# CVE-2026-107566: Missing Authorization in hMailServer > Authenticated users can forge email headers and DKIM signatures via Sieve or mail rules, allowing them to send mail as any address on hosted domains. - **Severity:** Medium 6.5 (`CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:H/A:N`) - **Exploited in the wild:** Not known - **Login needed:** Any signed-in user - **Attackable:** Over the internet - **Victim action needed:** Not needed - **Fix available:** No - **Published:** 19 Oct 2026 - **Updated:** 20 Oct 2026 - **Also known as:** GHSA-87p7-4v38-33c9 ## What is CVE-2026-107566? CVE-2026-107566 is a medium-severity missing authorization vulnerability in Progressive Robot Ltd hMailServer, published on 19 Oct 2026. Any signed-in user can exploit it over the network, with no action from the victim. ### Official description 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. Weakness: CWE-862 (Missing Authorization) ## Which versions are affected by CVE-2026-107566? ## How do I fix CVE-2026-107566? No fix has been published for CVE-2026-107566 yet. Limit exposure of the affected software until one is available. ## Is CVE-2026-107566 being exploited? There is no known exploitation of CVE-2026-107566 in the wild and no public exploit code. ## Fix prompt for AI coding agents ```text Check this codebase for CVE-2026-107566 and fix it if it is affected. What it is: CVE-2026-107566 is a medium-severity missing authorization vulnerability in Progressive Robot Ltd hMailServer, published on 19 Oct 2026. Any signed-in user can exploit it over the network, with no action from the victim. Exploitation: There is no known exploitation of CVE-2026-107566 in the wild and no public exploit code. Affected product: Progressive Robot Ltd hMailServer. No package-level version data is available, so check the vendor advisory linked from the reference page. Task: 1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies. 2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it. 3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead. 4. Run the tests and summarise what changed and why. 5. If nothing in the project is affected, say so and change nothing. Reference: https://insights.sbomify.com/CVE-2026-107566/ (Markdown: https://insights.sbomify.com/CVE-2026-107566/index.md) ``` ## Sources - CVE record: https://www.cve.org/CVERecord?id=CVE-2026-107566 - NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-107566 - OSV: https://osv.dev/vulnerability/CVE-2026-107566 --- From sbomify Insights: https://insights.sbomify.com/CVE-2026-107566/ Summaries are generated and may contain mistakes; the official description above is the record.