Archive for April 25th, 2008

Apr 25, '08

Recently, I was preparing to give a talk on web authentication so was looking at the source code of Wordpress, which I had just upgraded to version 2.5. Unfortunately, I found a rather nasty security hole, which has now been disclosed. If a Wordpress installation is configured to permit account creation, the vulnerability allows an attacker to gain administrator access.

The problem is to do with how cookies are generated. The authentication code was substantially overhauled for Wordpress 2.5, in part to deal with security problems in the password database. Now, the authentication cookies take the form of:

wordpress_.COOKIEHASH = USERNAME . | . EXPIRY_TIME . | . MAC

Where:
COOKIEHASH
MD5 hash of the site URL (to maintain cookie uniqueness)
USERNAME
The username for the authenticated user
EXPIRY_TIME
When cookie should expire, in seconds since start of epoch
MAC
HMAC-MD5(USERNAME . EXPIRY_TIME) under a key derived from a secret and USERNAME . EXPIRY_TIME.

This scheme is based on two papers: Dos and Don’ts of Client Authentication on the Web by Fu et al. and A Secure Cookie Protocol by Liu et al. However, there is a small difference and, as is common in cryptographic systems, small changes can have big impact.

The problem is that USERNAME and EXPIRY_TIME are not delimited when calculating the MAC. This means that a MAC for one cookie is valid for any other, provided that USERNAME . EXPIRY_TIME is unchanged. So an attacker can register a username starting with “admin”, log in as usual, then modify their cookie so it’s valid for the administrator account.

Fu et al. called this the “cryptographic splicing” attack in their paper (Section 3.3), and is one of the many ways they show how people can slip up when implementing web authentication. Unfortunately, dynamic website frameworks, especially PHP, offer little assistance to programmers trying to implement secure applications.

I will expand on this topic in a future post but in the mean time, if you run Wordpress, you really should upgrade to 2.5.1. While Wordpress 2.3.3 doesn’t have the problem described here, it is still not secure.

There is some more detail on the cookie vulnerability in my disclosure (CVE-2008-1930). Wordpress have mentioned it in their release announcement and I’ve also just sent it to the usual mailing lists.


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