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.
Written by anthropic/claude-haiku-5.5 from the sources below. Verify against them.
- Exploited
- No known exploitation
- Fix
- Fixed in 6.3.8
- Who is exposed
- Specific setups only
Do first: Check whether your setup matches what the bug needs. Runs hMailServer 6.3.6 or 6.3.7 A domain has a report mailbox set by an administrator (needed for the mailed TLS-RPT report path) For the password-protected reply route without an account: REST API turned on and a public base URL set
- Published
- Last modified
- Whether it needs action now or is a reference record, by fixed rules, with why. Last checked is when cve.now last re-read it.What do these mean? →
- Reference
- Last checked
- A 0-100 score for how much a CVE matters across the industry: exploitation, exposure, how widely the software is used, severity and whether a fix exists.What do these mean? →
- 20/100 · provisional
- How many real-world setups can hit the bug, from the vendor's advisory and CISA data. It does not know your setup.What do these mean? →
- Specific setups only
- Whether working attack code is public (Exploit-DB, Metasploit, Nuclei or GitHub). Public code makes attacks easier.What do these mean? →
- None on Exploit-DB, Metasploit or GitHub
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.
Read the original 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.
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.
cve.now Index
- Exploitation?/40Unknown. No CISA SSVC exploitation rating yet
- Exposure12/25Specific setups only, reachable over the network without a login
- Reach?/20Unknown. No tracked technology or package matched
- Severity7.5/10CVSS 7.5
- No patch0/5No signal
What would change the score
- Would score about 70 if added to CISA KEV
- Would score about 65 if CISA's SSVC rates exploitation active
What to do
Check whether your setup matches what the bug needs.
Runs hMailServer 6.3.6 or 6.3.7 A domain has a report mailbox set by an administrator (needed for the mailed TLS-RPT report path) For the password-protected reply route without an account: REST API turned on and a public base URL setUpgrade to a fixed version: 6.3.8.
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
Check your systems with an agent
Paste this into a coding or ops agent. It checks your systems read-only and reports a table per asset.
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.
Timeline
- CVE published
- First seen by cve.now
Affected versions
- Progressive Robot Ltd hMailServer
From 6.3.6 · through 6.3.7
Evidence 5
CVE record · 3
Establishes: The identifier and description.
- Record
- Record
- Link
Corrections
None so far.
Something wrong on this page? Report an error. Every fix is listed here.
Questions about CVE-2026-126623
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: cve.now feed data
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: gitlab.com, cve.org
Which versions are affected by CVE-2026-126623?
hMailServer: from 6.3.6 through 6.3.7.Source: cve.now feed data
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: gitlab.com, cve.org
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: gitlab.com, cve.org