OVERVIEW

Windows Media Player versions 7 and 8 are vulnerable to a directory
traversal attack when skin files (*.WMZ) are downloaded from Internet.
The vulnerability allows malicious users to upload an arbitrary file to
an arbitrary location when a victim user views a web page.

When Media Player 7 or 8 is installed, Internet Explorer opens skin files
without confirmation from the user. Thus, an attacker can exploit the
vulnerability when the victim visits a malicious web page. The ability to
upload files can be used to run arbitrary code on the victim system in
several ways.

As most other Internet Explorer vulnerabilites, this one can be exploited
via Outlook (Express) e-mail if the security zone setting is set to
"Internet zone". In recent versions, this is not the default case.

DETAILS

When Internet Explorer encounters a document having the MIME type
"application/x-ms-wmz", it starts up wmplayer.exe with the "/layout"
command line switch which instructs Media Player to download a skin file
from the specified URL to the Media Player's Skins folder. To prevent
certain Internet based attacks, the program uses a random element in the
download path so that the exact file name of the downloaded skin file
can't be guessed by a potential attacker.

Due to a flaw in Media Player this measure can be circumvented with
hex-encoded backslashes in the URL. If an appropriate URL is crafted,
the exact download folder can be chosen.

If the filename doesn't end with ".WMZ", Media Player normally adds this
extension to the file. However, if the Content-disposition HTTP header is
used in a certain way, this restriction can be circumvented and also the
extension can be freely chosen. The attacker may thus place files with any
name and extension to any location on the local disks (and network shares
the user has access write access to). The attacker can not automatically
overwrite previously existing files; in this case a confirmation is asked
from the user.

There are numerous ways of exploiting this vulnerability to run arbitrary
code:

• codebase related attacks can be done by placing a HTML help, Java
  applet, a script, or similar file to the local filesystem and
  redirect Internet Explorer to its location

• a configuration file with malicious content might be uploaded for a
  program which by default doesn't have a configuration file

• uploading a DLL or EXE file to a carefully chosen folder might cause
  Internet Explorer or other program to use the attacker-supplied DLL
  or EXE instead of the original file - e.g. a program might use a DLL
  uploaded to C:\WINNT instead of C:\WINNT\SYSTEM32 and vice versa.

• the attacker may place programs in the Startup folder so that it
  would be started on the next reboot

Finding other attack vectors is left as an excercise to the reader. The
demonstration I set up for the vendor uploads a Java class file to
%SYSTEMROOT\Java\Trustlib\ and uses an applet tag to start it. The class
becomes "trusted" due to its location and is allowed to contain native
DLL calls. Now it can e.g. download an EXE program from Internet and
start it.

Windows Media Player version 9 doesn't seem to contain the flaw.

If Windows Media Player is not installed and a WMZ file is encountered,
Internet Explorer will usually suggest an automatic installation of
version 7 (Install on Demand).

SOLUTION

Microsoft was notified about the vulnerability on March 14, 2003. A
bulletin and patch correcting the issue has been released. They are
available at

http://www.microsoft.com/technet/security/bulletin/MS03-017.asp

Microsoft has classified this vulnerability as critical.

It should be noted that changing File Types settings at My Computer ->
Tools -> Folder Options doesn't seem to work as an workaround. WMZ files
are opened automatically regardless of them. Disabling this behavior
can probably be done by manually editing the registry.

CREDITS

The vulnerability was discovered by Jouko Pynnönen of Online Solutions
Ltd, Finland.

–
Jouko Pynnonen Online Solutions Ltd Secure your Linux -
[email protected] http://www.solutions.fi http://www.secmod.com