Use of a less trusted source in the DKIM signing of Progressive Robot hMailServer 6.0.0 through 6.3.6 allows a remote unauthenticated attacker to have mail they write signed with the DKIM key of a domain the server hosts, so that it passes DKIM and DMARC for that domain at other receivers. With DKIM signing enabled for a hosted domain, the server signed every message on its first delivery pass as the domain named in its From header, before local delivery, whatever the message's origin and without checking that its sender had authenticated. A sender anywhere could therefore send the server, as ordinary inbound mail, a message whose From is any address at such a domain, and the server signed it with that domain's key. Every copy the server then sent on to an address outside carried a valid signature aligned with the forged From, whatever the domain's DMARC policy: an account's forward, a mail rule's forward or copy, a Sieve redirect, an alias or distribution list with an outside member, the mirror copy, a managed-list or journal copy, or a message released from quarantine. A signed copy can also be re-sent to other recipients while the domain's key is unchanged. Exploitation needs DKIM signing to be enabled for the hosted domain (it is off until configured) and, for the forgery to leave the server, a forward, alias, list or rule that carries the domain's mail to an outside address. The signer likewise signed a message submitted over authenticated SMTP as whatever hosted domain its From header named, a header authenticated SMTP does not hold to the submitting account, so that an account of one hosted domain could have mail written in the name of another hosted domain signed with that domain's key; this mattered where the server holds signed-in senders to their own envelope addresses (SmtpAuthenticatedSenderCheck, off by default), as the signature was then the remaining way for such mail to pass the other domain's DMARC policy.
Upgrade to hMailServer 6.3.7. It records on each message, and on every copy of it, whether the message came from outside: an SMTP session that did not authenticate and was not a submission the server's IP ranges allow without authentication, a download from an external account, or an imported file. It signs a message as its From domain only when an authenticated user submitted it (as another domain the server hosts only when the account may send as the address its From names), the server made it, or an IP range let it in as one of the server's own domains, so mail from outside and its forwards keep only the signatures they arrived with, and a signed-in account's message from an address at another hosted domain that the account may not send as is sent unsigned as that domain. Until then, do one of the following: publish a DMARC policy of quarantine or reject for each hosted domain the server signs for and set the server's DMARC failure score (DMARCFailureScore) at or above the spam delete threshold (SpamDeleteThreshold), so that such a message is refused on arrival; refuse it in an OnAcceptMessage event script when the client has not authenticated and the From header names a domain the server signs for; remove the forwards, rule forwards, Sieve redirects, alias and list members and mirror addresses that carry mail to outside addresses; or disable DKIM signing for the hosted domains, which alone of these also covers a signed-in account's submission. Rotate the DKIM key of any domain whose key may have signed forged mail.