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Public proof of conceptFix available

CVE-2021-36563

A public proof of concept exists; no exploitation attested.

  • Published2021-07-26
  • WeaknessCWE-79

Am I affected?

With a domain, we read the version its public headers and pages expose. A distribution build can carry the fix under an older number.

Description

The CheckMK management web console (versions 1.5.0 to 2.0.0) does not sanitise user input in various parameters of the WATO module. This 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), the XSS payload will be triggered when the user accesses some specific sections of the application. In the same sense a very dangerous potential way would be when an attacker who has the monitor role (not administrator) manages to get a stored XSS to steal the secretAutomation (for the use of the API in administrator mode) and thus be able to create another administrator user who has high privileges on the CheckMK monitoring web console. Another way is that persistent XSS allows an attacker to modify the displayed content or change the victim's information. Successful exploitation requires access to the web management interface, either with valid credentials or with a hijacked session.

Timeline

dates in UTC
  1. 2021-07-2618:15CVE publishedNVD

Versions per branch

NVD
Checkmk
BranchAffectedFixed in
2.0≥ 1.5.0, ≤ 2.0.0no fix published

as publishedRanges as NVD and OSV publish them; a distribution’s build can be fixed under an older number.

Distribution builds and vendor advisories

Packages from OSV.dev and products from the vendors' CSAF advisories.

version not verifiedVersions not verified: read as OSV.dev and the vendors publish them, not checked against each vendor's release list.

Packages

  • check-mkUbuntu:Pro:16.04:LTSall versions · fixed in 1.2.6p12-1ubuntu0.16.04.1+esm1
  • check-mkUbuntu:18.04:LTSall versions · fixed in 1.2.8p16-1ubuntu0.2

Full record on NVD

Fix

Advisories and patches

Detection

read 2026-11-01 01:02 UTC
Nuclei template
none so far
ProjectDiscovery
Exploit-DB
no entry
Exploit-DB
Proof of concept
Metasploit modules
none listed
Metasploit Framework

Sources

  • NVDCVSS 3.15.4read 2026-10-23
  • NVDCVSS 2.03.5read 2026-10-23
  • NVDweakness (CWE)CWE-79read 2026-10-23
  • FIRST EPSSEPSS score1.7 %2026-11-04
  • PoC-in-GitHubproof of concept1known since 2026-11-01
  • OSVaffected packages—

Identifiers

Technical details

CVSS v3 Vector
CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N
Published
2021-07-26
Last Modified
2026-06-17
NVD status
Modified

Frequently asked questions

What is CVE-2021-36563?

The CheckMK management web console (versions 1.5.0 to 2.0.0) does not sanitise user input in various parameters of the WATO module. This 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), the XSS payload will be triggered when the user accesses some specific sections of the application. In the same sense a very dangerous potential way would be when an attacker who has the monitor role (not administrator) manages to get a stored XSS to steal the secretAutomation (for the use of the API in administrator mode) and thus be able to create another administrator user who has high privileges on the CheckMK monitoring web console. Another way is that persistent XSS allows an attacker to modify the displayed content or change the victim's information. Successful exploitation requires access to the web management interface, either with valid credentials or with a hijacked session.

Is CVE-2021-36563 actively exploited?

Active exploitation of CVE-2021-36563 has not been confirmed. Its EPSS score was 1.7% on 2026-11-04, the estimated probability of exploitation in the next 30 days.

What is the CVSS score for CVE-2021-36563?

CVE-2021-36563 has a CVSS v3 base score of 5.4 (MEDIUM severity), with vector string CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N.

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