The ACL plugin in Dovecot before 1.1.4 treats negative access rights as if they are positive access rights, which allows attackers to bypass intended access restrictions.
https://www.redhat.com/archives/fedora-package-announce/2008-October/msg00844.html
https://www.redhat.com/archives/fedora-package-announce/2008-October/msg00816.html
http://www.vupen.com/english/advisories/2008/2745
http://www.ubuntu.com/usn/USN-838-1
http://www.dovecot.org/list/dovecot-news/2008-October/000085.html
http://security.gentoo.org/glsa/glsa-200812-16.xml
http://lists.opensuse.org/opensuse-security-announce/2009-02/msg00002.html