SquirrelMail is a webmail package written in PHP. It supports IMAP and SMTP and can optionally be installed with SQL support.
It was discovered that the sendmail.cf file is mishandled in a popen call.
A remote attacker, by enticing a user to open an e-mail attachment, could execute arbitrary shell commands.
There is no known workaround at this time.
Gentoo has discontinued support for SquirrelMail and recommends that users unmerge the package:
# emerge --unmerge "mail-client/squirrelmail"