---
id: CVE-2026-107572
title: >-
  Inefficient complexity in the Sieve filter evaluation of Progressive Robot
  hMailServer 6.2.24 through 6.3.5 allows an authenticated account holder to
  make the mail services unavailable with their own filter
summary: >-
  Inefficient complexity in the Sieve filter evaluation of Progressive Robot
  hMailServer 6.2.24 through 6.3.5 allows an authenticated account holder to
  make the mail services unavailable with their own filter. A ':matches' pattern
  was matc…
severity: medium
cvss: 6.5
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H'
cwe:
  - CWE-1333
vendor: Progressive Robot Ltd
product: hMailServer
affected:
  - hMailServer >= 6.2.24 < 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-2026-107572'
references:
  - url: 'https://gitlab.com/hmailserver/hmailserver/-/releases/v6.3.6'
    label: cve@gitlab.com
  - url: 'https://gitlab.com/hmailserver/hmailserver/-/work_items/75'
    label: cve@gitlab.com
tags:
  - nvd
  - cve.org
ssvc:
  exploitation: none
  automatable: 'no'
  technicalImpact: partial
  timestamp: '2026-10-08T17:35:04.470485Z'
ingestedAt: '2026-10-08T12:39:48.754Z'
---

## Overview

Inefficient complexity in the Sieve filter evaluation of Progressive Robot hMailServer 6.2.24 through 6.3.5 allows an authenticated account holder to make the mail services unavailable with their own filter. A ':matches' pattern was matched by a backtracking descent whose time grew with the matched value's length raised to the number of wildcards, so a pattern such as '*a*a*a*b' over 800 characters took 44 seconds; and 'deleteheader' erased the fields it removed one at a time, so removing many fields of one name over a message's header cost O(N^2) (80,000 fields took 18.8 seconds). Sieve filters run on the small, shared delivery thread pool, so an account holder whose active script does this, fed a few messages they can send themselves, empties the pool and stops delivery for the whole server. The script is the account holder's own and cannot be set for another user.

## Remediation

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