dbcve.org
CheckmkApplication

CVE-2025-64999

MEDIUM · 5.4 CVSS v3.1 Published 2026-02-26
Mitigation only
No fix yet — a mitigation exists. There is no fixed release. A documented workaround reduces exposure in the meantime.
See remediation →
58/100
Remediation priority · Elevated
Remotely reachable

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
Improper neutralization of input in Checkmk versions 2.4.0 before 2.4.0p22, and 2.3.0 before 2.3.0p43 allows an attacker that can manipulate a host's check output to inject malicious JavaScript into the Synthetic Monitoring HTML logs, which can then be accessed via a crafted phishing link.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

A stored XSS vulnerability in Checkmk's Synthetic Monitoring feature allows authenticated attackers to inject malicious JavaScript through manipulated host check output. The malicious payload gets stored in HTML logs and executes when users access the logs via a crafted phishing link.

MitigationUpdate Checkmk to version 2.4.0p22, 2.3.0p43, or later to patch the improper input neutralization vulnerability.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
CheckmkApplication
Affected:= 2.3.0= 2.4.0

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
Low
User interaction
Required
Scope
Changed
Confidentiality
Low
Integrity
Low
Availability
None

CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Identify Checkmk version
    Run 'cmk --version' or check /etc/check_mk/version file to determine the installed Checkmk version number
    Affected if The installed version is 2.3.0 or 2.4.0 (prior to patches 2.3.0p43 and 2.4.0p22)
  2. Confirm Synthetic Monitoring is in use
    Check if Synthetic Monitoring feature is configured - look for 'synthetic' or 'smtc' related configuration files in /omd/sites/*/etc/check_mk/ or via 'cmk -L' command listing active modules
    Affected if Synthetic Monitoring feature is enabled and configured with host checks
  3. Inspect HTML log storage
    Locate HTML log files for Synthetic Monitoring - typically in /omd/sites/*/var/log/ or /omd/sites/*/tmp/ directories, search for files containing 'synthetic' or 'log' in their paths
    Affected if HTML log files exist and contain stored host check output data
  4. Review host check output entries
    Examine the stored log files for any host check output entries that may contain unescaped HTML or script tags in the output fields
    Affected if Host check output in logs contains raw HTML characters or script tags that could indicate exploitation

You are affected if running Checkmk version 2.3.0 or 2.4.0 with Synthetic Monitoring enabled and stored HTML logs contain unescaped script content from host check output.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Mitigation available No clean upgrade yet — mitigate in the meantime
Mitigation

Update Checkmk to version 2.4.0p22, 2.3.0p43, or later to patch the improper input neutralization vulnerability.

Recommended fix High confidence

Checkmk 2.4.0p22 or 2.3.0p43

  1. Download Checkmk version 2.4.0p22 (for Checkmk 2.4.0 installations) or version 2.3.0p43 (for Checkmk 2.3.0 installations) from the official Checkmk download portal
  2. Back up your current Checkmk configuration and database before proceeding with the upgrade
  3. Stop the Checkmk services on your instance
  4. Install the appropriate patched version (2.4.0p22 or 2.3.0p43) using the standard Checkmk upgrade process
  5. Restart the Checkmk services
  6. Verify the upgrade was successful and the Synthetic Monitoring functionality is working correctly

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Checkmk Scoped from the published advisory
  • Consultation2.0 h
  • Implementation4.0 h
  • Testing3.0 h
  • Review / QA1.0 h
10.0 hours of engineering $1,750
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2025-64999 in production — separate from our analysis above.

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data