How Can Google Caja Help Restrict Access to Sensitive Browser Features in JavaScript?

Sandboxing JavaScript in Browsers to Restrict Access to Sensitive Features

To protect web applications from unauthorized access and manipulation, it's often desirable to restrict JavaScript's access to certain features. One such feature is the window object, which provides access to the browser's functionality.

Consider a scenario where you want to allow end users to define event handlers but prevent them from accessing window properties and functions. Here are a few approaches and their limitations:

• Redefining window.alert: This approach wouldn't work because other code running in the page may rely on alert.
• Sending event handler code to the server: While this ensures that the code runs under server control, it's not practical for event handlers that need to execute in the context of the page.

Fortunately, Google Caja provides a "source-to-source translator" known as Caja:

Caja: A Solution to JavaScript Sandboxing

Caja translates user-defined JavaScript code into a restricted sandboxed version. This ensures that the code can execute without accessing sensitive features:

• Caja isolates the sandboxed code in an iframe, preventing direct access to window properties.
• It provides restricted versions of native JavaScript objects, limiting access to sensitive functions.
• Developers can define security policies to further restrict sandboxed code.

For instance, to block access to alert using Caja, you would add the following configuration:

trustedUris: s:["off"]
trustedDomains: s:["off"]

This allows end users to define event handlers without the ability to call alert or other window functions.

Conclusion

Google Caja offers a secure and customizable way to sandbox JavaScript in browsers, allowing developers to restrict access to sensitive browser features and protect web applications from unauthorized manipulation.

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