CVE-2025-64999 is a cross-site scripting (XSS) vulnerability classified under CWE-79 affecting Checkmk, a monitoring software developed by Checkmk GmbH. The flaw exists in versions 2.4.0 prior to patch 2.4.0p22 and 2.3.0 prior to patch 2.3.0p43. The vulnerability stems from improper neutralization of user-controllable input during the generation of Synthetic Monitoring HTML logs. Specifically, an attacker with the ability to manipulate a host's check output can inject malicious JavaScript code into these logs. When a user accesses the compromised logs via a crafted phishing link, the injected script executes in the context of the victim's browser, potentially allowing session hijacking, credential theft, or further exploitation within the Checkmk web interface. The CVSS 4.0 base score is 7.3, reflecting network attack vector, low attack complexity, partial privileges required, user interaction needed, and high impact on confidentiality, integrity, and availability. Although no known exploits have been observed in the wild, the vulnerability poses a significant risk due to the widespread use of Checkmk in IT infrastructure monitoring. The vulnerability does not require elevated privileges to exploit but does require the attacker to influence host check outputs and the victim to click a malicious link. This scenario highlights the importance of securing input sources and user awareness against phishing attacks. No official patches were linked in the provided data, but updates to versions 2.4.0p22 and 2.3.0p43 or later are implied to remediate the issue.

The exploitation of CVE-2025-64999 can have severe consequences for organizations relying on Checkmk for infrastructure monitoring. Successful XSS attacks can lead to unauthorized access to sensitive monitoring data, session hijacking, and execution of arbitrary scripts within the context of the Checkmk web application. This can compromise the confidentiality and integrity of monitoring data and potentially disrupt availability if attackers manipulate monitoring outputs or escalate privileges. Given that monitoring systems often have privileged access and visibility into critical infrastructure, attackers could leverage this vulnerability as a foothold for lateral movement or further attacks. The requirement for user interaction (clicking a phishing link) somewhat limits the attack scope but does not eliminate risk, especially in environments with large user bases or less security awareness. The vulnerability affects multiple versions, indicating a broad attack surface. Organizations with unpatched Checkmk deployments face increased risk of targeted phishing campaigns exploiting this flaw, potentially impacting operational continuity and data security.

To mitigate CVE-2025-64999, organizations should immediately upgrade Checkmk installations to versions 2.4.0p22, 2.3.0p43, or later where the vulnerability is patched. In the absence of immediate patch availability, implement strict input validation and sanitization on all host check outputs to prevent injection of malicious scripts into Synthetic Monitoring HTML logs. Employ Content Security Policy (CSP) headers to restrict the execution of unauthorized scripts within the Checkmk web interface. Enhance user awareness training to recognize and avoid phishing attempts that could deliver malicious links exploiting this vulnerability. Monitor logs and network traffic for suspicious activities related to Synthetic Monitoring outputs and unusual user interactions. Limit privileges of users who can manipulate host check outputs to reduce the attack surface. Consider isolating the Checkmk web interface behind additional security layers such as web application firewalls (WAFs) configured to detect and block XSS payloads. Regularly audit and review monitoring configurations to ensure no untrusted input can be injected into web-accessible logs.