---
id: CVE-2026-107582
title: >-
  Inefficient algorithmic complexity in the REST API (6.3.3 through 6.3.5) and
  the IMAP PREVIEW response (6.2.22 through 6.3.5) of Progressive Robot
  hMailServer allows a remote unauthenticated attacker to make the webmail, the
  administrati…
summary: >-
  Inefficient algorithmic complexity in the REST API (6.3.3 through 6.3.5) and
  the IMAP PREVIEW response (6.2.22 through 6.3.5) of Progressive Robot
  hMailServer allows a remote unauthenticated attacker to make the webmail, the
  administrati…
severity: medium
cvss: 6.5
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H'
cwe:
  - CWE-407
vendor: Progressive Robot Ltd
product: hMailServer
affected:
  - hMailServer >= 6.2.22-pre1 < 6.3.6
published: '2026-10-08'
updated: '2026-10-08'
sourceUpdated: '2026-10-08T15:17:44.530'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-107582'
references:
  - url: 'https://gitlab.com/hmailserver/hmailserver/-/releases/v6.3.6'
    label: cve@gitlab.com
  - url: 'https://gitlab.com/hmailserver/hmailserver/-/work_items/67'
    label: cve@gitlab.com
tags:
  - nvd
  - cve.org
ssvc:
  exploitation: none
  automatable: 'no'
  technicalImpact: partial
  timestamp: '2026-10-08T14:18:32.670777Z'
ingestedAt: '2026-10-08T12:39:48.757Z'
---

## Overview

Inefficient algorithmic complexity in the REST API (6.3.3 through 6.3.5) and the IMAP PREVIEW response (6.2.22 through 6.3.5) of Progressive Robot hMailServer allows a remote unauthenticated attacker to make the webmail, the administration console and the REST API unavailable by sending a message. To show a snippet of each message in a folder's message list, the server decoded the character entity references of a message that has an HTML part and no text part with a string replacement whose work grew with the square of their number. A received HTML-only message holding a very large number of entity references therefore keeps one of the listener's four worker threads busy for minutes or longer each time the recipient's webmail lists the folder, without the message being opened, so that a few such listings leave the HTTP listener unable to answer anybody. The IMAP PREVIEW response read such text the same way, holding an IMAP thread for each client that asks for the preview of such a message. The flaw is in the server's shared string class, whose replace and remove both ran in quadratic time.

## Remediation

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