Horde-IMP is the Internet Messaging Program. It is written in PHP and provides webmail access to IMAP and POP3 accounts.
Horde-IMP fails to properly sanitize email messages that contain malicious HTML or script code.
By enticing a user to read a specially crafted e-mail, an attacker can execute arbitrary scripts running in the context of the victim's browser. This could lead to a compromise of the user's webmail account, cookie theft, etc.
There is no known workaround at this time.
All Horde-IMP users should upgrade to the latest stable version:
# emerge sync
# emerge -pv ">=www-apps/horde-imp-3.2.4"
# emerge ">=www-apps/horde-imp-3.2.4"