CVE-2026-15937 in Checkmk
Summary
by MITRE • 09/04/2026
Improper certificate validation in Checkmk <2.5.0p10 allows a relay and a push agent that share the same UUID to reuse each other's mTLS certificate to authenticate against agent receiver endpoints in either direction, because the endpoints do not verify that the certificate was issued by their own root certificate.
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Analysis
by VulDB Data Team • 09/04/2026
The vulnerability described involves a critical flaw in the mutual Transport Layer Security (mTLS) authentication mechanism within Checkmk versions prior to 2.5.0p10. This issue stems from improper validation of X.509 certificates during the TLS handshake process, specifically regarding the verification of certificate chains against trusted root authorities. In a properly secured mTLS environment, each endpoint must verify that the presented client certificate was issued by a Certificate Authority (CA) it explicitly trusts and recognizes as its own local or organizational root CA. However, in this flawed implementation, the agent receiver endpoints fail to enforce strict chain-of-trust validation relative to their specific configured root certificates. Instead, they appear to accept any validly signed mTLS certificate provided by an agent, regardless of whether that certificate was issued for a different segment of the infrastructure or intended for a different set of receivers.
This architectural oversight creates a significant security gap where relay agents and push agents sharing identical Universally Unique Identifiers (UUIDs) can exploit this lack of strict validation to authenticate against each other's endpoints. The UUID serves as an identifier within Checkmk, but in the context of mTLS, it should not be the sole determinant for trust; rather, cryptographic proof via certificate chain verification is required. Because the endpoint does not verify that the incoming certificate was issued by its own root CA, a relay agent possessing a valid mTLS certificate can present this same credential to a push agent receiver, and vice versa. The receiving end accepts the authentication because it validates the signature against some trusted store but fails to ensure that the trust anchor matches its specific operational context or designated root authority.
The operational impact of this vulnerability is severe, as it allows for unauthorized access to monitoring data and configuration updates. An attacker who compromises one agent with a known UUID could potentially impersonate another agent sharing that identifier across different parts of the infrastructure. This enables the interception, modification, or injection of malicious commands into the Checkmk monitoring system. Since mTLS is typically used to ensure both confidentiality and integrity in communications between agents and servers, bypassing this authentication mechanism undermines the entire security posture of the distributed monitoring architecture. It effectively breaks the isolation boundaries intended by UUID-based segmentation, allowing lateral movement within the monitored environment if an attacker gains access to a single agent's credentials.
From a standards perspective, this vulnerability aligns with CWE-295: Improper Certificate Validation and CWE-347: Improper Verification of Cryptographic Signature. The failure lies in not verifying that the certificate was issued by a trusted CA specific to the service being accessed. In terms of MITRE ATT&CK techniques, this flaw facilitates Initial Access through T1078: Valid Accounts or potentially Credential Access if certificates are stolen and reused improperly across segments. It also impacts Availability and Integrity if malicious data is injected into the monitoring pipeline.
To mitigate this risk, organizations running Checkmk versions earlier than 2.5.0p10 must upgrade to the patched version immediately where proper certificate chain validation against specific root CAs is enforced. Until an upgrade is possible, administrators should implement network-level controls such as firewall rules or micro-segmentation policies that restrict which agents can communicate with which receiver endpoints based on IP addresses rather than relying solely on application-layer authentication. Additionally, rotating all mTLS certificates and ensuring strict separation of trust anchors for different agent types (relay versus push) can reduce the blast radius if an attacker attempts to exploit this UUID reuse mechanism. Regular audits of certificate issuance policies should also be conducted to ensure that no single CA is inadvertently trusted by multiple distinct service endpoints without proper isolation controls.