source: https://www.securityfocus.com/bid/4220/info

ReBB is web forum software which will run on most Unix and Linux variants, as well as Microsoft Windows operating systems. It is written in PHP and may be back-ended by a number of databases.

ReBB allows users to include images in forum messages using image tags, with the following syntax:

[img]url of image[/img]

It is possible to inject arbitrary script code into forum messages via these image tags. Script code will be executed in the browser of the user viewing the forum message, in the context of the website running the vulnerable software. This may allow an attacker to steal cookie-based authentication credentials. 

[img]javascript:alert('test')[/img]