CVE-2026-107584 in hMailServer
Summary
by MITRE • 10/08/2026
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.
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Analysis
by VulDB Data Team • 10/08/2026
The vulnerability identified in Progressive Robot hMailServer versions 6.0.0 through 6.3.5 represents a critical failure in the implementation of Domain Name System Security Extensions (DNSSEC) validation specifically within the context of DNS-based Authentication of Named Entities for SMTP, commonly known as DANE. This flaw fundamentally undermines the security guarantees provided by RFC 7672, which relies on the integrity and authenticity of DNS records to establish secure TLS connections between mail servers. The core technical deficiency lies in how the server's validating DNSSEC resolver handles various failure modes during lookups for TLSA or MX records. Instead of adhering strictly to the strict validation policies required by DANE, the software incorrectly treats several types of invalid responses as if they indicated that the recipient domain was unsigned and therefore not requiring secure transport.
Specifically, the vulnerability manifests when a DNSSEC-validated lookup encounters conditions such as no answer returned, a SERVFAIL response code, or a malformed reply from the authoritative name servers. In these scenarios, rather than rejecting the connection attempt to prevent insecure delivery, hMailServer proceeds by assuming the domain lacks DANE support. Furthermore, if an answer is received but it does not contain the requested records and fails to provide NSEC or NSEC3 proofs of absence, the resolver also defaults to treating the domain as unsigned. This behavior violates the principle that a lack of verifiable proof should result in a failure state rather than a fallback to insecure methods. Additionally, starting from version 6.2.19, an even more severe flaw was introduced where the server would deliver mail to MX hosts derived from unvalidated MX lookups if those hosts were not named by the DNSSEC-validated MX record set. This creates a direct path for attackers to redirect traffic away from secure endpoints.
The operational impact of this vulnerability is significant and poses a severe risk to email confidentiality and integrity. An attacker who possesses the ability to manipulate, drop, or forge DNS answers along the resolution path can effectively disable DANE protections for any DNSSEC-signed recipient domain. This manipulation allows the attacker to force hMailServer into cleartext SMTP delivery, exposing sensitive communications to passive eavesdropping. More critically, if the server falls back to using an unvalidated MX record set or accepts a forged TLSA record due to the validation bypass, messages can be delivered to mail exchangers controlled by the attacker. In such cases, the attacker can present arbitrary certificates that appear valid to the client but are not trusted by the intended recipient's policy, enabling man-in-the-middle attacks where emails are read and modified without detection.
This vulnerability aligns with CWE-295 Improper Certificate Validation, as the system fails to correctly validate the authenticity of TLS certificates based on DNSSEC data. It also relates to CWE-749 Exposure of Sensitive Information to an Unauthorized Actor due to the potential interception of unencrypted email traffic. From a threat modeling perspective using MITRE ATT&CK techniques, this flaw facilitates Network Sniffing (T1040) and Man-in-the-Middle attacks (T1557), allowing adversaries to intercept and alter communications in transit. The exploitation requires an active position on the SMTP path or control over DNS resolution infrastructure, making it particularly dangerous for organizations relying on DANE for secure email transport.
Mitigation strategies must focus on immediate software updates and configuration hardening. Organizations running hMailServer versions 6.0.0 through 6.3.5 should upgrade to a patched version that correctly implements strict DNSSEC validation policies as defined by RFC 7672. Until an update is applied, administrators can consider disabling DANE entirely if the risk of cleartext delivery is deemed lower than the operational disruption caused by failed deliveries due to strict validation failures, although this reduces overall security posture. Network-level defenses such as DNS filtering and monitoring for anomalous DNSSEC response patterns can also help detect attempts to exploit this vulnerability. Long-term remediation involves ensuring that all components in the email infrastructure adhere strictly to standards-compliant implementations of DANE and DNSSEC, avoiding fallback behaviors that compromise security guarantees when validation errors occur.