Executive Summary

This vulnerability involves the exposure of SSH private keys in the HTML source code of the "Remote alert handlers (Linux)" rule page in Checkmk versions 2.4.0p18 and 2.3.0. An attacker who obtains these keys could potentially trigger predefined alert handlers on hosts where these handlers are deployed. However, the impact is limited because the authorized_keys file restricts which alert handlers can be executed with the key, preventing broader unauthorized access. [1]

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Impact Analysis

If exploited, this vulnerability could allow an attacker to trigger predefined alert handlers on affected hosts without proper authorization. This could lead to unauthorized execution of alert handling actions, potentially disrupting monitoring or alerting processes. However, the scope of this impact is limited due to restrictions in the authorized_keys file that control which handlers can be triggered. [1]

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Detection Guidance

You can detect this vulnerability by checking if the SSH private keys related to the Remote alert handlers (Linux) rule are exposed in the HTML source of the rule configuration page in your Checkmk installation. Additionally, verify if the alert handler SSH key is present in the authorized_keys file on affected hosts. Commands to assist detection include: 1) Using a web browser or curl to view the HTML source of the rule page and searching for SSH private keys, e.g., `curl -s https://your-checkmk-server/path-to-rule-page | grep 'PRIVATE KEY'`. 2) On hosts, check for the alert handler key in the authorized_keys file, e.g., `grep 'alert-handler-key' ~/.ssh/authorized_keys`. These steps help identify exposure and unauthorized keys related to this vulnerability. [1]

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Mitigation Strategies

Immediate mitigation steps include: 1) Updating Checkmk to version 2.5.0b1 or later, where the issue is fixed with no manual intervention needed. 2) If updating is not possible immediately, deactivate the Remote alert handlers (Linux) rule in your Checkmk configuration. 3) Roll out updated agents to hosts. 4) Verify and remove the alert handler SSH private key from the authorized_keys file on affected hosts to prevent unauthorized triggering of alert handlers. [1]

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