CVE-2026-24097: Checkmk Information Disclosure Flaw

CVE-2026-24097 Overview

CVE-2026-24097 is an improper permission enforcement vulnerability affecting Checkmk, a popular IT infrastructure monitoring solution. The vulnerability exists in the agent-receiver/register_existing endpoint, where authenticated users can enumerate existing hosts by observing different HTTP response codes returned by the application. This information disclosure flaw allows attackers to map out the monitored infrastructure without proper authorization.

Critical Impact

Authenticated attackers can enumerate host inventory within Checkmk deployments, potentially enabling reconnaissance for further targeted attacks against identified systems.

Affected Products

• Checkmk 2.4.0 (versions prior to 2.4.0p23)
• Checkmk 2.3.0 (versions prior to 2.3.0p43)
• Checkmk 2.2.0 (all versions - End of Life)

Discovery Timeline

• March 13, 2026 - CVE CVE-2026-24097 published to NVD
• March 18, 2026 - Last updated in NVD database

Technical Details for CVE-2026-24097

Vulnerability Analysis

This vulnerability falls under CWE-204 (Observable Response Discrepancy), where the application returns different HTTP response codes based on whether a host exists in the Checkmk inventory. When an authenticated user makes requests to the agent-receiver/register_existing endpoint, the server responds differently depending on the validity of the supplied host identifier. This response discrepancy can be leveraged to systematically enumerate all hosts managed by the Checkmk instance.

The flaw is particularly concerning in enterprise environments where Checkmk monitors sensitive infrastructure. An attacker with low-privilege authenticated access could compile a complete inventory of monitored systems, gaining valuable intelligence about network topology, server naming conventions, and infrastructure scope.

Root Cause

The root cause of this vulnerability is insufficient access control validation in the agent-receiver registration endpoint. The endpoint fails to return consistent HTTP responses regardless of whether a host exists or not, creating an observable side-channel that leaks information about the internal host inventory. Proper implementation should return identical responses for both existing and non-existing hosts to prevent enumeration attacks.

Attack Vector

The attack requires network access and valid authentication credentials to the Checkmk instance. An attacker can exploit this vulnerability by:

1. Authenticating to the Checkmk web interface with any valid user account
2. Sending HTTP requests to the /agent-receiver/register_existing endpoint with various host identifiers
3. Analyzing the HTTP response codes to determine which hosts exist in the system
4. Building a comprehensive list of monitored hosts for further reconnaissance or targeted attacks

The vulnerability can be exploited remotely over the network by any authenticated user, regardless of their permission level within the Checkmk authorization model.

Detection Methods for CVE-2026-24097

Indicators of Compromise

• High volume of HTTP requests to the /agent-receiver/register_existing endpoint from a single user session
• Sequential or pattern-based host identifier probing in request parameters
• Unusual access patterns to the agent-receiver endpoint outside of normal agent registration workflows
• Authenticated users accessing the registration endpoint who do not have legitimate agent administration responsibilities

Detection Strategies

• Monitor web application logs for repeated requests to the agent-receiver/register_existing endpoint with varying host parameters
• Implement rate limiting and anomaly detection for the affected endpoint to identify enumeration attempts
• Configure SIEM rules to alert on suspicious patterns of 404 vs 200 response codes for registration requests
• Deploy web application firewall rules to detect and block host enumeration attack patterns

Monitoring Recommendations

• Enable detailed access logging for all Checkmk API endpoints, particularly the agent-receiver component
• Establish baseline metrics for normal agent registration activity and alert on deviations
• Monitor for authenticated users accessing administrative endpoints outside their normal scope of responsibilities
• Correlate agent-receiver endpoint access with user role assignments to identify potential abuse

How to Mitigate CVE-2026-24097

Immediate Actions Required

• Upgrade Checkmk 2.4.0 installations to version 2.4.0p23 or later
• Upgrade Checkmk 2.3.0 installations to version 2.3.0p43 or later
• Migrate from Checkmk 2.2.0 immediately as this version has reached End of Life and will not receive security patches
• Review access logs for any evidence of prior exploitation attempts against the agent-receiver endpoint

Patch Information

Checkmk has released security patches addressing this vulnerability. Detailed information about the fix is available in the Checkmk Werk 18993. Organizations should apply the appropriate patch based on their installed version:

Workarounds

• Implement network segmentation to restrict access to Checkmk administrative interfaces to trusted networks only
• Review and minimize user accounts with access to the Checkmk web interface, applying the principle of least privilege
• Deploy web application firewall rules to rate-limit requests to the /agent-receiver/register_existing endpoint
• Consider implementing additional authentication requirements for administrative API endpoints if patching cannot be performed immediately

# Example: Restrict access to Checkmk at the reverse proxy level
# Add to Apache configuration for Checkmk site
<Location "/agent-receiver">
    # Require specific IP ranges for agent registration
    Require ip 10.0.0.0/8
    Require ip 192.168.0.0/16
</Location>