Not Failing Securely ('Failing Open') in hMailServer

Overview

Not failing securely in the anti-virus attachment-stripping action of Progressive Robot hMailServer 6.0.0 through 6.3.6 allows a remote unauthenticated attacker to have a message that the server's virus scanner found infected delivered to the recipient's mailbox unchanged. With virus scanning enabled and its action set to remove attachments and deliver the text (the default action deletes the message), the server keeps one text part of an infected message and replaces every attachment with a note. A message whose single part is not text - a program or an archive sent as the whole message, a message carried whole, a file declared an attachment, or an S/MIME message signed as one part, whose content the recipient's mail client opens - had no part the action could pick out, and was delivered as it arrived, with the content the scanner had found and nothing but a line in the application log to show it. Any sender can write such a message.

Affected

• hMailServer >= 6.0.0 <= 6.3.6

Remediation

Upgrade to hMailServer 6.3.7, in which the action replaces a message whose one part is an attachment, an S/MIME message signed as one part among them, with its note, as it replaces a message with no text part, and deletes a message it cannot read, or whose stripped form it cannot write over the message's file, as the delete action does, sending its recipients the message-deleted notice. A message that is one text part, and an encrypted message, are still delivered as they arrived. Until then, set the anti-virus action to delete the whole message, the default, wherever an infected message must not be delivered.