Authentication is not required to exploit this vulnerability.
The specific flaw exists within the imapd process responsible for handling remote IMAP requests. The process does not properly validate IMAP commands and arguments. Supplying an overly long command followed by an invalid argument can cause an exploitable overflow to occur. This vulnerability can be leveraged to execute arbitrary code.
Disclosure Timeline:
2008-11-10 - Vulnerability reported to vendor
2011-02-07 - Coordinated public release of advisory