Read the docs
TI-2026-69496 Aliases CVE-2026-103649

Missing timeouts in hMailServer on Linux enable denial of service

Priority LOW 38CWE-1088Published · Updated

Key facts

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

Attack profile from CVSS 3.1 · CNA. Fix status from CNA.

Summary

Linux builds of Progressive Robot hMailServer 6.3.0 through 6.3.5 lack network timeouts, so a remote peer that accepts a connection and sends nothing can hold server threads indefinitely (CWE-1088). An attacker who can make the server deliver mail to a domain they control, such as by sending a message that bounces, can exhaust delivery threads and stop outbound mail delivery. The MTA-STS policy fetch is enabled by default, and the DANE TLSA query, OAuth2 token request, ACME client, and ManageSieve and metrics listeners are also affected. Windows builds are not affected.

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

Description from the CNA

Missing network timeouts in the Linux builds of Progressive Robot hMailServer 6.3.0 through 6.3.5 allow a remote attacker to hold server threads indefinitely and so stop outbound mail delivery (denial of service). The server set its socket timeouts in the form Windows takes, which Linux refuses, and its HTTPS clients read without a deadline, so a peer that accepts a connection and then sends nothing held the waiting thread for as long as the connection stayed open. The MTA-STS policy fetch, enabled by default, is made during outbound delivery to mta-sts.<recipient domain>, so anyone who can make the server deliver mail to a domain they control - for example as the envelope sender of a message that bounces - can hold delivery threads until outbound delivery stops. The same flaw affects the DANE TLSA query, the OAuth2 token request, the ACME client, and the ManageSieve and metrics listeners, which a silent client stops from serving anyone else. Windows builds are not affected.

Why it matters now

Exploitation None reported CISA SSVC · not in CISA KEV
EPSS 0.0027 19th 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 LtdhMailServer6.3.0 to before 6.3.66.3.6Patch availableCNA
  • hMailServer VendorProgressive Robot Ltd Affected 6.3.0 to before 6.3.6 Fixed in 6.3.6 Patch available CNA

Remediation

Upgrade hMailServer on Linux to version 6.3.6. The sources list no further fix details beyond this version.

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
    Show all 23 records →

Timeline

  1. 2026-10-08 Threatist record created from CVE-2026-103649

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-2026-69564 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
See all 41 →

Sources

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