Insufficient preservation of the original sender in the forwarding of Progressive Robot hMailServer 6.0.0 through 6.3.6 allows a remote unauthenticated attacker to have mail they write, naming in its From an address at a domain the server hosts, delivered to other receivers as passing that domain's DMARC policy. When the server sent on a copy of mail that came from outside - an account's forward, a mail rule's forward, a Sieve redirect, a list's copy, or an alias or list expanding to an outside address - it could give the copy an SMTP identity in the organisation of the domain the copy's From names, whatever the message's origin: with the Sender Rewriting Scheme or the rewrite of the envelope sender on forwarding enabled (both off by default), an address at the forwarding domain; for a list, its bounce address; for a message with an empty envelope sender, none, under the server's own host name; and an envelope sender the message came with at a subdomain of a hosted domain was kept. Where that identity's SPF record authorises the server, other receivers' SPF check of the copy passed for a domain aligned with the From, and DMARC passed with it, although the message had not passed DMARC when the server received it. Exploitation needs a forward, redirect, alias or list that carries the hosted domain's mail to an outside address, and one of: SRS or the envelope rewrite enabled; a list whose bounce address is in the domain's organisation; an envelope sender at a subdomain whose SPF record authorises the server; or, for an empty envelope sender, an SPF record that authorises the server under its host name. The DKIM route to the same result is CVE-2026-107567.
Upgrade to hMailServer 6.3.7, in which a copy of mail from outside 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, a pass every copy of the message carries. Otherwise a forward, a Sieve redirect or a rule's forward keeps the envelope sender the message came with rather than an SRS or rewritten one; a copy whose identity would still be in that organisation - an empty envelope sender under a host name of it, or an envelope sender at the server's host name or in a hosted domain's organisation - is not sent to an outside address, nor is the message where an alias or a list expands to one; a managed list does not distribute the posting; and a distribution list's bounce address is not applied to it. Until then: keep SRS and the rewrite of the envelope sender on forwarding off where they are not needed; turn on the account option not to forward messages flagged as spam (ForwardAbortSpamFlagged) for accounts that forward to outside addresses, which does not hold a Sieve redirect; and set the DMARC test's score (DMARCFailureScore) at or above the spam delete threshold, so that a message that fails a published quarantine or reject policy is refused, or quarantined, on arrival. Neither of the last two holds mail in the name of a domain whose DMARC policy is none.