Improper certificate validation in hMailServer webmail
Key facts
- Attack vector
- Network
- Privileges required
- None
- User interaction
- Required
- Attack complexity
- High
- Impact type
- Spoofing
- Confidentiality · integrity · availability
- High · Low · None
- Fix status
- Fixed
Attack profile from CVSS 3.1 · CNA. Fix status from CNA. Impact type from CWE-295.
Summary
Progressive Robot hMailServer from 6.3.2 before 6.3.6 has improper certificate validation (CWE-295) in its webmail. When a recipient opens a signed message, the webmail keeps the signer's certificate for encrypting replies, whether or not the server trusts its chain, filed under the first address the certificate lists rather than the From address. A remote unauthenticated attacker can have later encrypted mail sent to that address also encrypted to the attacker's key, so a holder of that key who obtains a copy can read it. The user must open a signed message from the attacker.
Written with Claude Haiku 5.5 from the sources below · 2026-10-29
Description from the CNA
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.
Why it matters now
SSVC decision points
CISA ADP · SSVC 2.0.3
- Exploitation
- None
- Automatable
- No
- Technical impact
- Partial
The decision (Track, Attend, Act) also depends on your mission and well-being impact; see CISA's SSVC guide.
Affected products
| Vendor | Product | Affected versions | Fixed in | Status | Source |
|---|---|---|---|---|---|
| Progressive Robot Ltd | hMailServer | 6.3.2 to before 6.3.6 | 6.3.6 | Patch available | CNA |
- hMailServer VendorProgressive Robot Ltd Affected 6.3.2 to before 6.3.6 Fixed in 6.3.6 Patch available CNA
Remediation
Upgrade Progressive Robot hMailServer to version 6.3.6.
Written with Claude Haiku 5.5 from the sources below · 2026-10-29
- Fix · CNA hMailServer (Progressive Robot Ltd) Fixed in 6.3.6
Fixed in the same release
Records fixed by the same vendor advisory or release, as the CNA lists them.
- Release ·
Progressive Robot hMailServer 6.3.6
23 records Highest priority Priority LOW 23 LOW
Other records, highest priority first
- TI-2026-69567 Inefficient processing in hMailServer stalls mail deliveryPriority LOW 39
- TI-2026-69565 Infinite loop in hMailServer MIME handling lets attackers stop mailPriority LOW 39
- TI-2026-69564 Inefficient DKIM and ARC checks in hMailServer cause denial of servicePriority LOW 39
Timeline
- 2026-10-08 Threatist record created from CVE-2026-107587
References
More Progressive Robot Ltd vulnerabilities
- TI-2026-64884 Improper Neutralization of Directives in Dynamically Evaluated Code ('Eval Injection') in hMailServer CVSS 3.1CRITICAL 9.8
- TI-2026-76418 Unchecked loop limit in hMailServer lets remote attackers stop mail CVSS 3.1HIGH 7.5
- TI-2026-69567 Inefficient processing in hMailServer stalls mail delivery CVSS 3.1HIGH 7.5
- TI-2026-69565 Infinite loop in hMailServer MIME handling lets attackers stop mail CVSS 3.1HIGH 7.5
- TI-2026-69564 Inefficient DKIM and ARC checks in hMailServer cause denial of service CVSS 3.1HIGH 7.5
- TI-2026-69562 Inefficient JSON parsing in hMailServer lets attackers halt mail CVSS 3.1HIGH 7.5
Sources
This product uses the NVD API but is not endorsed or certified by the NVD. Sources and licences