CVE-2026-15576: Checkmk Authentication Bypass Vulnerability

CVE-2026-15576 Overview

CVE-2026-15576 is an improper authentication vulnerability in the agent receiver component of Checkmk versions prior to 2.5.0p10. The flaw allows an unauthenticated remote attacker to bypass mutual TLS (mTLS) client certificate verification on relay endpoints by supplying a fixed placeholder identity in the request URL. Successful exploitation yields limited impact on integrity and availability, with no impact on confidentiality. Only the Cloud, Ultimate, and Ultimate MT editions expose relay endpoints and are therefore affected. The weakness is categorized under [CWE-306: Missing Authentication for Critical Function].

Critical Impact
Unauthenticated network attackers can bypass mTLS client certificate verification on relay endpoints in Checkmk Cloud, Ultimate, and Ultimate MT editions, affecting integrity and availability.

Affected Products

Checkmk Cloud Edition versions prior to 2.5.0p10
Checkmk Ultimate Edition versions prior to 2.5.0p10
Checkmk Ultimate MT Edition versions prior to 2.5.0p10

Discovery Timeline

2026-08-21 - CVE-2026-15576 published to NVD
2026-08-26 - Last updated in NVD database

Technical Details for CVE-2026-15576

Vulnerability Analysis

The agent receiver in Checkmk exposes relay endpoints that are intended to authenticate clients using mutual TLS. Mutual TLS requires both the server and the client to present valid X.509 certificates and prove possession of the associated private keys. In the affected releases, the agent receiver accepts a fixed placeholder identity supplied through the request URL and treats the request as authenticated without enforcing certificate verification.

This authentication gap lets an unauthenticated remote attacker interact with relay endpoints as if it were a legitimately enrolled relay client. The advisory scopes the impact to limited integrity and availability effects, with no direct confidentiality exposure. The affected code paths only exist in the Cloud, Ultimate, and Ultimate MT editions because other editions do not expose relay functionality.

Root Cause

The root cause is missing authentication on a security-critical code path [CWE-306]. The agent receiver trusts a client-supplied identifier embedded in the URL instead of deriving identity from the presented client certificate. When the placeholder value is supplied, the mTLS verification step is effectively skipped, breaking the authentication guarantee that relay endpoints depend on.

Attack Vector

The vulnerability is exploitable over the network without authentication or user interaction. An attacker sends a crafted HTTPS request to an exposed agent receiver relay endpoint and supplies the fixed placeholder identity in the URL. The server processes the request without validating a client certificate, allowing the attacker to invoke relay functionality. Refer to the Checkmk Patch Announcement for the vendor's technical description of the affected endpoints and the corrective changes.

Detection Methods for CVE-2026-15576

Indicators of Compromise

Requests to agent receiver relay endpoints that contain the fixed placeholder identity string described in the Checkmk advisory.
Successful HTTP responses from relay endpoints where the TLS session did not include a validated client certificate.
Unexpected relay registration or relay-related state changes on Checkmk Cloud, Ultimate, or Ultimate MT sites.

Detection Strategies

Review agent receiver access logs for requests to relay endpoints originating from IP addresses outside the expected relay population.
Correlate TLS handshake metadata with application-layer identity values to flag mismatches between the URL identity and the presented client certificate.
Compare the running Checkmk version against 2.5.0p10 and enumerate sites still exposing the vulnerable code path.

Monitoring Recommendations

Forward agent receiver and reverse proxy logs to a centralized SIEM and alert on relay endpoint access from unknown sources.
Monitor for anomalous spikes in relay endpoint traffic or new relay identifiers appearing in operational data.
Track outbound behavior from Checkmk hosts for signs of tampering with monitoring data integrity or availability.

How to Mitigate CVE-2026-15576

Immediate Actions Required

Upgrade all Checkmk Cloud, Ultimate, and Ultimate MT sites to version 2.5.0p10 or later without delay.
Restrict network access to the agent receiver to trusted management networks using firewall rules or reverse proxy allowlists.
Audit existing relay registrations and revoke any that cannot be attributed to a known, authorized system.

Patch Information

Checkmk addressed the issue in version 2.5.0p10. Full remediation details are provided in the Checkmk Patch Announcement. Editions other than Cloud, Ultimate, and Ultimate MT are not affected because they do not expose relay endpoints.

Workarounds

Block external access to agent receiver relay endpoints at the network perimeter until the patch is applied.
Terminate TLS at an upstream reverse proxy that enforces client certificate validation before forwarding traffic to the agent receiver.
Disable or avoid deploying relay functionality on affected editions if it is not required in the environment.

bash

# Configuration example: restrict agent receiver access with iptables
# Replace 10.0.0.0/24 with the trusted management network for relays
iptables -A INPUT -p tcp --dport 8000 -s 10.0.0.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 8000 -j DROP