Mailman is a Python based mailing list server with an extensive web interface.
Mailman fails to properly handle standards-breaking RFC 2231 formatted headers. Furthermore, Moritz Naumann discovered several XSS vulnerabilities and a log file injection.
An attacker could exploit these vulnerabilities to cause Mailman to stop processing mails, to inject content into the log file or to execute arbitrary scripts running in the context of the administrator or mailing list user's browser.
There is no known workaround at this time.
All Mailman users should upgrade to the latest version:
# emerge --sync
# emerge --ask --oneshot --verbose ">=net-mail/mailman-2.1.9_rc1"