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Reflected Cross-Site Scripting vulnerability in W3 Total Cache plugin
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Sipke Mellema, July 2016

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Abstract
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A Cross-Site Scripting vulnerability was found in the W3 Total Cache
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 URL provided by an attacker.

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OVE ID
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OVE-20160719-0004

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Tested versions
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This issue was successfully tested on W3 Total Cache version 0.9.4.1.

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Fix
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This issue is resolved in W3 Total Cache version 0.9.5.

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Details
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https://sumofpwn.nl/advisory/2016/reflected_cross_site_scripting_vulnerability_in_w3_total_cache_plugin.html

The issue exists in the file inc/options/support/form.php and is caused by the lack of output encoding on the request_id request parameter. The vulnerable code is listed below.

<input type="hidden" name="request_type" value="<?php echo $request_type; ?>" />
<input type="hidden" name="request_id" value="<?php echo $request_id; ?>" />
<input type="hidden" name="payment" value="<?php echo $payment; ?>" />

Proof of concept


Have an authenticated admin visit the URL:

http://<target>/wp-admin/admin.php?page=w3tc_support&request_type=bug_report&request_id="><script>alert('sumofpwn.nl');</script>

A pop-up box should appear, meaning the JavaScript contained in the request_id request parameter was executed by the browser.

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Summer of Pwnage (https://sumofpwn.nl) is a Dutch community project. Its
goal is to contribute to the security of popular, widely used OSS
projects in a fun and educational way.