In this post, we will explore a vulnerability present in Nunjucks templating engine versions prior to 3.2.4. This exploitation allows an attacker to inject cross-site scripting (XSS) payloads into a web application that uses the affected version of Nunjucks.

Background

Nunjucks is a powerful templating engine that offers a simple, yet powerful, syntax to create reusable HTML templates for web applications. Nunjucks provides a feature called autoescaping, which is designed to automatically escape potentially unsafe characters when rendering user-generated content. In some cases, this feature could prevent the injection of malicious scripts and other attacks.

Vulnerability Details

The vulnerability stems from the way Nunjucks handles user-controlled content within its templates when autoescaping is enabled. If two user-controlled parameters exist on the same line within a Nunjucks template, an attacker can bypass the autoescaping feature by using a backslash \ character. This allows them to inject a malicious script into the application, potentially compromising user data or executing unwanted actions on the user's behalf.

Nunjucks fixed this issue in version 3.2.4 by improving the way they handle autoescaping and escaping other potentially unsafe characters.

Consider the following Nunjucks snippet

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>{{ title }}</title>
</head>
<body>
    <h1>{{ username }}</h1>
    <p>{{ content }}</p>
</body>
</html>

In this example, assume title, username, and content are provided by the user via a form submission or other input. An attacker could craft a payload using the backslash \ character to bypass the autoescaping and inject an XSS payload. For instance, they could set username to:

" onmouseover="alert('XSS') "

...and content to

\

With these values, the generated template would now become

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>{{ title }}</title>
</head>
<body>
    <h1>" onmouseover="alert('XSS') "\</h1>
    <p>{{ content }}</p>
</body>
</html>

When rendered, the above template would execute the malicious XSS payload when a user hovers over the username element, displaying an alert with the message "XSS".

While this may seem harmless, it exemplifies the potential for more damaging payloads, such as stealing user information or hijacking user sessions.

Mitigation

To mitigate this vulnerability, ensure that your application is using Nunjucks version 3.2.4 or later. This version has improved autoescaping functionality that protects against this specific XSS bypass technique.

References

- Nunjucks Documentation
- GitHub Commit Fixing the Issue
- NPM Package for Nunjucks

Conclusion

It's important to keep your software dependencies up-to-date and follow security best practices when using third-party libraries. By understanding the vulnerabilities present in older versions of Nunjucks and mitigating them by upgrading to a secure version, you can protect your users and maintain a more secure application environment.

Timeline

Published on: 11/26/2024 12:15:18 UTC
Last modified on: 11/27/2024 17:15:05 UTC