---
id: CVE-2026-107575
title: >-
  Inefficient algorithmic complexity in the SPF macro expansion of Progressive
  Robot hMailServer 6.3.4 and 6.3.5 allows a remote unauthenticated attacker to
  consume worker-thread time by publishing a crafted SPF record
summary: >-
  Inefficient algorithmic complexity in the SPF macro expansion of Progressive
  Robot hMailServer 6.3.4 and 6.3.5 allows a remote unauthenticated attacker to
  consume worker-thread time by publishing a crafted SPF record. RFC 7208
  section 7.…
severity: medium
cvss: 5.3
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L'
cwe:
  - CWE-407
vendor: Progressive Robot Ltd
product: hMailServer
affected:
  - hMailServer >= 6.3.4 < 6.3.6
published: '2026-10-08'
updated: '2026-10-08'
sourceUpdated: '2026-10-08T21:02:43.860'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-107575'
references:
  - url: 'https://gitlab.com/hmailserver/hmailserver/-/releases/v6.3.6'
    label: cve@gitlab.com
  - url: 'https://gitlab.com/hmailserver/hmailserver/-/work_items/74'
    label: cve@gitlab.com
tags:
  - nvd
  - cve.org
ssvc:
  exploitation: none
  automatable: 'yes'
  technicalImpact: partial
  timestamp: '2026-10-08T14:16:55.314371Z'
ingestedAt: '2026-10-08T12:39:48.755Z'
---

## Overview

Inefficient algorithmic complexity in the SPF macro expansion of Progressive Robot hMailServer 6.3.4 and 6.3.5 allows a remote unauthenticated attacker to consume worker-thread time by publishing a crafted SPF record. RFC 7208 section 7.1 requires a name too long to look up to lose whole labels from the left; the server did this by removing one label at a time and copying the rest of the name each time, so the work grew with the square of the expansion. An attacker who publishes an SPF record for a domain they control, with a mechanism whose domain-spec expands through macros to a name far longer than 253 characters, makes the SPF check of a message from that domain take several seconds. The expansion is bounded by SPF's own per-term and per-macro limits, so the loss of availability is partial.

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.