# CVE-2026-126616: hMailServer auto-ban bypass via UTF-8 character handling

Source: https://junglewise.ai/threats/cve-2026-126616-the-auto-ban-of-progressive-robot-hmailserver-6-2-0-through-6-3  
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-2026-126616 · Severity: medium · CVSS 5.3 · Published 2026-11-11

hMailServer is an email server that includes an auto-ban feature to lock out addresses after repeated failed login attempts. A character encoding mismatch between the database and authentication system allows attackers to bypass this protection and continue guessing passwords indefinitely from the same address without being locked out. This enables persistent brute-force attacks against email accounts.

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

## Quick answers

**Is CVE-2026-126616 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-126616 being exploited in the wild.

**How severe is CVE-2026-126616?** Medium severity, with a CVSS score of 5.3, published 11 November 2026.

**Which products and versions are affected?** Progressive Robot hMailServer 6.2.0 through 6.3.3.

**Is there a fix for CVE-2026-126616?** No fixed version is recorded yet. Check the vendor's advisory in the references below. 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-2026-126616 attacked?** It is attacked over the network, in a hosted service.

**Am I affected by CVE-2026-126616?** 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.

## Technical details

The vulnerability stems from a mismatch between MySQL tables created in utf8 character set (which cannot store characters outside the Basic Multilingual Plane) and SASL PLAIN usernames decoded as UTF-8 (which can contain such characters). When a username with an out-of-plane character is used in the failed logon that would trigger a ban, the database rejects the INSERT of the auto-ban rule due to character encoding constraints. The count of failed attempts is cleared but the address is not added to the ban list, resetting the attack counter and allowing the attacker unbounded login attempts. The vulnerability requires MySQL or MariaDB backend and affects multiple authentication protocols (POP3, IMAP, SMTP AUTH PLAIN where enabled).

## Affected products

- Progressive Robot hMailServer 6.2.0 through 6.3.3

## 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)