XBOX  Security

                      -= Security  Advisory =-

     Advisory: XBOX Dashboard local vulnerability
 Release Date: 2003/07/04
Last Modified: 2003/07/04
       Author: Stefan Esser []

  Application: Microsoft XBOX Dashboard (up to today)
     Severity: A vulnerability within the XBOX Dashboard allows to
               totally compromise the security features of the XBOX.
         Risk: Critical
Vendor Status: Vendor is not willing to talk about XBOX vulnerabilities.


   The XBOX Dashboard is what appears when you turn the XBOX on without a 
   disc in the DVD drive. It will let you adjust system settings, manage
   your save games, play and rip audio CDs and configure your XBOX Live
   account. It is the heart of the XBOX and its most vulnerable point, 
   because it lacks several security restrictions which are enforced on 
   games. This includes the lack of the reboot-on-eject-button "feature",
   which is obligatory for all games.
   The existance of an exploitable vulnerability within the dashboard could
   totally compromises the XBOX security system. It will make the box 
   independent from Microsoft signed code and therefore this information is
   released to the public now on the 4th of July 2003, the day of the XBOX
   Microsoft knows that a vulnerability within the XBOX dashboard could 
   have serious impact. This is underlined by the fact that the dashboard 
   checks most of its files against an internal stored SHA1 hash value 
   before it uses them. 
   For an unknown reason this check is not performed on the audio (.wav) 
   and font (.xtf) files. Unfourtunately for Microsoft there exists an 
   exploitable integer underflow vulnerabilitiy within the font file loader
   which can be exploited with a malformed font file. When the XTF header 
   is processed the dashboards reads a 4 byte blocksize field from the font
   file. This is expected to represent the size of some datablock including
   the 4 bytes of the size field itself. The blocksize is then allocated 
   and the sizefield is copied into the  beginning of the buffer. This is 
   already a possible overflow bug when the field contains the values 0..3.
   Due to memory alignment this is not exploitable. But then the blocksize
   is decreased by 4 because the dashboard wants to read the rest of  the 
   block into memory. Obviously values of 0..3 will underflow when 
   decreased by 4 and this results in the dashboard wanting to read up to 
   ~4 gigabytes of data from the font file in a f.e. 3 bytes buffer.
   Because the XBOX malloc()/free() implementation is also storing control 
   information inbound and is similiar to the Windows 2000/XP heap 
   allocators this bug is exploitable and allows execution of arbitrary 
   code. The attached proof of concept code shows that exploiting is 
   possible with offsets that are equal on all dashboards and XBOX versions
   BTW: the dashboard loads its font files directly after the XBOX start 
        animation. This means the exploit does not need any user 
        interaction and when the code is executed only part of the 
        dashboard background is on screen.

Proof of Concept:

   Attached you will find a proof of concept exploit which will start 
   linux. To install it you have to rename the 2 XBOX font files within the
   font directory of the dashboard partition and then copy ernie.xtf and 
   bert.xtf into this directory. (If you have an XBOX with an older 
   dashboard the font directory does not exist and you must do the renaming
   and file adding work in the main directory). Once the new fonts are in 
   place you copy the default.xbe (which is a copy of xbeboot) into the 
   main directory and add your favourite linux to it.

Trustworthy Computing:

   Trustworthy Computing at its best. Nearly 2 Years ago I reported an SSL 
   vulnerability within IE to Microsoft. 1 month later I released 
   information about this bug to the public because MS did absolutely 
   nothing. The vulnerability was nearly forgotten, it only exists on the 
   list of 19 unpatched IE vulnerabilities anymore. But this is wrong, the
   vulnerability was indeed fixed with one of the many IE patches in the 
   middle of last year. Well is secretly fixing bugs without an official 
   advisory trustworthy?

Anticipated Questions:

   Q1: How do I get the files onto the harddisk?
   A1: There are several ways. You could f.e. install the files with the 
       Mechassault or 007 hacks. This requires one of the games and the 
       files on a memorycard. The other way is to open the box and do the
       harddisk swap trick which is described all over the net.

   Q2: This vulnerability is in the dashboard, isn't it? So Microsoft can
       simply update the dashboard with XBOX Live or with the help of new
   A2: Yes Microsoft could try to upgrade the dashboard and fix the 
       vulnerability with such an update, but keep in mind that this 
       vulnerability is like a "local root" hole. You can do nearly 
       everything with it and this includes redirecting reads and writes to
       the xboxdash.xbe file. Additionally people who do not play games on 
       their box will not be reachable with such updates. And groups who 
       pirate games can always disable the update feature.
   Q3: Well but MS can make the kernel block the vulnerable dashboard.
   A3: Indeed they can. But until boxes with new kernels reach the market 
       we will have the end of this year (You can still get 1.0 boxes in 
       shops over here) and they can only fix the bugs they know about.
   Q4: Is it possible to play "backed-up" games with this?
   A4: Yes it is possible to play pirated games by using this vulnerability
       but my proof of concept code will not allow this. You have to change 
       the exploit to patch the kernel in memory. This is not very hard and 
       I am not going to help you with this.
   Q5: Can I go "Live" with this hack?
   A5: You have full control over the box with this vulnerability. You can 
       modify the exploit to allow XBOX Live playing but this will only 
       start a cat & mouse game with Microsoft.

   Q6: I have read that I can solder my mainboard with this hack...
   A6: This exploit has nothing to do with soldering, It will just run 
       everything you want on unmodded (and even unopened) XBOXes. Infact 
       when this hack is installed you do not need to solder anything to 
       get your homebrew or whatever applications to run.
Copyright 2003 Stefan Esser. All rights reserved.