source: http://www.securityfocus.com/bid/6204/info

A HTML injection vulnerability has been discovered in Mhonarc.

An attacker may exploit this issue by sending a specially constructed email containing malicious HTML code in the header section. When the vulnerable Mhonarc client converts the message to HTML, any malicious HTML code will be executed within the context of the displayed web page.

To: <someone@example.com>
From: <hacker@example.com>
Header<SCRIPT>hello</SCRIPT>def: whatever