CVE-2026-4637
Received Received - Intake

Reflected XSS in Paessler PRTG Network Monitor

Publication date: 2026-09-24

Last updated on: 2026-09-24

Assigner: SEC Consult Vulnerability Lab

Description
Paessler PRTG Network Monitor before version 26.2.120.1449 is affected by a reflected Cross-Site Scripting (XSS) vulnerability. When a request is made for a non-existent resource ending in \".htm\", the web interface returns an HTTP 403 \"Forbidden Path\" error page that echoes the requested URL path into the HTML response body without proper output encoding or sanitization. An unauthenticated, remote attacker can craft a URL containing an HTML/JavaScript payload in the path (e.g. https:////welcome.htm) and, once a victim with an active PRTG session opens the crafted link, execute arbitrary JavaScript in the security context of the PRTG web interface. Because the PRTG session cookie is not protected with the HttpOnly attribute, successful exploitation allows the attacker to read and exfiltrate the victim's session cookie, potentially leading to session hijacking.

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Meta Information

Published
2026-09-24
Last Modified
2026-09-24
Generated
2026-09-24
AI Q&A
2026-09-24
EPSS Evaluated
N/A
NVD
CVE-2026-4637
EUVD
EUVD-2026-85840

Affected Vendors & Products

Showing 1 associated CPE
Vendor Product Version / Range
paessler prtg_network_monitor to 26.2.120.1449 (exc)

Helpful Resources

Exploitability

CWE
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KEV
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CWE ID Description
CWE-79 The product does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users.

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Executive Summary

This is a reflected Cross-Site Scripting (XSS) vulnerability in Paessler PRTG Network Monitor before version 26.2.120.1449. When a non-existent resource ending in .htm is requested, the web interface returns an error page that includes the requested URL path in the response without proper encoding. An attacker can craft a malicious URL with an HTML/JavaScript payload in the path. If a victim with an active PRTG session opens this link, the attacker can execute arbitrary JavaScript in the PRTG web interface context.

Detection Guidance

To detect this vulnerability, monitor HTTP 403 error responses for non-existent .htm resources in PRTG Network Monitor. Check if the error page echoes the requested URL path without proper encoding. Use network traffic analysis tools like Wireshark or tcpdump to inspect HTTP responses for reflected XSS patterns in error pages.

Impact Analysis

An attacker could steal the victim's session cookie due to the missing HttpOnly attribute, leading to session hijacking. This allows the attacker to read and exfiltrate sensitive data, perform actions on behalf of the victim, or gain unauthorized access to the PRTG web interface. The impact includes potential data breaches, unauthorized system access, and further exploitation of the network.

Compliance Impact

This vulnerability could lead to unauthorized access to sensitive data, violating confidentiality requirements under GDPR and HIPAA. If exploited, it may result in data breaches, triggering mandatory breach notifications and potential fines. Organizations must ensure systems are patched to maintain compliance with these regulations.

Mitigation Strategies

Immediately upgrade PRTG Network Monitor to version 26.2.120.1449 or later. For hosted customers, the update was applied automatically. For self-managed systems, download and install the patch from Paessler's official site. Ensure session cookies are set with the HttpOnly attribute to prevent session hijacking.

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