CVE-2026-107587: Improper Certificate Validation in hMailServer
Improper certificate validation in the webmail of Progressive Robot hMailServer 6.3.2 through 6.3.5 allows a remote unauthenticated attacker to have S/MIME-encrypted mail that the account later sends to another correspondent also encrypted to the attacker's key. When its recipient opened a signed message, the webmail kept the signer's certificate for encrypting replies whether or not the server found its chain trusted, under the first e-mail address the certificate listed rather than the message's From address, and beside any certificate already held for that address. Encrypted mail later sent from the webmail to that address was encrypted to every certificate held for it, so a holder of the kept certificate's key who obtains a copy of such a message can read it.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
Progressive Robot hMailServerto a version that resolves this vulnerability.Fixed in 6.3.6 - Compensating control
Until upgrading, check the certificates listed for an address before sending encrypted mail from the webmail, or encrypt the message in a mail program instead.
- Operational
After upgrading, review the correspondents' certificates on the webmail Security page and remove any certificates that should not be there.
Event History
Frequently Asked Questions
Which deployments are affected?
The issue affects hMailServer 6.3.2 through 6.3.5 and involves the webmail S/MIME handling path. The provided release reference identifies version 6.3.6 as the relevant fixed release.
What must happen before an attacker can read an encrypted reply?
An unauthenticated attacker must cause a recipient to open a signed message carrying the attacker-controlled certificate. The webmail must retain that certificate, and a user must later send S/MIME-encrypted mail from webmail to the certificate email address; the attacker also needs to obtain a copy of that encrypted message.
Why does an untrusted certificate create exposure?
Webmail retains the signer certificate for reply encryption even if its trust chain is not trusted. It stores the certificate under the first email address listed in the certificate rather than the message From address, and later encrypts mail to every certificate stored for that address.