CVE-2026-15576 in Checkmk
Summary
by MITRE • 08/21/2026
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.
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Analysis
by VulDB Data Team • 08/21/2026
The vulnerability identified involves an improper authentication flaw within the agent receiver component of Checkmk versions prior to 2.5.0p10. This security defect specifically impacts the mutual TLS client certificate verification mechanism used for authenticating relay endpoints. In a properly secured environment, mutual TLS requires both the server and the client to present valid certificates during the handshake process, ensuring that only authorized entities can communicate with the service. However, in this affected scenario, an unauthenticated remote attacker is able to bypass these critical authentication checks by supplying a fixed placeholder identity within the request URL. This manipulation allows the attacker to impersonate legitimate relay endpoints without possessing the required cryptographic credentials, effectively circumventing the intended access controls designed to protect the integrity of the monitoring infrastructure.
The operational impact of this vulnerability centers on limited but significant risks to data integrity and service availability. Because the authentication bypass is restricted to specific editions—namely Cloud, Ultimate, and Ultimate MT—the scope of potential exploitation is confined to environments utilizing these advanced features that expose relay endpoints. While the attacker cannot necessarily achieve full system compromise or arbitrary code execution through this flaw alone, they can interfere with the flow of monitoring data. By injecting false metrics or disrupting legitimate agent communications, an adversary could degrade the accuracy of system observability and potentially cause denial-of-service conditions for affected services. This undermines trust in the monitored environment's health status and operational visibility.
From a classification perspective, this vulnerability aligns with CWE-287, which denotes Improper Authentication, as it involves failing to properly verify the identity of an entity attempting to access resources. Furthermore, the technique employed by attackers to bypass authentication mechanisms via manipulated request parameters is consistent with tactics observed in ATT&CK framework entries related to credential manipulation or resource hijacking, specifically those involving API abuse and unauthorized access through misconfigured security controls. The reliance on a fixed placeholder identity suggests a failure in input validation logic where expected cryptographic proofs are ignored if specific URL patterns are matched, highlighting a gap in the implementation of mutual TLS protocols within the agent receiver module.
Mitigation strategies must prioritize immediate patching to version 2.5.0p10 or later, which resolves this authentication bypass by enforcing strict certificate verification regardless of request parameters. For organizations unable to apply patches immediately due to operational constraints, network-level controls such as firewall rules should be implemented to restrict access to relay endpoints exclusively from known and trusted IP addresses. Additionally, monitoring logs for unusual patterns involving placeholder identities in URL requests can aid in early detection of exploitation attempts. It is also advisable to review the configuration of mutual TLS settings across all Cloud, Ultimate, and Ultimate MT deployments to ensure that no alternative bypass methods exist through similar parameter manipulations or misconfigured certificate authorities.