source: https://www.securityfocus.com/bid/10439/info

SquirrelMail is reported to be prone to an email header HTML injection vulnerability. This issue is due to a failure of the application to properly sanitize user-supplied email header strings.

An attacker can exploit this issue to gain access to an unsuspecting user's cookie based authentication credentials; disclosure of personal email is possible. Other attacks are also possible. 

Content-Type: application/octet-stream"<script>window.alert(document.cookie)</script>"; name=top_secret.pdf