CVE-2026-107567

Status: DeferredHigh (7.5)—

Severity7.5High · CVSS 3.1

Exploitation probability0.17 %higher than 5% of CVEs

Active exploitationNonot in the CISA catalog

Public exploitNot knownno public code found

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.

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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.

CVSS

Base score: 7.5
Version: 3.1
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

Exploitation probability (EPSS)

Probability of exploitation in the next 30 days: 0.17 %
Percentile among all scored CVEs: 5
Score date: 10/20/2026

EPSS (Exploit Prediction Scoring System, FIRST) estimates how likely a vulnerability is to be exploited in the wild within 30 days. It complements CVSS (impact) and CISA KEV (confirmed exploitation).

ATT&CK techniques

How this vulnerability is exploited and what the attacker gains, in MITRE ATT&CK terms.

ExploitationT1190 Exploit Public-Facing Applicationinitial access60 %

Inferred by deterministic rules from the CVSS vector and CWE. Indicative only.

ATT&CK mitigations that cover these techniques

Vulnerability Scanning Privileged Account Management Network Segmentation Limit Access to Resource Over Network Filter Network Traffic Application Isolation and Sandboxing

Affected technologies (1)

⚠ AI-inferred from the description — NVD hasn't analyzed this CVE yet, these aren't verified CPEs.

Hmailserver — Hmailserver^AI

CWEs

CWE-348

References

Show 2 references

Raw JSON (NVD)

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{
  "id": "CVE-2026-107567",
  "cveTags": [],
  "metrics": {
    "cvssMetricV31": [
      {
        "type": "Secondary",
        "source": "cve@gitlab.com",
        "cvssData": {
          "scope": "UNCHANGED",
          "version": "3.1",
          "baseScore": 7.5,
          "attackVector": "NETWORK",
          "baseSeverity": "HIGH",
          "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N",
          "integrityImpact": "HIGH",
          "userInteraction": "NONE",
          "attackComplexity": "LOW",
          "availabilityImpact": "NONE",
          "privilegesRequired": "NONE",
          "confidentialityImpact": "NONE"
        },
        "impactScore": 3.6,
        "exploitabilityScore": 3.9
      }
    ]
  },
  "affected": [
    {
      "source": "cve@gitlab.com",
      "affectedData": [
        {
          "repo": "https://gitlab.com/hmailserver/hmailserver",
          "vendor": "Progressive Robot Ltd",
          "product": "hMailServer",
          "versions": [
            {
              "status": "affected",
              "version": "6.0.0",
              "versionType": "semver",
              "lessThanOrEqual": "6.3.6"
            }
          ],
          "defaultStatus": "unaffected"
        }
      ]
    }
  ],
  "published": "2026-10-19T16:17:01.587",
  "references": [
    {
      "url": "https://gitlab.com/hmailserver/hmailserver/-/releases/v6.3.7",
      "source": "cve@gitlab.com"
    },
    {
      "url": "https://gitlab.com/hmailserver/hmailserver/-/work_items/84",
      "source": "cve@gitlab.com"
    }
  ],
  "vulnStatus": "Deferred",
  "weaknesses": [
    {
      "type": "Primary",
      "source": "cve@gitlab.com",
      "description": [
        {
          "lang": "en",
          "value": "CWE-348"
        }
      ]
    }
  ],
  "descriptions": [
    {
      "lang": "en",
      "value": "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."
    }
  ],
  "lastModified": "2026-10-19T21:14:14.150",
  "sourceIdentifier": "cve@gitlab.com"
}