SektionEins GmbH www.sektioneins.de -= Security Advisory =- Advisory: PunBB Blind Password Recovery Vulnerability Release Date: 2008/02/20 Last Modified: 2008/02/20 Author: Stefan Esser [stefan.esser[at]sektioneins.de] 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 Reference: http://www.sektioneins.de/advisories/advisory-012008-punbb-blind-password-recovery-vulnerability.html Overview: Quote from http://punbb.org/ "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. Details: 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 firstname.lastname@example.org 19. February 2008 - PunBB developers released PunBB 1.2.17 20. February 2008 - Public Disclosure Recommendation: 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: http://punbb.org/downloads.php CVE Information: The Common Vulnerabilities and Exposures project (cve.mitre.org) has not assigned a name to this vulnerability yet. GPG-Key: http://www.sektioneins.de/sektioneins-signature-key.asc 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.