VulnerabilityPublished 11 Nov 2026

CVE-2026-126626: Progressive Robot hMailServer · Overly Restrictive Account Lockout Mechanism

Affects Progressive Robot hMailServerSource CVE Database V5

Compare

Not known to be exploited · CVSS 5.3 Medium.

Data as of 11 Nov 2026, 08:50 UTC

CVSS 3.1 base score

5.3Medium

How severe the flaw is on the 0–10 CVSS scale.

Thin evidence

One scorer or none

CVSS scores by source
CNA (GitLab)	CVSS 3.1	5.3

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

Attack Vector: Network
Attack Complexity: Low
Privileges Required: None
User Interaction: None
Scope: Unchanged
Confidentiality: None
Integrity: None
Availability: Low

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable component is bound to the network stack and the set of possible attackers extends beyond the other options listed, up to and including the entire Internet. Such a vulnerability is often termed 'remotely exploitable' and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: Low: Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success against the vulnerable component.
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without any interaction from any user.
Scope: Unchanged: An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
Confidentiality: None: There is no loss of confidentiality within the impacted component.
Integrity: None: There is no loss of integrity within the impacted component.
Availability: Low: Performance is reduced or there are interruptions in resource availability. Even if repeated exploitation of the vulnerability is possible, the attacker does not have the ability to completely deny service to legitimate users. The resources in the impacted component are either partially available all of the time, or fully available only some of the time, but overall there is no direct, serious consequence to the impacted component.

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 11 Nov 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponized: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 10 Nov 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponized: A Metasploit exploit module or Exploit-DB entry exists (Metasploit scanner modules do not count), or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Affected software

Product	Affected	Fixed in
Progressive Robot LtdhMailServer	Affected >=6.3.3 <=6.3.7	Fixed in–

AI-suggested

Administrators should be aware that the auto-ban feature is enabled by default and that trusted proxies' IP addresses can be banned due to aggregated failed login attempts. Mitigation options include disabling the web services listener if not needed (it is off by default), adjusting MaxInvalidLogonAttempts and AutoBanMinutes settings to reduce the risk of unintended bans, or configuring AutoBanNeverBan to exempt trusted proxy IP addresses. Monitoring and adjusting proxy trust settings may also help. Check vendor advisories for any future patches or official fixes.

Description

CVE-2026-126626 is a vulnerability in Progressive Robot Ltd's hMailServer versions 6.3.3 through 6.3.7 involving an overly restrictive account lockout mechanism. The web services listener counts failed sign-in attempts from trusted proxies against the proxy's own IP address rather than the individual client IPs. This can cause legitimate mail clients behind the proxy to be auto-banned from accessing SMTP, POP3, IMAP, and ManageSieve services for a default period of 60 minutes after three failed attempts within 30 minutes. The issue affects setups using trusted proxies, including those on the same machine or listed in WebServicesTrustedProxies, and results in denial of service for legitimate users behind those proxies.

Impact

This vulnerability can cause denial of service for legitimate mail clients behind trusted proxies by auto-banning the proxy's IP address from accessing mail services (SMTP, POP3, IMAP, ManageSieve) after a small number of failed authentication attempts. This affects both remote proxies and local proxies on the same machine, potentially disrupting mail delivery and access for multiple users. There is no direct impact on confidentiality or integrity, but availability is affected.

Summary

The vulnerability arises because the hMailServer web services listener, introduced in versions 6.3.3 through 6.3.7, counts failed CardDAV, CalDAV, or Autodiscover sign-in attempts from trusted proxies against the proxy's IP address rather than the originating client IP. The listener trusts proxies on the same machine or named in WebServicesTrustedProxies and uses the X-Forwarded-Proto header to determine HTTPS but does not trust X-Forwarded-For, causing all failed attempts from clients behind the proxy to be aggregated and attributed to the proxy's IP. When the number of failed attempts reaches the MaxInvalidLogonAttempts threshold (default 3 within 30 minutes), the proxy's IP is auto-banned from mail listeners for AutoBanMinutes (default 60). This can lead to denial of service for all clients behind the proxy, including local clients if the proxy is on the same machine (loopback address). The auto-ban feature is enabled by default, and no addresses are exempt by default.

CWE-645Overly Restrictive Account Lockout Mechanism (opens in a new tab)
Base weaknessLikelihood of exploit: High
The product contains an account lockout protection mechanism, but the mechanism is too restrictive and can be triggered too easily, which allows attackers to deny service to legitimate users by causing their accounts to be locked out.
Architecture and Design
Implement more intelligent password throttling mechanisms such as those that take IP address into account, in addition to the login name.
Architecture and Design
Implement a lockout timeout that grows as the number of incorrect login attempts goes up, eventually resulting in a complete lockout.
Architecture and Design
Consider alternatives to account lockout that would still be effective against password brute force attacks, such as presenting the user machine with a puzzle to solve (making it do some computation).
OWASP ASVS 4.0.3
V2.8.5 (opens in a new tab)
V2.8.4 (opens in a new tab)
V9.2.3 (opens in a new tab)
Attack patterns
Inducing Account Lockout (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

Community reviews

Loading reviews…

Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 11 Nov 2026, 08:50 UTC

WeaknessesCWE-645

Technical details

References

Community reviews

Related threats