source that should not be trusted.
• It writes that data to the page of the page without proper verification or encoding.

How Dome-based XSS works

Here is a simplified workflow of a DOM-based XSS attack:

1. An attacker acts a malicious URL crafts that consists of a payload.
2. The victim clicks on the URL.
3. The browser loads the page, and the embedded JavaScript reads the input (e.g., from `document.location.hash`).
4. Without sanitization, the JavaScript writes this value into the DOM (e.g., using `innerHTML`).
5. The payload executes in the context of the user's browser.

Example URL:

https://example.com/page#<script>alert('XSS')</script>

If the JavaScript does:

let hash = location.hash;

document.getElementById("output").innerHTML = hash;

The script is then executed directly, causing DOM-based XSS.

Major sources and sinks in dome-based XSS

In DOM-based XSS, we refer to "sources" as places where data is read and "sink" as places where data outputs. If the data flows from an unsafe source without an unprotected sink without proper hygiene, it becomes exploited.

Common Sources:

• `document.URL`
• `document.location`
• `document.referrer`
• `window.name`
• `location.hash`

Common Sinks:

• `element.innerHTML`
• `document.write()`
• `element.outerHTML`
• `setTimeout()`, `setInterval()` (with string arguments)

• `eval()`

Example of real world: Github's DOM-based XSS (2012)

In 2012, Github received the DOM-based XSS vulnerability in its search feature. The application was passing the URL pieces using the JavaScript and putting them in the dome with 'innerHTML,' allowing the attackers to allow the arbitrary scripts to inject and execute the arbitrary script when visited a specially designed URL.

The problem was quickly packed, but it also highlighted the importance of not relying on any user-control input in client-side operation.

Why Dome-based XSS is dangerous

• Bypassing the traditional filter: as it is completely in the browser, it cannot be logged or intercepted by a server-based filter.
• Hard to detect: standard safety scanners often recall dome-based issues without crawling or JavaScript execution.
• Client-side exposure: As more argument is pushed to client-side (spa, PWA), the surface of the attack increases.
• Series with other attacks: CSRF, clickjacking, and can be paired with others.

DOM-Based Technology

1. Manual code review

• Check the use of unsafe sources and sinks in JavaScript files.
• For direct assignment, see for `Innerhtml`, eval ', etc.

2. Static analysis equipment

• Tools such as Eslint, Sonarqube, and Semgrep can detect unsafe code patterns.

3. Dynamic test equipment

• Use browser extensions such as Dom invader or XSS-Detect of Burp Suite.
• Fazing client-side JavaScript.

4. Browser developer equipment

• Observe dynamic Dome changes and manually test script execution.

How to stop dome-based XSS

1. Avoid dangerous tasks

• Do not use `Innerhtml`, 'Document.Write (), or Eval ()` with uneven data.
• Use 'TextContent' or Setattribute () for a safe DOM manipulation.

2. Use reliable libraries

• Libraries such as dompurify can clean HTML content firmly.
• Use framework such as reacts, angular, or vue -like reactions providing XSS security by default.

3. Input verification and relevant output encoding

• Come to the user input on both the server and client sides.
• Always encode the data before putting it in DOM.

4. Apply material safety policy (CSP)

• Prevent the execution of unauthorized scripts by applying a strict CSP header.

5. Safe JavaScript architecture

• Design an application to reduce direct dome manipulation.
• Use a model-view separation to safely handle the user input.

Common Mistakes Leading to DOM-Based XSS

• Blindly trusting `location.search` or `location.hash` without sanitization.
• Inserting user input directly into HTML via `innerHTML`.
• Using `eval()` with user data.
• Allowing third-party scripts with high privileges.

DOM-based XSS (Spas) in single page applications

Spas rely too much on client-side rendering, increasing sensitivity to DOM-based XSS. Developers need:

• Use the url -avoided routing library.
• Sanitize dynamic content before rendering.
• Avoid using 'dangerouslySetInnerHTML` in framework such as framework as long as necessary.

DOM-Based XSS business effects

• Brand Damage: A visible script attack may immediately ruin the user trust.
• Data Breach: Malibly scripts can cause sensitive information.
• Regulatory Risk: Violation of data protection laws (eg, GDPR, CCPA).
• Revenue loss: loss of users due to the perception of insecurity.

Best practice for developers

• Always users consider input to be incredible.
• The favoritic framework that survives HTML, automatically.
• Include security lines and pre-commit hooks.
• Regular third-party script and audit of libraries.
• Conduct regular security training.

conclusion

DOM-based XSS is a sophisticated and secret web vulnerability that targets client-side script. As the modern web apps rapidly move towards JavaScript-darling architecture, the risk of DOM-based cross-site scripting increases rapidly.

Understanding the attack vector, general developer loss and effective prevention, you can secure your applications and protect users from client-side abuse. Do not wait for a violation to fix your code. Make safe from the beginning.

Stay safe, code responsibly, and always validate the user input.