XPCNativeWrapper: Difference between revisions

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===Scenario 2===
===Scenario 2===
The same analogy applies to calling methods on an untrusted object. Consider the case where privileged code wants to get access to the body element of a document.
The same analogy applies to calling methods on an untrusted object. Consider the case where privileged code wants to get access to the body element of a document:
<pre>
<pre>
var bd = document._content.document.getElementsByTagName('body')[0];
var bd = document._content.document.getElementsByTagName('body')[0];
</pre>
</pre>
It may also look harmless, but an attacker could execute malicious code if they overwrote the getElementsByTagName method with another:
It may also look harmless, but an attacker could execute malicious code if they overwrote the getElementsByTagName method with their own method:
<pre>
<pre>
document.getElementsByTagName = function() {
document.getElementsByTagName = function() {
Line 60: Line 60:
};
};
</pre>
</pre>
In both scenarios, the malicious code gets executed with elevated privileges.


==References==
==References==

Revision as of 10:56, 22 March 2005

This page is part of the extension development documentation project.

Ask your questions in MozillaZine Forums. Also try browsing example code.

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.

Prerequisite

Before using XPCNativeWrapper, you need to import/reference the script file to make it available. 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 only and the desired methods are specified by their name followed by a pair of empty brackets. Each property or method should be specified in its own separate parameter. The returned object can then be used just like the untrusted object, in that methods and properties can be called on it directly:

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

Caveat

In most cases, XPCNativeWrapper can be used effortlessly and without much modification to existing code. However one area to beware of is when using a wrapped object with the instanceof check and typeof check. Wrapped objects are no longer instances of the untrusted object and may also be of a different type.

Attack Scenarios

To illustrate the importance of using XPCNativeWrapper to improve security, consider the following two scenarios:

Scenario 1

Certain privileged code wants to modify the title property of the document. It executes the following:

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

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

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

Scenario 2

The same analogy applies to calling methods on an untrusted object. Consider the case where privileged code wants 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 their own method:

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

In both scenarios, the malicious code gets executed with elevated privileges.

References

  1. XPCNativeWrapper.js