Plesk 8.6.0, when short mail login names (SHORTNAMES) are enabled, allows remote attackers to bypass authentication and send spam e-mail via a message with (1) a base64-encoded username that begins with a valid shortname, or (2) a username that matches a valid password, as demonstrated using (a) SMTP and qmail, and (b) Courier IMAP and POP3.
CVE Vulnerability