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.
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
- Not reported
Do first: Upgrade to a fixed version: 6.3.8.
- 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? →
- 25/100 · 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? →
- Unclear
- 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
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.
Read the original 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.
cve.now Index
- Exploitation?/40Unknown. No CISA SSVC exploitation rating yet
- Exposure17/25No login or user action needed, reachable over the network without a login · Unknown: Advisories don't say whether default setups are affected
- 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
- Would score about 30 if advisories confirm default setups are exposed
What to do
Upgrade 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-2026-126613 (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.
Timeline
- CVE published
- First seen by cve.now
Affected versions
- Progressive Robot Ltd hMailServer
From 6.3.2 · through 6.3.7
Evidence 4
CVE record · 2
Establishes: The identifier and description.
- Record
- Record
Corrections
None so far.
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Questions about CVE-2026-126613
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: cve.now feed data
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: gitlab.com, cve.org
Which versions are affected by CVE-2026-126613?
hMailServer: from 6.3.2 through 6.3.7.Source: cve.now feed data
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: gitlab.com, cve.org
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: gitlab.com, cve.org