CVE-2026-107568: Auth Bypass (CVSS 6.5)

Description

Missing authorization in the Sieve notify action (6.2.22 through 6.3.6) and in the mail-rule Reply action (6.0.0 through 6.3.6) of Progressive Robot hMailServer allows an authenticated user to send mail through the server as any address. A Sieve notify sends a notification message, and its :from tag names the sender; the server set the notification's From header to that :from as the script wrote it, without checking that the address belongs to the account - the check that the Sieve vacation action applies to its own :from, and that the server applies to the sender of mail queued over COM or sent from the webmail. An account that can store a Sieve script (over ManageSieve, the REST API's self-service filter, or the administration interface) could therefore have the server send a notification from any address, on this server or anywhere; and where the claimed address is at a domain the server hosts with DKIM signing enabled, the server signed the notification with that domain's DKIM key, because it signs by the From header's domain, producing forged mail aligned to pass the domain's DMARC policy. The notify action and this behaviour were introduced in 6.2.22. In the mail-rule engine, a rule's Reply was likewise sent from whatever address the action named, whoever the rule belonged to, so that an account acting on its own rules over COM, a domain administrator, or an API key confined to a domain (over the REST API from 6.3.4) could have a domain's or an account's rule reply from any address, signed, where that address is at a domain the server hosts with DKIM signing enabled, with that domain's key.

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CVSS v3.1 Score

6.5

MEDIUM

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

EPSS — Exploit Prediction

0.0024

Probability of exploitation

0.16%

Percentile rank

EPSS estimates the probability that this vulnerability will be exploited in the wild within the next 30 days. A higher score means more likely to be exploited.

Weakness Type (CWE)

CWE-862 Missing Authorization

References

Other References

https://gitlab.com/hmailserver/hmailserver/-/releases/v6.3.7 https://gitlab.com/hmailserver/hmailserver/-/work_items/82

Frequently Asked Questions

What is CVE-2026-107568? +

Missing authorization in the Sieve notify action (6.2.22 through 6.3.6) and in the mail-rule Reply action (6.0.0 through 6.3.6) of Progressive Robot hMailServer allows an authenticated user to send mail through the server as any address. A Sieve notify sends a notification message, and its :from tag names the sender; the server set the notification's From header to that :from as the script wrote it, without checking that the address belongs to the account - the check that the Sieve vacation action applies to its own :from, and that the server applies to the sender of mail queued over COM or sent from the webmail. An account that can store a Sieve script (over ManageSieve, the REST API's self-service filter, or the administration interface) could therefore have the server send a notification from any address, on this server or anywhere; and where the claimed address is at a domain the server hosts with DKIM signing enabled, the server signed the notification with that domain's DKIM key, because it signs by the From header's domain, producing forged mail aligned to pass the domain's DMARC policy. The notify action and this behaviour were introduced in 6.2.22. In the mail-rule engine, a rule's Reply was likewise sent from whatever address the action named, whoever the rule belonged to, so that an account acting on its own rules over COM, a domain administrator, or an API key confined to a domain (over the REST API from 6.3.4) could have a domain's or an account's rule reply from any address, signed, where that address is at a domain the server hosts with DKIM signing enabled, with that domain's key. It has a CVSS v3.1 base score of 6.5 (MEDIUM).

How severe is CVE-2026-107568? +

CVE-2026-107568 has a CVSS v3.1 score of 6.5 out of 10, rated MEDIUM. This is a medium-severity vulnerability that should be remediated as part of regular maintenance. The EPSS score is 0.0024, placing it in the 0th percentile for exploitation probability.

How do I check if I'm vulnerable to CVE-2026-107568? +

You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.

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