RE: Internet Explorer and Opera local zone restriction bypass

["Thor Larholm" <thor () pivx com>]

There was not a lot of details in your post, so I will try to verify and clarify your findings. First things first, 
this is not a problem with Microsofts Internet Explorer, but with Macromedia and their Flash player.

I could reproduce this issue successfully with a fresh install of the latest Flash player, version 6.0.65.0, on fully 
patched versions of both IE6SP1 and Windows XP Pro.

There are two completely new issues at hand here.

The first issue is that Macromedia Flash allows you to store arbitrary content in a known location, that is 
%APPDATA%\Macromedia\Flash Player\YOURDOMAINNAME.TLD\YOURDOMAINNAME.sol. All flash cookies (which is what you set in 
your example, not browser cookies) from YOURDOMAINNAME.TLD are stored in this file.

The issue is caused by Macromedias decision to store the contents of your Flash cookie in plaintext in this .SOL file. 
When IE later reads the file the "magic filetype" feature of Explorer reads the first 256 bytes, finds HTML content and 
determines to render the file as HTML since the target application is the browser, including your scripting.

Being able to store arbitrary content in a known location is vital to any of the current range of IE exploits.

Flash itself is a binary format, so this complete issue can easily be fixed by Macromedia by applying the same level of 
binary formatting to its Flash cookie contents, to provide slight obfuscation of the contents of Flash cookies when 
storing them on disk so Explorer does not misread its datatype.

End-users can protect themselves against this exploit by changing how much data Flash applications are allowed to store 
on disk by going to http://www.macromedia.com/support/flashplayer/help/settings/global_storage.html and moving the 
slider all the way down, equivelant to checking the "Never Ask Again" checkbox on the page. When an updated version of 
the Flash player that fixes this is available, it is equally easy to change the setting back.

System administrators can edit the file %APPDATA%\Macromedia\Flash Player\maromedia.com\support\sys\settings.sol and 
change the bytes at positions c7 and c8 to contain BF and F0, respectively (ASCII ¿ and ð), to restrict data storage 
for Flash applications as an end-user would above. If you want to restore the file to default settings (for storing 
100KB data) change the bytes back to 40 and 59, respectively (ASCII @ and Y).

This is also why several people have said they could not reproduce the issue. They were either not logged in with the 
Administrator account, which your POC required, or they did not have the Macromedia Flash player installed.

A similar issue was found way back with ID3 tags in Winamp and RealPlayer media files, and has been found on several 
occasions where a third-party non-Microsoft application allows you to store arbitrary content in a known location.


The second issue is that IE lets you redirect to local files. This was restricted in IE6 SP1. While going over the 
source code in your POC, we discovered that it inadvertently redirects to a local file, despite the apparent 
restriction.

When IE encounters a redirect such as the following

Content-Location: file://c:/somefile.html

it will disallow the action and not follow the redirect. However, your POC has one important alteration, which is the 
following

Content-Location: file:///c:/somefile.html

Did you notice that slight difference? Adding another slash to the URL circumvents the initial restriction, and when IE 
finally decides to load the URL in another part of its code it removes any excess slashes and properly loads 
file://c:/somefile.html

The restriction imposed by IE6 SP1 is imposed on all local protocols, such as file:// and res://, and this new way to 
circumvent it equally applies to all local protocols. This means that you don't have to know the location of a specific 
file, but instead can open a ressource file available on all systems, such as

Content-Location: res:///browselc.dll/mb404.htm

Of course, since you could not inject any code in the ressource file you will now have to use another cross-domain 
scripting vulnerability in place of the Macromedia Flash vulnerability you identified in the first issue. On the 
positive side, it also means that you no longer have to guess the users Windows Logon name.


In summary, when Macromedia changes their Flash player to no longer store Flash cookies in plaintext in a known 
location, this will no longer be an issue. All of the currently unpatched cross-domain scripting vulnerabilities are 
having patches produced, and since they have no easy POC exploits I doubt we will see any malicious use of the local 
file redirection variation you found.



Regards
Thor Larholm
PivX Solutions, LLC - Senior Security Researcher
http://pivx.com/larholm/ - Get our research, join our mailinglist



-----Original Message-----
From: Mindwarper * [mailto:mindwarper () linuxmail org] 
Sent: Friday, October 24, 2003 6:53 AM
To: bugtraq () securityfocus com
Subject: Internet Explorer and Opera local zone restriction bypass

<snip http://www.securityfocus.com/archive/1/342317/2003-10-22/2003-10-28/0>

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