With read-only access to the Wordpress database, it is possible to generate a valid login cookie for any account, without resorting to a brute force attack. This allows a limited SQL injection vulnerability to be escalated into administrator access.
Of course, entering your password after each action that requires authorization would be exceptionally tedious. So, after logging in, Wordpress presents the client with two cookies:
The cookie names contains the MD5 hash (6092...1a5f) of the blog URL. The value of wordpressuser_... is the login name, and the value of wordpresspass is the double-MD5 hash of the user password.
Wordpress will permit access to a given user account if the wordpressuserpass_... cookie matches the hash of the specified user's wp_users.user_pass database entry.
In other words, the database contains MD5(password) and the cookie contains MD5(MD5(password)). It is thus trivial to convert a database entry into an authentication cookie.
At this point the vulnerability should be clear. If an attacker can gain read access to the wp_user table, for example due to a publicly visible backup or SQL injection vulnerability, a valid cookie can be generated for any account.
This applies even if the user's password is sufficiently complex to resist brute force and rainbow table attacks. While it should be computationally infeasible to go backwards from MD5(password) to password, the attacker needs only to go forwards.
The exploitation steps are therefore:
1) Find the hash of the blog URL: Either just look at the URL, or create an account to get a user cookie
2) Read the user_pass entry from wp_users table: Look for backups, perform SQL injection, etc...
3) Set the following cookies:
4) You have admin access to the blog
A remote attacker, with read access to the password database can gain administrator rights. This may be used in conjunction with an SQL injection attack, or after locating a database backup.
An attacker who has alternatively compromised the database of one Wordpress blog can also gain access to any other whose users have the same password on both.
- Protect the Wordpress database, and do not allow backups to be released.
- Keep your Wordpress installation up to date. This should reduce the risk that your database will be compromised.
- Do not share passwords across different sites.
- If you suspect a database to be compromised, change all passwords to different ones. It is not adequate to change the passwords to the same ones, since Wordpress does not "salt"  the password database.
- Remove write permissions on the Wordpress files for the system account that the webserver runs as. This will disable the theme editor, but make it more difficult to escalate Wordpress administrator access into the capability to execute arbitrary code
- Configure the webserver to not execute files in any directory writable by the webserver system account (e.g. the upload directory).
The problem occurs because it is easy to go from the password hash in the database to a cookie (i.e the application of MD5 is the wrong way around). The simplest fix is to store MD5(MD5(password)) in the database, and make the cookie MD5(password). This still makes it infeasible to retrieve the password from a cookie, but means that it is also infeasible to generate a valid cookie from the database entry.
However, there are other vulnerabilities in the Wordpress cookie and password handling, which should be resolved too:
- Passwords are unsalted , leaving them open to brute force, rainbow table and other attacks .
- It is impossible to revoke a cookie without changing the user's password.
- Cookies do not contain an expiry time, so are always valid (until the user's password changes)
- There ought to be an option to limit cookies to a particular IP address or range.
2007-10-29: email@example.com notified; no response
2007-11-02: firstname.lastname@example.org notified; Confirmation of active exploitation requested by Wordpress
2007-11-02: Confirmation sent; no response
2007-11-19: Advisory released to full-disclosure and BugTraq