# CVE-2021-36563: Cross-site scripting in n/a

> Stored XSS in CheckMK WATO module allows attackers to steal API secrets and escalate privileges to administrator.

- **Severity:** Medium 5.4 (`CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Over the internet
- **Victim action needed:** A user has to click or open something
- **Fix available:** Yes
- **Published:** 26 Jul 2021
- **Updated:** 4 Aug 2024
- **Also known as:** GHSA-8r25-xwf2-j5q3, UBUNTU-CVE-2021-36563

## What is CVE-2021-36563?

CVE-2021-36563 is a medium-severity cross-site scripting vulnerability in Checkmk Checkmk, published on 26 Jul 2021. Any signed-in user can exploit it over the network, and only if a user opens a file or follows a link.

### Official 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.

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

## Which versions are affected by CVE-2021-36563?

It affects 1 product.


Products: Checkmk Checkmk.

## How do I fix CVE-2021-36563?

Upgrade to a fixed version.

## Is CVE-2021-36563 being exploited?

There is no known exploitation of CVE-2021-36563 in the wild and no public exploit code.

Public exploit code:

- https://github.com/Edgarloyola/CVE-2021-36563 (github_poc): https://github.com/Edgarloyola/CVE-2021-36563

## 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-2021-36563 and fix it if it is affected.

What it is: CVE-2021-36563 is a medium-severity cross-site scripting vulnerability in Checkmk Checkmk, published on 26 Jul 2021. Any signed-in user can exploit it over the network, and only if a user opens a file or follows a link.
Exploitation: There is no known exploitation of CVE-2021-36563 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-2021-36563/ (Markdown: https://insights.sbomify.com/CVE-2021-36563/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-36563
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-36563
- OSV: https://osv.dev/vulnerability/CVE-2021-36563

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From sbomify Insights: https://insights.sbomify.com/CVE-2021-36563/
Summaries are generated and may contain mistakes; the official description above is the record.