plugins/acl/acl-backend-vfile.c in Dovecot 1.2.x before 1.2.15 and 2.0.x before 2.0.5 interprets an ACL entry as a directive to add to the permissions granted by another ACL entry, instead of a directive to replace the permissions granted by another ACL entry, in certain circumstances involving the private namespace of a user, which allows remote authenticated users to bypass intended access restrictions via a request to read or modify a mailbox.