# CVE-2026-126618: hMailServer account password brute-force via auto-ban bypass

Source: https://junglewise.ai/threats/cve-2026-126618-the-auto-ban-of-progressive-robot-hmailserver-6-0-0-b1-through-6  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-11-11. Updated: 2026-11-11  
License: CC BY 4.0 with a link back

CVE-2026-126618 · Severity: medium · CVSS 5.3 · Published 2026-11-11

hMailServer, an email server, implements automatic IP banning after repeated failed login attempts to prevent password guessing attacks. An attacker can bypass this protection by crafting usernames with specific properties (length, special characters, or non-ASCII bytes) that cause the ban system to fail silently, allowing unlimited password guesses across multiple IP addresses on SMTP, IMAP, POP3, and web service protocols.

Technologies: [Progressive Robot hMailServer](https://junglewise.ai/threats/technologies/hmailserver). Vendors: [Progressive Robot](https://junglewise.ai/threats/vendors/progressive-robot).

## Quick answers

**Is CVE-2026-126618 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-126618 being exploited in the wild.

**How severe is CVE-2026-126618?** Medium severity, with a CVSS score of 5.3, published 11 November 2026.

**Which products and versions are affected?** Progressive Robot hMailServer 6.0.0-B1 through 6.3.7.

**Is there a fix for CVE-2026-126618?** No fixed version is recorded yet. Restrict SMTP, IMAP, POP3, and web service authentication to trusted networks or require additional authentication factors (e.g., multi-factor authentication) to reduce exposure to brute-force attacks while a patch is applied. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.

**How is CVE-2026-126618 attacked?** It is attacked over the network, in a hosted service.

**Am I affected by CVE-2026-126618?** Check whether you run Progressive Robot hMailServer at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix


Restrict SMTP, IMAP, POP3, and web service authentication to trusted networks or require additional authentication factors (e.g., multi-factor authentication) to reduce exposure to brute-force attacks while a patch is applied.

## Technical details

The vulnerability stems from a flaw in the auto-ban mechanism's IP range naming logic: when a username exceeds 90 characters or contains NUL bytes, the database insertion of the auto-ban IP range fails (due to truncation or statement parsing), leaving the attacker's address unbanned and its failed-attempt counter reset. An unauthenticated remote attacker on the SMTP, IMAP, POP3, ManageSieve, CalDAV/CardDAV, or REST API ports can trigger this by cycling through password guesses under maliciously-crafted usernames, receiving a fresh set of attempts after each ban-system failure. The impact scales with the number of available IP addresses the attacker can use.

## Affected products

- Progressive Robot hMailServer 6.0.0-B1 through 6.3.7

## Timeline

- 2026-11-11: disclosed

## Related threats

- [Progressive Robot hMailServer denial of service in web services listener](https://junglewise.ai/threats/cve-2026-126626-the-web-services-listener-of-progressive-robot-hmailserver-6-3-3)
- [hMailServer Windows installer privilege escalation via insecure folder permissions](https://junglewise.ai/threats/cve-2026-126625-incorrect-default-permissions-in-the-windows-installer-of)
- [hMailServer Windows installer privilege escalation in Bin folder](https://junglewise.ai/threats/cve-2026-126624-unverified-ownership-in-the-windows-installer-of-progressive)
- [Progressive Robot hMailServer inefficient JSON parsing DoS](https://junglewise.ai/threats/cve-2026-126623-inefficient-algorithmic-complexity-in-the-json-reader-of)
- [Progressive Robot hMailServer incorrect default installer permissions](https://junglewise.ai/threats/cve-2026-126622-incorrect-default-permissions-in-the-windows-installer-of)