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PatchSiren cyber security CVE debrief

CVE-2026-9549 Checkmk GmbH CVE debrief

CVE-2026-9549 is a MEDIUM severity vulnerability in Checkmk versions before 2.5.0p5, 2.4.0p31, 2.3.0p48, and all 2.2.0 versions. This vulnerability allows an administrator who can configure active or custom checks to inject malicious HTML or JavaScript into check output that executes in the browser of an admin or a user with host read permissions when they run the check on the service discovery page.

Vendor
Checkmk GmbH
Product
Checkmk
CVSS
MEDIUM 4.8
CISA KEV
Not listed in stored evidence
Original CVE published
2026-06-08
Original CVE updated
2026-07-23
Advisory published
2026-06-08
Advisory updated
2026-07-23

Who should care

Administrators and users of Checkmk versions before 2.5.0p5, 2.4.0p31, 2.3.0p48, and all 2.2.0 versions who have host read permissions or can configure active or custom checks.

Technical summary

The vulnerability is caused by a lack of proper input validation and sanitization in the service discovery active check output. An attacker with the ability to configure active or custom checks can inject malicious HTML or JavaScript code into the check output.

Defensive priority

MEDIUM

Recommended defensive actions

  • Upgrade to Checkmk version 2.5.0p5 or later, 2.4.0p31 or later, or 2.3.0p48 or later.
  • Restrict access to active and custom check configuration to trusted administrators.
  • Monitor service discovery page output for suspicious activity.

Evidence notes

The CVE-2026-9549 vulnerability was published on 2026-06-08T13:16:34.030Z and modified on 2026-06-08T15:53:09.253Z. The vulnerability has a CVSS score of 4.8 and is classified as MEDIUM severity.

Sources and references

Verified primary and authoritative sources

  • CVE-2026-9549 CVE Program record

    Publisher, destination, and source semantics verified

    URL: https://www.cve.org/CVERecord?id=CVE-2026-9549

    CVE Program - Official CVE Program record with source-provided CVE metadata.

  • CVE-2026-9549 NVD vulnerability detail

    Publisher, destination, and source semantics verified

    URL: https://nvd.nist.gov/vuln/detail/CVE-2026-9549

    NIST National Vulnerability Database - Official NIST NVD detail page and source-specific vulnerability assessment.

Supplemental references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.