XPCNativeWrapper: Difference between revisions

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To illustrate the importance of using XPCNativeWrapper, consider the following scenarios:
To illustrate the importance of using XPCNativeWrapper, consider the following scenarios:


#An extension is written to modify the title property of the document.
===Scenario 1===
An extension is written to modify the title property of the document.
<pre>
<pre>
document._content.document.title = 'Hello World';
document._content.document.title = 'Hello World';
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</pre>
</pre>


#The same analogy applies to calling methods on an untrusted object. Consider the following scenario:
===Scenario 2===
The same analogy applies to calling methods on an untrusted object. Consider the following scenario:


An extension tries to get access to the body element of a document.
An extension tries to get access to the body element of a document.

Revision as of 09:58, 22 March 2005

This page is part of the extension development documentation project.

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Note: development documentation is in process of being moved to Mozilla Development Center (MDC).

XPCNativeWrapper is a Javascript object that should be used whenever privileged code is used to access unprivileged code. It is used to create a security wrapper that is analogous to a sandbox.

Basic Usage

Using XPCNativeWrapper is very simple. You create an instance of XPCNativeWrapper with the untrusted object as the first parameter and the desired properties and methods of the untrusted object as the additional parameters.

Prelude

Before using XPCNativeWrapper, you need to import/reference the script to make it available for scripting. In your main XUL page or overlay page, add the following to the top of the page. For Firefox, add:

<script type="application/x-javascript" src="chrome://global/content/XPCNativeWrapper.js"/>

For the Mozilla Suite add:

<script type="application/x-javascript" src="chrome://communicator/content/XPCNativeWrapper.js"/>

Example

This example creates a wrapper around the document object so that it can safely read the title of the document and call one of its methods:

doc = window._content;
doc2 = new XPCNativeWrapper(doc, 'title', 'getElementsByTagName()');

Notice how the desired properties are specified by their name and the desired methods are specified by their name followed a pair of empty brackets. The returned object can be used just like the untrusted object. Methods and properties can be called from it directly:

var tt = doc2.title;
var bd = doc2.getElementsByTagName('body')[0];

Attack Scenarios

To illustrate the importance of using XPCNativeWrapper, consider the following scenarios:

Scenario 1

An extension is written to modify the title property of the document.

document._content.document.title = 'Hello World';

On the surface it looks harmless enough. However, an attacker could execute malicious code if they added a getter to the title property:

document.title getter = function() {
  alert('code executed');  //malicious code...
};

Scenario 2

The same analogy applies to calling methods on an untrusted object. Consider the following scenario:

An extension tries to get access to the body element of a document.

var bd = document._content.document.getElementsByTagName('body')[0];

It may also look harmless, but an attacker could execute malicious code if they overwrote the getElementsByTagName method with another:

document.getElementsByTagName = function() {
  alert('code executed');  //malicious code...
};

References

  1. XPCNativeWrapper.js