Description
Inefficient algorithmic complexity in the inbound DKIM and ARC signature verification of Progressive Robot hMailServer 6.0.0 through 6.3.5 allows a remote unauthenticated attacker to make the mail services unavailable by sending a message. Building the canonical header and choosing the header fields named in a signature's h= tag took time growing with the square of the message's header: the 'simple' canonicalisation prepended each continuation line of a folded field to the lines already gathered, and both canonicalisations searched the gathered fields from the bottom for each h= name and erased the match from the middle of the list. A message whose header holds very many fields, or a field folded over very many lines, with a DKIM-Signature the attacker signs for a domain they control, keeps a worker thread busy for tens of seconds per signature; up to ten signatures are evaluated per message by each of the DKIM and DMARC tests, on the threads that serve delivery and SMTP.
Published: 2026-10-08
Score: 7.5 High
EPSS: n/a
KEV: No
AI Analysis

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Remediation

Vendor Solution

Upgrade to hMailServer 6.3.6, which builds the canonical header in one pass and chooses the h= fields from an index by name. Until then, lower the maximum message size, which bounds the cost.

Tracking

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Advisories

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History

Thu, 08 Oct 2026 15:15:00 +0000

Type Values Removed Values Added
Metrics ssvc

{'options': {'Automatable': 'yes', 'Exploitation': 'none', 'Technical Impact': 'partial'}, 'version': '2.0.3'}


Thu, 08 Oct 2026 13:30:00 +0000

Type Values Removed Values Added
First Time appeared Progressive Robot
Progressive Robot hmailserver
Vendors & Products Progressive Robot
Progressive Robot hmailserver

Thu, 08 Oct 2026 12:00:00 +0000

Type Values Removed Values Added
Description Inefficient algorithmic complexity in the inbound DKIM and ARC signature verification of Progressive Robot hMailServer 6.0.0 through 6.3.5 allows a remote unauthenticated attacker to make the mail services unavailable by sending a message. Building the canonical header and choosing the header fields named in a signature's h= tag took time growing with the square of the message's header: the 'simple' canonicalisation prepended each continuation line of a folded field to the lines already gathered, and both canonicalisations searched the gathered fields from the bottom for each h= name and erased the match from the middle of the list. A message whose header holds very many fields, or a field folded over very many lines, with a DKIM-Signature the attacker signs for a domain they control, keeps a worker thread busy for tens of seconds per signature; up to ten signatures are evaluated per message by each of the DKIM and DMARC tests, on the threads that serve delivery and SMTP.
Title Inefficient Algorithmic Complexity in hMailServer
Weaknesses CWE-407
References
Metrics cvssV3_1

{'score': 7.5, 'vector': 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H'}


Subscriptions

Progressive Robot Hmailserver
cve-icon MITRE

Status: PUBLISHED

Assigner: GitLab

Published:

Updated: 2026-10-08T14:15:09.064Z

Reserved: 2026-10-08T10:51:45.640Z

Link: CVE-2026-107576

cve-icon Vulnrichment

Updated: 2026-10-08T14:15:05.682Z

cve-icon NVD

Status : Received

Published: 2026-10-08T12:17:15.487

Modified: 2026-10-08T15:17:43.817

Link: CVE-2026-107576

cve-icon Redhat

No data.

cve-icon OpenCVE Enrichment

Updated: 2026-10-08T13:30:18Z

Weaknesses
  • CWE-407

    Inefficient Algorithmic Complexity