CVE-2025-64999: Checkmk XSS Vulnerability

CVE-2025-64999 Overview

CVE-2025-64999 is a Cross-Site Scripting (XSS) vulnerability affecting Checkmk, a popular IT infrastructure monitoring solution. The vulnerability stems from improper neutralization of input in the Synthetic Monitoring HTML logs feature. An attacker who can manipulate a host's check output can inject malicious JavaScript code, which is then rendered when a victim accesses the logs through a crafted phishing link.

This stored XSS vulnerability allows attackers to execute arbitrary JavaScript in the context of authenticated users' browser sessions, potentially leading to session hijacking, credential theft, or further attacks against the monitoring infrastructure.

Critical Impact

Attackers can inject malicious JavaScript into Synthetic Monitoring HTML logs, enabling session hijacking, credential theft, and unauthorized actions within the Checkmk web interface when victims access crafted phishing links.

Affected Products

• Checkmk version 2.4.0 before 2.4.0p22
• Checkmk version 2.3.0 before 2.3.0p43

Discovery Timeline

• 2026-02-26 - CVE CVE-2025-64999 published to NVD
• 2026-02-26 - Last updated in NVD database

Technical Details for CVE-2025-64999

Vulnerability Analysis

This vulnerability is classified under CWE-79 (Improper Neutralization of Input During Web Page Generation), commonly known as Cross-Site Scripting (XSS). The flaw exists in how Checkmk processes and displays host check output within the Synthetic Monitoring HTML logs feature.

When host check output is rendered in the web interface, the application fails to properly sanitize or encode user-controllable input. This allows an attacker with the ability to influence check output data to embed malicious JavaScript payloads that persist in the logs. The attack requires user interaction—specifically, a victim must access the malicious content through a crafted phishing link, making this a stored XSS attack with a social engineering component.

The network-accessible nature of Checkmk deployments means this vulnerability can be exploited remotely, though the attacker needs some level of authenticated access to manipulate host check outputs.

Root Cause

The root cause is insufficient input validation and output encoding in the Synthetic Monitoring HTML logs component. When rendering check output data in the browser, the application does not properly neutralize special HTML characters or JavaScript code, allowing injected scripts to execute in the victim's browser context.

Attack Vector

The attack follows a multi-stage process:

1. Initial Access: The attacker gains the ability to manipulate a host's check output, either through compromising a monitored host or exploiting other access to inject data into check results
2. Payload Injection: Malicious JavaScript is embedded within the check output data that gets stored in the Synthetic Monitoring logs
3. Social Engineering: The attacker crafts a phishing link pointing to the affected log entry and distributes it to target victims
4. Execution: When an authenticated Checkmk user clicks the phishing link and views the logs, the malicious JavaScript executes in their browser session

The vulnerability mechanism involves the improper handling of check output data within the Synthetic Monitoring HTML logs rendering process. Detailed technical information is available in the Checkmk Work Item #19238.

Detection Methods for CVE-2025-64999

Indicators of Compromise

• Unusual JavaScript code patterns in Synthetic Monitoring log entries (e.g., <script> tags, event handlers like onerror, onload)
• Suspicious check output containing HTML entities or encoded JavaScript payloads
• Unexpected outbound network connections from user browsers after accessing monitoring logs
• Reports of phishing links directing users to specific Checkmk log URLs

Detection Strategies

• Implement web application firewall (WAF) rules to detect and block XSS patterns in inbound requests to Checkmk
• Monitor audit logs for unusual access patterns to Synthetic Monitoring log endpoints
• Deploy browser-based XSS protection and Content Security Policy (CSP) headers to limit script execution
• Review check output data sources for signs of compromise or manipulation

Monitoring Recommendations

• Enable verbose logging for the Checkmk web interface to capture request details and potential attack attempts
• Configure alerts for anomalous patterns in check output data, particularly those containing HTML or script-like content
• Monitor for phishing campaigns targeting your organization that reference internal Checkmk URLs
• Implement user behavior analytics to detect unusual access patterns to monitoring logs

How to Mitigate CVE-2025-64999

Immediate Actions Required

• Upgrade Checkmk 2.4.0 installations to version 2.4.0p22 or later immediately
• Upgrade Checkmk 2.3.0 installations to version 2.3.0p43 or later immediately
• Review Synthetic Monitoring logs for any suspicious content that may indicate prior exploitation
• Educate users about phishing risks and warn against clicking suspicious links to monitoring interfaces

Patch Information

Checkmk has released security patches addressing this vulnerability. Organizations should upgrade to the following patched versions:

• For the 2.4.x branch: Upgrade to 2.4.0p22 or later
• For the 2.3.x branch: Upgrade to 2.3.0p43 or later

Detailed patch information is available in the Checkmk Work Item #19238.

Workarounds

• Restrict access to Synthetic Monitoring logs to only essential personnel until patches can be applied
• Implement strict Content Security Policy (CSP) headers to mitigate JavaScript execution from untrusted sources
• Deploy network-level access controls to limit who can reach the Checkmk web interface
• Consider temporarily disabling the Synthetic Monitoring feature if it is not critical to operations

Access restriction and CSP configuration should be applied at the web server level hosting the Checkmk installation. Consult the Checkmk Work Item #19238 for specific configuration guidance related to this vulnerability.