Internet Explorer Local Zone Restriction Bypass (Exploit)
28 Oct. 2003
Summary
Microsoft Internet Explorer does not allow local file access by a remote host by default. By creating an iframe that points on a specially crafted CGI script (using the location header to confuse IE), it is possible to cause IE to execute any local file through the iframe with local zone restrictions. This then allows remote arbitrary file execution on the victim without having the victim do a thing besides loading the page.
Vulnerable systems:
* Internet Explorer 6.0 SP1 and 6.0
* Flash player version 6.0.65.0
Technical details:
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 mis-read its datatype.
A similar issue was found 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, Thor 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
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 do not have to know the location of a specific file, but instead can open a resource 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 resource 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 un-patched cross-domain scripting vulnerabilities are having patches produced, and since they have no easy POC exploits Thor doubts we will see any malicious use of the local file redirection variation Mind Warper found.
Workaround:
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, equivalent 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 ?), 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).
