We have identified and fixed a cross-site request forgery (XSRF) vulnerability which may affect Confluence instances in a public environment. The XSRF vulnerability is exposed in the Confluence Social Bookmarking plugin. Note that the Social Bookmarking plugin is disabled by default. If you do not have this plugin enabled, your site will not be affected.

An attacker might take advantage of the vulnerability to trick users into unintentionally submitting malicious data. An XSRF attack works by exploiting the trust that a site has for the user. If a user is logged into Confluence and an attacker tricks their browser into making a request to a Confluence URL, then the task is performed as the logged in user.

You can read more about XSRF attacks at cgisecurity and other places on the web:
http://www.cgisecurity.com/csrf-faq.html

Please refer to the security advisory for details of the vulnerability, risk assessment and mitigation strategies:
https://confluence.atlassian.com/x/aAI5Dg

The page also includes instructions for upgrading the plugin.