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.
Written by anthropic/claude-haiku-5.5 from the sources below. Verify against them.
- Exploited
- No known exploitation
- Fix
- Fixed in 2.5.0p19, 2.4.0p40
- Who is exposed
- Specific setups only
Do first: Check whether your setup matches what the bug needs. The Robotmk test service in Checkmk Synthetic Monitoring is in use. A monitored host must be compromised to add crafted agent output, for example through a malicious agent plug-in or spool file. Automatic service discovery is enabled for the host, or a monitoring admin confirms adding Robotmk services.
- Published
- Last modified
- Whether it needs action now or is a reference record, by fixed rules, with why. Last checked is when cve.now last re-read it.What do these mean? →
- Reference
- Last checked
- A 0-100 score for how much a CVE matters across the industry: exploitation, exposure, how widely the software is used, severity and whether a fix exists.What do these mean? →
- 17/100 · provisional
- How many real-world setups can hit the bug, from the vendor's advisory and CISA data. It does not know your setup.What do these mean? →
- Specific setups only
- Whether working attack code is public (Exploit-DB, Metasploit, Nuclei or GitHub). Public code makes attacks easier.What do these mean? →
- None on Exploit-DB, Metasploit or GitHub
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.
Read the original 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.
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.
cve.now Index
- Exploitation?/40Unknown. No CISA SSVC exploitation rating yet
- Exposure8/25Specific setups only, reachable over the network
- Reach?/20Unknown. No tracked technology or package matched
- Severity9.4/10CVSS 9.4
- No patch0/5No signal
What would change the score
- Would score about 67 if added to CISA KEV
- Would score about 62 if CISA's SSVC rates exploitation active
- Would score about 24 if CISA's SSVC rates it automatable
What to do
Check whether your setup matches what the bug needs.
The Robotmk test service in Checkmk Synthetic Monitoring is in use. A monitored host must be compromised to add crafted agent output, for example through a malicious agent plug-in or spool file. Automatic service discovery is enabled for the host, or a monitoring admin confirms adding Robotmk services.Upgrade to a fixed version: 2.5.0p19, 2.4.0p40.
Check your systems with an agent
Paste this into a coding or ops agent. It checks your systems read-only and reports a table per asset.
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.
Timeline
- CVE published
- First seen by cve.now
- Posted on X by cve.now
Affected versions
- Checkmk GmbH Checkmk
From 2.5.0 · fixed in 2.5.0p19; From 2.4.0 · fixed in 2.4.0p40
Evidence 3
CVE record · 2
Establishes: The identifier and description.
- Record
- Record
Corrections
None so far.
Something wrong on this page? Report an error. Every fix is listed here.
Questions about CVE-2026-120495
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: cve.now feed data
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: checkmk.com
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: cve.now feed data
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: checkmk.com
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: checkmk.com