High · CVSS 7.4CVE-2026-107584

Not Failing Securely ('Failing Open') in hMailServerCVE-2026-107584

Answer in brief

CVE-2026-107584 records a High severity (CVSS 7.4) vulnerability in Not Failing Securely ('Failing Open') in hMailServer. The current sources do not mark it as known exploited. The current feed maps Progressive Robot Ltd/hMailServer (generic). Check affected ranges and fixed versions before updating.

Analysis pending evidence review

HOL Guard separates source facts from reviewed analysis. See the methodology.

Published Oct 8, 2026Updated Oct 8, 2026Source checked Oct 8, 2026First seen by HOL Oct 8, 2026Material review Oct 8, 2026

Upstream Advisory

Key facts

Risk: High · CVSS 7.4
Exploitation: Not marked as known exploited
Affected software: 1 mapped package or product
Fix availability: Available

Why this deserves its current priority

CVSS is 7.4. The current sources do not mark it as known exploited. Treat this as a source-backed prioritization signal, not a statement about your environment.

Analysis status

Analysis pending evidence review

Factual feed record only; HOL analysis is not approved for indexing. Read the methodology.

Affected scope and exposure questions

The current feed maps Progressive Robot Ltd/hMailServer (generic). Check affected ranges and fixed versions before updating.

Mapped affected packages and fixed versions
Package	Affected range	Fixed version
Progressive Robot Ltd/hMailServergeneric	>=6.0.0 <6.3.6	6.3.6

Recommended response

Check inventory. Check lockfiles and deployed manifests for Progressive Robot Ltd/hMailServer.
Review the reported fix. Source-reported solution: Upgrade to hMailServer 6.3.6, which treats an incomplete, unsigned or unproven DNSSEC answer for a TLSA or MX lookup under a signed zone as indeterminate, does not use a mail exchanger whose lookup was indeterminate, defers the delivery when none is left, delivers only to mail exchangers named by a DNSSEC-validated MX record set, and (for a host whose DNS servers are IPv6 only) asks those IPv6 servers. Until then, for each recipient domain whose mail must be protected, an SMTP route that names its mail exchangers with the connection security 'STARTTLS (required)', with remote certificate verification on (VerifyRemoteSslCertificate=1), prevents delivery in cleartext or to another host for that domain; and the server's DNS resolver should be one reached over a trusted path, such as a validating resolver on the same host.

Evidence timeline and material changes

Published upstream
Oct 8, 2026
Evidence: source:cvelist:source_dates:source-dates:record
Source modified
Oct 8, 2026
Evidence: source:cvelist:source_dates:source-dates:record
First seen by HOL
Oct 8, 2026

Sources and claim methodology

Upstream source description

Progressive Robot hMailServer 6.0.0 through 6.3.5 fails open when applying DANE (RFC 7672) to outbound SMTP delivery. The server's validating DNSSEC resolver treated a TLSA or MX lookup that did not complete (no answer, SERVFAIL, a malformed reply), an answer without the requested records and without an NSEC/NSEC3 proof of their absence, and an answer whose records carried no applicable RRSIG as if the recipient domain were unsigned, and from 6.2.19 it also delivered to mail exchangers taken from an unvalidated MX lookup that the DNSSEC-validated MX record set did not name. An attacker who can drop, forge or strip DNS answers on the path to the server's resolver, at the resolver, or between the resolver and the recipient domain's name servers, and who holds an active position on the SMTP path, can thereby disable DANE for a DNSSEC-signed recipient domain and cause messages to be delivered in cleartext or to a host of the attacker's choosing with an arbitrary certificate, where they can be read and modified.

Quoted source text, attributed separately from HOL analysis.