Use of Less Trusted Source in hMailServer

Overview

Use of a less trusted source for the envelope sender in the external-account download of Progressive Robot hMailServer 6.0.0 through 6.3.6 allows a remote unauthenticated attacker to have mail they write in the name of a domain the server hosts sent on by the server with an envelope sender at that domain, so that it passes the domain's SPF and DMARC at other receivers. A message the server downloads from an external POP3 or IMAP account has no SMTP envelope of its own, and the server took the address in its From header as its envelope sender, whatever that address was. A forward or redirect of the downloaded message to an outside address then left with that address as its envelope sender, which the Sender Rewriting Scheme leaves alone for a hosted domain, so that the receiver's SPF check passed for the forged domain, aligned even under strict alignment, with no non-default setting, host name record or DMARC policy needed. An attacker needs to know a mailbox elsewhere that an account here collects and forwards to an outside address; an account holder who adds an external account of their own can collect from a server of their own, which matters where SmtpAuthenticatedSenderCheck holds signed-in senders to their own addresses.

Affected

• hMailServer >= 6.0.0 <= 6.3.6

Remediation

Upgrade to hMailServer 6.3.7, in which a download does not take an address hosted on the server from its From as its envelope sender - it keeps the empty envelope sender, as a download whose From names no address does - and a forward or redirect of it is held to the rule 6.3.7 applies to every copy of mail from outside: it is sent on with an envelope sender, or under a host name, in the organisation of a domain its From names only when the message passed DMARC for that organisation when it arrived, which a download never has. Until then, do not forward or redirect to an outside address the mail of an account that collects an external mailbox where that mail's From can name a domain the server hosts; the spam checks of an external account are off unless they are set, and a download has no DMARC verdict of its own to hold it.