source: https://www.securityfocus.com/bid/42406/info

TagCloud is prone to an HTML-injection vulnerability because it fails to properly sanitize user-supplied input before using it in dynamically generated content.

Successful exploits will allow attacker-supplied HTML and script code to run in the context of the affected browser, potentially allowing the attacker to steal cookie-based authentication credentials or to control how the site is rendered to the user. Other attacks are also possible.

TagCloud version 2.0 is vulnerable; other versions may also be affected. 

The following example input is available:

'Topic' Field: <script>javascript:alert("lolcats")</script>