CVE-2026-15576: Agent receiver accepts mTLS requests without a client certificate
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.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
Checkmkto a version that resolves this vulnerability.Fixed in 2.5.0p10
Event History
Frequently Asked Questions
Which deployments are exposed to this issue?
Only Checkmk Cloud, Ultimate, and Ultimate MT editions are affected because they expose relay endpoints. Other editions do not expose relay endpoints and are not affected.
What does an attacker need to exploit the vulnerability?
An unauthenticated remote attacker must be able to send requests to a relay endpoint and supply a fixed placeholder identity in the request URL. No valid mTLS client certificate is required.
Are affected deployments safe if mutual TLS is configured?
No. The flaw bypasses mutual TLS client-certificate verification at the agent receiver for relay endpoints.
What is the impact of successful exploitation?
Successful exploitation has limited impact on integrity and availability.