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CheckmkApplication

CVE-2021-40906

MEDIUM · 6.1 CVSS v3.1 Published 2022-03-25
Fix available
A fix is available. Upgrade to 1.6.0 or later.
See remediation →
68/100
Remediation priority · Elevated
Remotely reachable No privileges

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
CheckMK Raw Edition software (versions 1.5.0 to 1.6.0) does not sanitise the input of a web service parameter that is in an unauthenticated zone. This Reflected XSS allows an attacker to open a backdoor on the device with HTML content and interpreted by the browser (such as JavaScript or other client-side scripts) or to steal the session cookies of a user who has previously authenticated via a man in the middle. Successful exploitation requires access to the web service resource without authentication.

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

CheckMK Raw Edition versions 1.5.0 to 1.6.0 contains a reflected XSS vulnerability where a web service parameter in an unauthenticated zone does not sanitize user input. This allows attackers to inject malicious HTML or JavaScript that executes in victim browsers, enabling session cookie theft or establishing browser-based backdoors.

MitigationUpgrade to a patched version of CheckMK or apply vendor-provided security updates. Implement WAF rules to filter malicious XSS payloads on the affected endpoint.

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:>= 1.5.0, < 1.6.0= 1.6.0
CheckmkApplication
Affected:= 1.6.0b10= 1.6.0b11= 1.6.0p10= 1.6.0p17= 1.6.0p18

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
None
User interaction
Required
Scope
Changed
Confidentiality
Low
Integrity
Low
Availability
None

CVSS:3.1/AV:N/AC:L/PR:N/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 the installed CheckMK version
    Access the CheckMK web interface or check the version using the command line (omd version or omd status). Compare the version number against the affected range: 1.5.0 to 1.6.0 (including 1.6.0, 1.6.0b10, 1.6.0b11, 1.6.0p10, 1.6.0p17, 1.6.0p18).
    Affected if The installed version falls within 1.5.0 to 1.6.0 inclusive.
  2. Confirm web interface accessibility
    Determine if the CheckMK web interface is exposed to network access, particularly on ports 80/443 or the configured web server port. Verify whether the interface is accessible without authentication.
    Affected if The web interface is accessible from a network where attackers could send requests.
  3. Locate unauthenticated web service endpoints
    Identify web service URLs or API endpoints that do not require authentication. Common paths include /webapi/, /api/, or similar service endpoints. Review the CheckMK documentation or enumerate accessible endpoints on the unauthenticated zone.
    Affected if Unauthenticated web service endpoints exist and are reachable.
  4. Inspect web parameters for lack of input sanitization
    Test web service parameters by submitting benign HTML/JavaScript payloads (such as <script>alert(1)</script>) in query parameters or form inputs. Observe whether the payload is reflected unsanitized in the response.
    Affected if User-supplied input in web service parameters is reflected in HTTP responses without encoding or sanitization.

You are affected if your CheckMK version is between 1.5.0 and 1.6.0 and the unauthenticated web service endpoint is accessible and reflects user input without sanitization.

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
Upgrade available Upgrade to 1.6.0 or later
Fixed in 1.6.0
Interim mitigation

Upgrade to a patched version of CheckMK or apply vendor-provided security updates. Implement WAF rules to filter malicious XSS payloads on the affected endpoint.

Recommended fix Moderate confidence

Checkmk Raw Edition 2.0.0 or later (latest stable release)

  1. 1. Back up your current Checkmk configuration and data before proceeding with any upgrade.
  2. 2. Identify your current Checkmk version by checking the About page in the Checkmk web interface.
  3. 3. Download the latest stable version of Checkmk Raw Edition from the official Checkmk download page (checkmk.com).
  4. 4. Review the Checkmk migration guide for your target version to understand any required migration steps.
  5. 5. Execute the upgrade following Checkmk's standard upgrade procedure for your deployment type.
  6. 6. After upgrade, verify that the web service parameter vulnerability is resolved by testing the affected parameter with XSS payload.
  7. 7. Confirm normal Checkmk functionality after upgrade.
Caveat Review Checkmk migration documentation - major version upgrades (1.6 to 2.x) may require configuration adjustments and testing

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

Fix this in Checkmk Scoped from the published advisory
  • Consultation2.0 h
  • Implementation3.0 h
  • Testing3.0 h
  • Review / QA2.0 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-2021-40906 in production — separate from our analysis above.

No notes yet

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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