What’s Vulnerable › Vulnerabilities

CVE-2026-4637: Reflected Cross-Site Scripting via URL Path in Paessler PRTG Network Monitor

• Severity: medium (CVSS 5.1 v4.0)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.6% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-09-24 · updated 2026-09-24
• Weakness: CWE-79

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.

Affected products

• Paessler GmbH PRTG Network Monitor: before 26.2.120.1449 — fixed in 26.2.120.1449

How to fix

Upgrade to a fixed version: Paessler GmbH PRTG Network Monitor 26.2.120.1449. Check the vendor advisory for mitigations.

References

• https://paessler.freshdesk.com/en/support/solutions/articles/76000088640
• https://r.sec-consult.com/paessler
• https://www.paessler.com/de/download/
• https://www.paessler.com/prtg/prtg-network-monitor
• https://sec-consult.com/vulnerability-lab/advisory/multiple-vulnerabilities-in-paessler-prtg-network-monitor/

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