---
title: "CVE-2026-104659: Progressive Robot hMailServer - Authentication Bypass"
description: "Progressive Robot hMailServer 6.0.0 through 6.3.5 contains a broken authentication caused by missing Host header validation and lack of throttling on failed admin sign-ins in the REST API listener, letting remote attackers perform brute-force attacks via DNS rebinding, exploit requires attacker to control a web page opened in the administrator's browser."
url: https://panop.io/threat-heartbeat/CVE-2026-104659/
---

# CVE-2026-104659 Progressive Robot hMailServer - Authentication Bypass

Progressive Robot hMailServer 6.0.0 through 6.3.5 contains a broken authentication caused by missing Host header validation and lack of throttling on failed admin sign-ins in the REST API listener, letting remote attackers perform brute-force attacks via DNS rebinding, exploit requires attacker to control a web page opened in the administrator's browser.

CVSS base score 7.5 /10

High

CISA KEV

Not listed

Public exploit

None known

Authentication

Not required

EPSS

0.14% chance of exploitation in 30 days

[View the NVD record ↗](https://nvd.nist.gov/vuln/detail/CVE-2026-104659)

## Details

Type

Authentication Bypass

Impact

Broken Authentication

CVSS vector

CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:H

Published

8 October 2026

Updated

15 October 2026

Weakness

[CWE-346](https://cwe.mitre.org/data/definitions/346.html)

## Vendor fix

Update to a version later than 6.3.5 or apply patches that validate Host headers and implement throttling on failed sign-ins.

Patching closes this CVE. It does not tell you which of your systems run the affected version, or what else is exposed alongside them.

## References

- [https://nvd.nist.gov/vuln/detail/CVE-2026-104659](https://nvd.nist.gov/vuln/detail/CVE-2026-104659)
- [https://gitlab.com/hmailserver/hmailserver/-/work\_items/57](https://gitlab.com/hmailserver/hmailserver/-/work_items/57)
- [https://gitlab.com/hmailserver/hmailserver/-/releases/v6.3.6](https://gitlab.com/hmailserver/hmailserver/-/releases/v6.3.6)

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## What this record can't tell you

- Whether you run the affected software, and where
- Whether an attacker can actually reach and exploit it
- The misconfigurations that never get a CVE number
- What it would cost the business, ranked into what to fix first

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