# CVE-2021-40906: Cross-site scripting in n/a

> Reflected XSS in CheckMK Raw Edition allows attackers to execute scripts, steal session cookies, or open backdoors via unauthenticated web service parameters.

- **Severity:** Medium 6.1 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** A user has to click or open something
- **Fix available:** Yes
- **Published:** 25 Mar 2022
- **Updated:** 9 Jul 2026
- **Also known as:** GHSA-jv73-q4rg-p7qh, UBUNTU-CVE-2021-40906

## What is CVE-2021-40906?

CVE-2021-40906 is a medium-severity cross-site scripting vulnerability in Checkmk Checkmk, published on 25 Mar 2022. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.

### Official description

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.

Weakness: CWE-79 (Improper Neutralization of Input During Web Page Generation)

## Which versions are affected by CVE-2021-40906?

It affects 2 products.


Products: Checkmk Checkmk, Tribe29 Checkmk.

## How do I fix CVE-2021-40906?

Upgrade to a fixed version.

## Is CVE-2021-40906 being exploited?

There is no known exploitation of CVE-2021-40906 in the wild and no public exploit code.

Public exploit code:

- https://github.com/Edgarloyola/CVE-2021-40906 (github_poc): https://github.com/Edgarloyola/CVE-2021-40906

## Other vulnerabilities in Checkmk Checkmk

- [CVE-2025-39666](https://insights.sbomify.com/CVE-2025-39666/): omd: Local privilege escalation when executing omd commands as root (critical 9.3)
- [CVE-2024-8606](https://insights.sbomify.com/CVE-2024-8606/): Fix 2FA bypass via RestAPI (critical 9.2)
- [CVE-2022-46836](https://insights.sbomify.com/CVE-2022-46836/): PHP code injection in watolib (critical 9.1)
- [CVE-2025-32919](https://insights.sbomify.com/CVE-2025-32919/): Privilege Escalation in Windows License plugin for Checkmk Windows Agent (high 8.8)
- [CVE-2024-28827](https://insights.sbomify.com/CVE-2024-28827/): Privilege escalation in Windows agent (high 8.8)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-40906 and fix it if it is affected.

What it is: CVE-2021-40906 is a medium-severity cross-site scripting vulnerability in Checkmk Checkmk, published on 25 Mar 2022. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Exploitation: There is no known exploitation of CVE-2021-40906 in the wild and no public exploit code.

Affected product: Checkmk Checkmk. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2021-40906/ (Markdown: https://insights.sbomify.com/CVE-2021-40906/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-40906
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-40906
- OSV: https://osv.dev/vulnerability/CVE-2021-40906

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-40906/
Summaries are generated and may contain mistakes; the official description above is the record.