CVE-2026-107566: Missing Authorization in hMailServer
Missing authorization in the Sieve editheader action (6.2.22 through 6.3.6) and in the mail-rule Set header value action (6.0.0 through 6.3.6) of Progressive Robot hMailServer allows an authenticated user to make the server send mail as any address, DKIM-signed where the address is at a domain the server signs for. A Sieve script's editheader (addheader/deleteheader) could add or delete any header field except Received and Return-Path, and the server applies a script's header edits before it queues a redirect, so the redirected copy is sent as the script left it. The server signs a message's first delivery pass as the domain named in its From header wherever it signs DKIM for that domain and the message carries no signature of it. A script could therefore delete and re-add From - another account's address, or anyone's at a hosted domain - and redirect, so the server sent the message in that name and, for a hosted domain, signed it with that domain's DKIM key, a forgery aligned to pass the domain's DMARC; or, where another sender's domain is one the server signs for, delete the DKIM-Signature that sender's message arrived with, alter a field, and redirect, so the server re-signed the altered message as that sender's domain. editheader placed no restriction on From, Sender, Resent-From, Resent-Sender or DKIM-Signature. The editheader action was introduced in 6.2.22. In the mail-rule engine, a domain's or an account's rule's Set header value action could likewise set From, Sender, Resent-From, Resent-Sender or DKIM-Signature ahead of a forward or a copy, 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 the server forward or copy a message in the name of any address, signed, where that address is at a domain the server hosts with DKIM signing enabled, with that domain's key.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
Progressive Robot hMailServerto a version that resolves this vulnerability.Fixed in 6.3.7 - Configuration
Keep ManageSieve off by setting ManageSieveServerPort=0, the default, or bind it only to the loopback interface where accounts do not need to set Sieve scripts.
ManageSieve ManageSieveServerPort = 0 - Configuration
Turn the REST API off by setting RestApiPort=0, the default, where the webmail filter editor is not wanted.
hMailServer REST API RestApiPort = 0 - Operational
Review active Sieve scripts for addheader or deleteheader operations on From, Sender, Resent-From, Resent-Sender, or DKIM-Signature followed by a redirect, and review account and domain rules for Set header value actions on those fields.
Event History
Frequently Asked Questions
Which deployments are exposed?
The affected functionality is present in hMailServer 6.2.22 through 6.3.6 for Sieve editheader actions, and 6.0.0 through 6.3.6 for mail-rule Set header value actions. The highest-impact case is a server that signs DKIM for hosted domains, because altered messages can be signed as those domains.
What access does an attacker need?
An attacker needs to be an authenticated user. They can use Sieve editheader actions or mail-rule header-setting actions to alter message headers and redirect the resulting message.
What can an attacker do with this access?
They can cause the server to send mail with a forged From address, including another account or an address at a hosted domain. For domains the server signs, the redirected message can receive a DKIM signature aligned with the forged From domain and potentially pass DMARC.
Can an attacker modify an existing signed message?
Yes. If the server signs for the original sender's domain, an attacker can remove the existing DKIM-Signature, alter a field, and redirect the message so that the server signs it again.