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

> An attacker can execute arbitrary JavaScript via a crafted URL, stealing session cookies and hijacking user accounts in PRTG Network Monitor.

- **Severity:** Medium 5.1 (`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`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** A user has to click or open something
- **Fix available:** No
- **Published:** 24 Sep 2026
- **Updated:** 24 Sep 2026
- **Also known as:** GHSA-xp6x-3pcm-f3ph

## What is CVE-2026-4637?

CVE-2026-4637 is a medium-severity cross-site scripting vulnerability in Paessler GmbH PRTG Network Monitor, published on 24 Sep 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.

### Official 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.

Weakness: CWE-79 (Improper Neutralization of Input During Web Page Generation)

## Which versions are affected by CVE-2026-4637?


## How do I fix CVE-2026-4637?

No fix has been published for CVE-2026-4637 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-4637 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-4637 exists.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-4637 and fix it if it is affected.

What it is: CVE-2026-4637 is a medium-severity cross-site scripting vulnerability in Paessler GmbH PRTG Network Monitor, published on 24 Sep 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-4637 exists.

Affected product: Paessler GmbH PRTG Network Monitor. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-4637/ (Markdown: https://insights.sbomify.com/CVE-2026-4637/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-4637
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-4637
- OSV: https://osv.dev/vulnerability/CVE-2026-4637

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-4637/
Summaries are generated and may contain mistakes; the official description above is the record.