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TI-2026-69564 Aliases CVE-2026-107576

Inefficient DKIM and ARC checks in hMailServer cause denial of service

Priority LOW 39CWE-407Published · Updated

Key facts

Attack vector
Network
Privileges required
None
User interaction
None
Attack complexity
Low
Impact type
Denial of service
Confidentiality · integrity · availability
None · None · High
Fix status
Fixed

Attack profile from CVSS 3.1 · CNA. Fix status from CNA. Impact type from CWE-407.

Summary

Progressive Robot Ltd hMailServer from 6.0.0 before 6.3.6 is affected. Its inbound DKIM and ARC signature verification uses an inefficient algorithm (CWE-407) whose processing time grows with the square of the message's header size. A remote unauthenticated attacker can send a message with many header fields or a heavily folded field, along with a DKIM-Signature for a domain they control, to keep worker threads busy for tens of seconds per signature and make the mail services unavailable.

Written with Claude Haiku 5.5 from the sources below · 2026-10-29

Description from the CNA

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.

Why it matters now

Exploitation None reported CISA SSVC · not in CISA KEV
EPSS 0.00428 37th percentileScored 2026-11-07
Fix Fixed hMailServer (Progressive Robot Ltd): 6.3.6

SSVC decision points

CISA ADP · SSVC 2.0.3

Exploitation
None
Automatable
Yes
Technical impact
Partial

The decision (Track, Attend, Act) also depends on your mission and well-being impact; see CISA's SSVC guide.

Affected products

VendorProductAffected versionsFixed inStatusSource
Progressive Robot LtdhMailServer6.0.0 to before 6.3.66.3.6Patch availableCNA
  • hMailServer VendorProgressive Robot Ltd Affected 6.0.0 to before 6.3.6 Fixed in 6.3.6 Patch available CNA

Remediation

Update hMailServer to version 6.3.6, which fixes the flaw.

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-2026-69562 Inefficient JSON parsing in hMailServer lets attackers halt mailPriority LOW 39
    Show all 23 records →

Timeline

  1. 2026-10-08 Threatist record created from CVE-2026-107576

References

Other 2

  • 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-2026-69562 Inefficient JSON parsing in hMailServer lets attackers halt mail CVSS 3.1HIGH 7.5
  • TI-2026-76416 hMailServer signs forged mail with hosted domain DKIM keys CVSS 3.1HIGH 7.5
See all 41 →

Sources

This product uses the NVD API but is not endorsed or certified by the NVD. Sources and licences