---
id: CVE-2026-107574
title: >-
  Inefficient algorithmic complexity in the JSON reader of Progressive Robot
  hMailServer allows a remote unauthenticated attacker to make the mail services
  unavailable
summary: >-
  Inefficient algorithmic complexity in the JSON reader of Progressive Robot
  hMailServer allows a remote unauthenticated attacker to make the mail services
  unavailable. Reading a JSON object kept the first of each duplicated member
  name by…
severity: high
cvss: 7.5
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H'
cwe:
  - CWE-407
vendor: Progressive Robot Ltd
product: hMailServer
affected:
  - hMailServer >= 6.2.28 < 6.3.6
published: '2026-10-08'
updated: '2026-10-08'
sourceUpdated: '2026-10-08T15:17:43.573'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-107574'
references:
  - url: 'https://gitlab.com/hmailserver/hmailserver/-/releases/v6.3.6'
    label: cve@gitlab.com
  - url: 'https://gitlab.com/hmailserver/hmailserver/-/work_items/76'
    label: cve@gitlab.com
tags:
  - nvd
  - cve.org
ssvc:
  exploitation: none
  automatable: 'yes'
  technicalImpact: partial
  timestamp: '2026-10-08T14:15:32.913666Z'
ingestedAt: '2026-10-08T12:39:48.754Z'
---

## Overview

Inefficient algorithmic complexity in the JSON reader of Progressive Robot hMailServer allows a remote unauthenticated attacker to make the mail services unavailable. Reading a JSON object kept the first of each duplicated member name by searching the members already read, so an object of N distinct member names cost O(N^2): 1 MB took 8.9 seconds and 4 MB took 300 seconds against the affected code. A remote attacker reaches the reader without an account by mailing a crafted TLS-RPT report (up to 16 MB after decompression) to a hosted domain's published report mailbox, which is read on a delivery thread; a few such reports hold every delivery thread (ten by default), so the server delivers no mail, local or outbound, for over an hour per report. A signed-in account reaches the same 16 MB body through the webmail's own REST routes, holding the REST API's own worker threads. Where no report mailbox is configured, the unauthenticated REST sign-in routes that read a JSON body are capped at 64 KB and are not affected.

## Remediation

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