Description
Inefficient algorithmic complexity in the bounce and complaint processing of Progressive Robot hMailServer 6.3.4 and 6.3.5 allows a remote unauthenticated attacker to stop mail delivery by sending messages, when bounce processing or complaint processing is enabled or a mailing list is managed by the server (none is by default). The readers of incoming delivery status notifications (RFC 3464) and abuse feedback reports (RFC 5965) removed the blank lines at the start of the returned headers part two bytes at a time, copying the rest of the part each time, so their work grew with the square of the number of blank lines. A message shaped like such a report, whose headers part begins with a very large number of blank lines within the reader's 2 MB limit, keeps a delivery thread busy for over a minute while it is delivered, and a few such messages a minute keep every delivery thread busy.
Published: 2026-10-08
Score: 7.5 High
EPSS: < 1% Very Low
KEV: No
Impact: Denial of Service via excessive resource consumption
Action: Patch
AI Analysis

Impact

An inefficient algorithm used to read bounce notices and abuse reports in hMailServer 6.3.4 and 6.3.5 can consume quadratic time as the number of leading blank lines grows. Sending a specially crafted message that contains a very large number of blank lines in the headers of an incoming delivery status notification or abuse feedback report will cause the server to spend over a minute processing the delivery thread. The effect is a denial‑of‑service, preventing mail delivery to legitimate users while the thread is busy.

Affected Systems

Progressive Robot hMailServer versions 6.3.4 and 6.3.5 are affected whenever bounce processing or complaint processing is enabled or mailing lists are managed. These features are disabled by default, but can be turned on by the system administrator.

Risk and Exploitability

The vulnerability is scored CVSS 7.5 and currently has no EPSS score available. It is not listed in the CISA KEV catalog. It can be exploited by any remote unauthenticated attacker who has the ability to send SMTP messages to the server, making the likely attack vector an untrusted email boundary. The high resource consumption risk can affect the availability of the mail server, especially under load or if multiple crafted messages are sent in quick succession.

Generated by OpenCVE AI on October 8, 2026 at 13:21 UTC.

Remediation

Vendor Solution

Upgrade to hMailServer 6.3.6, whose report readers cut the leading blank lines off once. Until then, turn bounce processing, complaint processing and list management off (the default).

OpenCVE Recommended Actions

  • Upgrade hMailServer to version 6.3.6, which removes the inefficiency in the bounce and complaint readers.
  • If an upgrade is not possible, temporarily disable bounce processing, complaint processing, and list management to prevent the denial of service.
  • Apply network‑level rate limiting or greylisting on SMTP connections to reduce the impact of repeated crafted messages.

Generated by OpenCVE AI on October 8, 2026 at 13:21 UTC.

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 bounce and complaint processing of Progressive Robot hMailServer 6.3.4 and 6.3.5 allows a remote unauthenticated attacker to stop mail delivery by sending messages, when bounce processing or complaint processing is enabled or a mailing list is managed by the server (none is by default). The readers of incoming delivery status notifications (RFC 3464) and abuse feedback reports (RFC 5965) removed the blank lines at the start of the returned headers part two bytes at a time, copying the rest of the part each time, so their work grew with the square of the number of blank lines. A message shaped like such a report, whose headers part begins with a very large number of blank lines within the reader's 2 MB limit, keeps a delivery thread busy for over a minute while it is delivered, and a few such messages a minute keep every delivery thread busy.
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: 2026-10-08T14:18:19.902Z

Reserved: 2026-10-08T10:52:00.639Z

Link: CVE-2026-107579

cve-icon Vulnrichment

Updated: 2026-10-08T14:18:16.724Z

cve-icon NVD

Status : Deferred

Published: 2026-10-08T12:17:15.940

Modified: 2026-10-08T21:02:43.860

Link: CVE-2026-107579

cve-icon Redhat

No data.

cve-icon OpenCVE Enrichment

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

Weaknesses
  • CWE-407

    Inefficient Algorithmic Complexity