Courier-IMAP is an IMAP server which is part of the Courier mail system. It provides access only to maildirs.
CJ Kucera has discovered that some Courier-IMAP scripts don't properly handle the XMAILDIR variable, allowing for shell command injection.
A remote attacker could send specially crafted login credentials to a Courier-IMAP server instance, possibly leading to remote code execution with root privileges.
There is no known workaround at this time.
All Courier-IMAP users should upgrade to the latest version:
# emerge --sync
# emerge --ask --oneshot --verbose ">=net-mail/courier-imap-4.0.6-r2"