CVE-2026-126626: hMailServer Vulnerability (CVSS 5.3)

Description

The web services listener of Progressive Robot hMailServer 6.3.3 through 6.3.7 counts every failed CardDAV, CalDAV or Autodiscover sign-in from a proxy it trusts - one on the same machine, or one named in WebServicesTrustedProxies, whose X-Forwarded-Proto it believes - against the proxy's own address, which allows a remote unauthenticated attacker who can reach the proxy to have that address banned from the server's mail listeners. The listener, off by default (WebServicesHttpPort and WebServicesHttpsPort 0), signs an account in with HTTP Basic over HTTPS for CardDAV from 6.3.3 and for CalDAV and Outlook Autodiscover from 6.3.4. Behind a TLS-terminating proxy, which the server trusts when it is on the same machine or named in WebServicesTrustedProxies, the listener takes the client's HTTPS from the proxy's X-Forwarded-Proto header, but it does not believe X-Forwarded-For and sees the proxy's address for every client. Each failed sign-in counted towards the auto-ban against that address, so that failed sign-ins through the proxy from anybody, under any user name, reaching MaxInvalidLogonAttempts (3 within 30 minutes by default) wrote an IP range that refused the proxy's address at the SMTP, POP3, IMAP and ManageSieve listeners for AutoBanMinutes (60 by default), and could write it again each time it lapsed. For a proxy on the same machine that address is the loopback address, so that a webmail on that machine, local scripts that submit mail and the other local clients of the mail listeners were shut out; for a proxy at another address that also carries mail, every mail client behind it was. Such a proxy forwarding to the listener's HTTPS port, re-encrypting, was counted against in the same way, and listing it in WebServicesTrustedProxies did not spare it. The auto-ban is on by default and AutoBanNeverBan, which spares an address, is empty by default; the listener itself went on serving the proxy.

Metrics

CVSS 3.1

5.3/10

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

EPSS Probability

0.52%

44.8th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

CWE-645

Affected Software

Source: CNA advisory (CVE.org). NVD analysis pending.

Vendor	Product	Versions
Progressive Robot Ltd	hMailServer	>= 6.3.3, <= 6.3.7

References

Timeline

Published: Nov 11, 2026
Last Modified: Nov 12, 2026
Status: Deferred

Frequently Asked Questions

What is CVE-2026-126626?

The web services listener of Progressive Robot hMailServer 6.3.3 through 6.3.7 counts every failed CardDAV, CalDAV or Autodiscover sign-in from a proxy it trusts - one on the same machine, or one named in WebServicesTrustedProxies, whose X-Forwarded-Proto it believes - against the proxy's own address, which allows a remote unauthenticated attacker who can reach the proxy to have that address banned from the server's mail listeners. The listener, off by default (WebServicesHttpPort and WebServicesHttpsPort 0), signs an account in with HTTP Basic over HTTPS for CardDAV from 6.3.3 and for CalDAV and Outlook Autodiscover from 6.3.4. Behind a TLS-terminating proxy, which the server trusts when it is on the same machine or named in WebServicesTrustedProxies, the listener takes the client's HTTPS from the proxy's X-Forwarded-Proto header, but it does not believe X-Forwarded-For and sees the proxy's address for every client. Each failed sign-in counted towards the auto-ban against that address, so that failed sign-ins through the proxy from anybody, under any user name, reaching MaxInvalidLogonAttempts (3 within 30 minutes by default) wrote an IP range that refused the proxy's address at the SMTP, POP3, IMAP and ManageSieve listeners for AutoBanMinutes (60 by default), and could write it again each time it lapsed. For a proxy on the same machine that address is the loopback address, so that a webmail on that machine, local scripts that submit mail and the other local clients of the mail listeners were shut out; for a proxy at another address that also carries mail, every mail client behind it was. Such a proxy forwarding to the listener's HTTPS port, re-encrypting, was counted against in the same way, and listing it in WebServicesTrustedProxies did not spare it. The auto-ban is on by default and AutoBanNeverBan, which spares an address, is empty by default; the listener itself went on serving the proxy.

How severe is CVE-2026-126626?

CVE-2026-126626 has a CVSS score of 5.3/10 (MEDIUM severity). The EPSS model estimates a 0.52% probability of exploitation in the next 30 days.

How do I fix CVE-2026-126626?

Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.

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