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Cross-Site Scripting in Calendar WordPress Plugin
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Remco Vermeulen, July 2016

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Abstract
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A Cross-Site Scripting vulnerability was found in the Calendar 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-20160725-0008

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Tested versions
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This issue was successfully tested on Calendar WordPress Plugin version
1.3.7.

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Fix
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This issue is resolved in Calendar version 1.3.8.

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

For any page containing a calendar the GET request parameters are appended as hidden inputs when the calendar is configured to have a date switcher. The input isn't properly sanitised and allows an attacker to add attributes to the input, including events.

In order to exploit this issue, the attacker has to lure/force a logged on WordPress Administrator into opening a malicious website. In addition, the calendar must configured to have a date switcher. By default this is disabled.
Proof of concept

Given the post hello-world, which has a calendar, visit the page with the following URL:

http://<target>/2016/06/22/hello-world/?foo=whoeiii\%22%20accesskey=x%20onclick=alert(1);//

The key combination CTRL + ALT + x triggers the onclick event.


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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.