source: https://www.securityfocus.com/bid/16680/info

HTML::BBCode is prone to an HTML-injection vulnerability. This issue is due to a failure in the application to properly sanitize user-supplied input before using it in dynamically generated content. 

Attacker-supplied HTML and script code would be executed in the context of the affected site, potentially allowing for the theft of cookie-based authentication credentials. An attacker could also exploit this issue to control how the site is rendered to the user; other attacks are also possible.

Examples have been provided:

[img]javascript:alert(123)[/img]

[url=javascript:alert(123)]Click me[/url]