------------------------------------------------------------------------ Cross-Site Scripting in Calendar WordPress Plugin ------------------------------------------------------------------------ Remco Vermeulen, July 2016 ------------------------------------------------------------------------ Abstract ------------------------------------------------------------------------ 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. ------------------------------------------------------------------------ OVE ID ------------------------------------------------------------------------ OVE-20160725-0008 ------------------------------------------------------------------------ Tested versions ------------------------------------------------------------------------ This issue was successfully tested on Calendar WordPress Plugin version 1.3.7. ------------------------------------------------------------------------ Fix ------------------------------------------------------------------------ This issue is resolved in Calendar version 1.3.8. ------------------------------------------------------------------------ Details ------------------------------------------------------------------------ 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. ------------------------------------------------------------------------ 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.