# CVE-2026-15576: Agent receiver accepts mTLS requests without a client certificate

> Unauthenticated attackers can bypass mTLS verification on Checkmk relay endpoints by using a placeholder identity, affecting specific enterprise editions.

- **Severity:** Medium 6.9 (`CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:L/VA:L/SC:N/SI:N/SA:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 21 Aug 2026
- **Updated:** 21 Aug 2026
- **Also known as:** GHSA-8fww-64wc-46pf, UBUNTU-CVE-2026-15576

## What is CVE-2026-15576?

CVE-2026-15576 is a medium-severity authentication bypass vulnerability in Checkmk, published on 21 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

Improper authentication in the agent receiver of Checkmk <2.5.0p10 allows an unauthenticated remote attacker to bypass mutual TLS client certificate verification of relay endpoints by supplying a fixed placeholder identity in the request URL, resulting in limited impact on integrity and availability. Only the Cloud, Ultimate and Ultimate MT editions are affected, as other editions do not expose relay endpoints.

Weakness: CWE-306 (Missing Authentication for Critical Functionality)

## Which versions are affected by CVE-2026-15576?


## How do I fix CVE-2026-15576?

No fix has been published for CVE-2026-15576 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-15576 being exploited?

There is no known exploitation of CVE-2026-15576 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-15576 and fix it if it is affected.

What it is: CVE-2026-15576 is a medium-severity authentication bypass vulnerability in Checkmk, published on 21 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-15576 in the wild and no public exploit code.

Affected product: 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-2026-15576/ (Markdown: https://insights.sbomify.com/CVE-2026-15576/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-15576
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-15576
- OSV: https://osv.dev/vulnerability/CVE-2026-15576

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-15576/
Summaries are generated and may contain mistakes; the official description above is the record.