Advisory 01/2008: PunBB Blind Password Recovery Vulnerability

                          SektionEins GmbH

                      -= Security  Advisory =-

     Advisory: PunBB Blind Password Recovery Vulnerability
 Release Date: 2008/02/20
Last Modified: 2008/02/20
       Author: Stefan Esser [stefan.esser[at]]

  Application: PunBB <= 1.2.16
     Severity: Weak random numbers lead to a blind password recovery
               vulnerability that allows account takeover
         Risk: High
Vendor Status: Vendor has released PunBB 1.2.17 which fixes this issue


   Quote from
   "PunBB is a fast and lightweight PHP-powered discussion board.
    It is released under the GNU General Public License. Its primary
    goals are to be faster, smaller and less graphically intensive as
    compared to other discussion boards. PunBB has fewer features
    than many other discussion boards, but is generally faster and
    outputs smaller, semantically correct XHTML-compliant pages."

   PunBB comes with a password reset feature that allows resetting a
   forgotten password. When a password reset is requested an email
   is sent to the user containing a new random password and an
   activation link that needs to be visited in order for the password
   change to become effective.

   Unfortunately it is possible due to several weak random numbers
   to determine the new random password and the activation link
   from the outside. This allows taking over any account on the
   forum including the administrator account.


   PunBB's password reset functionality uses internally mt_rand() to
   generate a new password and a new activation link that are both
   send to the user by email.

   Unfortunately PunBB initialises the mersenne twister random number
   generator on every request with a number between 0 and 1.000.000,
   depending on the current microsecond. This means there are only
   one million possible new passwords and new activation links. It
   would be possible to bruteforce this limited area, but the amount
   of time and traffic that would be required is huge.

   Because of this a better one shot solution was developed that
   allows to determine the new password and the new activation link
   from the result of the request that triggered the password reset.

   To understand how this is possible it is necessary to know that
   during the installation PunBB creates a "random" cookie seed that
   is used to store login data in the cookie during a visit. This
   cookie seed generation is not really random, because it is more
   or less the MD5 hash of the current timestamp. This means it is
   easily bruteforceable when the attacker has his own user account
   at the forum. He just needs to use his own login cookie and then
   check all seconds backwards from the date the admin account was
   created (see in memberlist).

   The second component required for the attack to work is PunBB's
   habit to return a cookie with a randomly generated password, when
   it receives a wrong login cookie. Because the cookie seed is known
   it can be used to check which one of the one million possible
   passwords was generated. By knowing the password we know the
   seed used in the call to mt_srand() which lets us predict all
   random numbers during the request.

   It should be obvious that using this attack on the request that
   triggers the password reset allows to blindly determine the new
   password and the new activation link in a few seconds. Both can
   then be used to takeover the attacked account.

Proof of Concept:

   SektionEins GmbH is not going to release a proof of concept
   exploit for this vulnerability.

Disclosure Timeline:

   15. February 2008 - Notified
   19. February 2008 - PunBB developers released PunBB 1.2.17
   20. February 2008 - Public Disclosure


   It is strongly recommended to upgrade to the latest version of
   PunBB which also fixes additional vulnerabilities reported by
   third parties.

   Grab your copy at:

CVE Information:

   The Common Vulnerabilities and Exposures project ( has
   not assigned a name to this vulnerability yet.


   pub  1024D/48A1DB12 2007-10-04 SektionEins GmbH - Signature Key
   Key fingerprint = 4462 A777 4237 E292 F52D  5AFE 7C9C C1AF 48A1 DB12

Copyright 2008 SektionEins GmbH. All rights reserved.