---
id: CVE-2021-40906
title: >-
  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
summary: >-
  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 an…
severity: medium
cvss: 6.1
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N'
cwe:
  - CWE-79
vendor: checkmk
product: checkmk
affected:
  - 'checkmk >= 1.5.0, < 1.6.0'
  - checkmk = 1.6.0
  - checkmk = 1.6.0b10
  - checkmk = 1.6.0b11
  - checkmk = 1.6.0p10
  - checkmk = 1.6.0p17
  - checkmk = 1.6.0p18
patched:
  - checkmk 1.6.0
published: '2022-03-25'
updated: '2026-07-05'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2021-40906'
references:
  - url: 'https://github.com/Edgarloyola/CVE-2021-40906'
    label: cve@mitre.org
  - url: 'http://checkmk.com'
    label: af854a3a-2127-422b-91ae-364da2661108
  - url: 'https://github.com/Edgarloyola/CVE-2021-40906'
    label: af854a3a-2127-422b-91ae-364da2661108
tags:
  - nvd
  - exploit-available
epss: 0.00989
epssPercentile: 0.61045
ingestedAt: '2026-07-06T17:03:24.117Z'
exploits:
  github: 1
  githubRepos:
    - 'https://github.com/Edgarloyola/CVE-2021-40906'
  checkedAt: '2026-09-24T07:52:48.878Z'
exploitAvailable: true
---

## Overview

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.

## Affected

- `checkmk >= 1.5.0, < 1.6.0`
- `checkmk = 1.6.0`
- `checkmk = 1.6.0b10`
- `checkmk = 1.6.0b11`
- `checkmk = 1.6.0p10`
- `checkmk = 1.6.0p17`
- `checkmk = 1.6.0p18`

## Remediation

Upgrade past the affected range:

- `checkmk 1.6.0`