[ISecAuditors Security Advisories] WordPress MU HTTP Header XSS Vulnerability

2009-03-1200:00:00

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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)
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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.

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