CVE-2026-126626VulnerabilityPublished 11 Nov 2026

The web services listener of Progressive Robot hMailServer 6.3.3 through 6.3.7 counts every failed CardDAV, CalDAV or

Source GCVE Database

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Authoritative record:CVE-2026-126626· Medium 5.3

Not known to be exploited · CVSS 5.3 Medium.

Data as of 11 Nov 2026, 10:04 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.

ghsa-874v-49pf-3wjr

What to do

Affected software

Product	Affected	Fixed in
Progressive Robot LtdhMailServer	Affected >=6.3.3 <=6.3.7	Fixed in–
GitHubhmailserver/hmailserver`pkg:github/hmailserver/hmailserver`Other threats	Affected >=6.3.3 <=6.3.7	Fixed in–

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

AI-suggested

The web services listener is off by default (WebServicesHttpPort and WebServicesHttpsPort set to 0), so disabling it prevents exposure. If enabled, administrators should consider removing proxies from WebServicesTrustedProxies or adjusting auto-ban settings to prevent unintended bans. No other specific mitigations are documented.

Description

Progressive Robot hMailServer versions 6.3.3 through 6.3.7 have a vulnerability in the web services listener related to how failed CardDAV, CalDAV, or Autodiscover sign-ins from trusted proxies are counted. The listener counts failed sign-ins against the proxy's IP address rather than the actual client, potentially causing the proxy's address to be auto-banned from the server's mail listeners. This can result in denial of service for legitimate mail clients behind the proxy. The listener is off by default and uses HTTP Basic authentication over HTTPS for these services. The auto-ban feature is enabled by default, and no patch or fixed versions are explicitly stated.

Impact

This vulnerability allows a remote unauthenticated attacker who can reach a trusted proxy to cause denial of service by triggering auto-ban of the proxy's IP address on the mail server's listeners. Legitimate mail clients behind the proxy or local services on the same machine may be blocked from accessing mail services for the duration of the ban. There is no direct confidentiality or integrity impact reported. The CVSS score is 5.3 (medium), reflecting availability impact only.

Summary

The web services listener in Progressive Robot hMailServer versions 6.3.3 through 6.3.7 counts every failed CardDAV, CalDAV, or Autodiscover sign-in attempt from a proxy it trusts against the proxy's own IP address. This occurs because the listener trusts the X-Forwarded-Proto header from proxies on the same machine or listed in WebServicesTrustedProxies but does not trust X-Forwarded-For, causing all client requests to appear as coming from the proxy's IP. When failed sign-ins reach the MaxInvalidLogonAttempts threshold (default 3 within 30 minutes), the proxy's IP is auto-banned from SMTP, POP3, IMAP, and ManageSieve listeners for AutoBanMinutes (default 60). This can cause denial of service for all mail clients behind the proxy or local services if the proxy is on the same machine.

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

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Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 11 Nov 2026, 10:04 UTC

WeaknessesCWE-645

Technical details

References

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