SquirrelMail is a webmail package written in PHP. It supports IMAP and SMTP protocols.
SquirrelMail is vulnerable to several cross-site scripting issues, most reported by Martijn Brinkers.
By enticing a user to read a specially-crafted e-mail or using a manipulated URL, 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 SquirrelMail users should upgrade to the latest version:
# emerge --sync
# emerge --ask --oneshot --verbose ">=mail-client/squirrelmail-1.4.4"
Note: Users with the vhosts USE flag set should manually use webapp-config to finalize the update.