Is CVE-2026-4637 exploited in the wild?

No. Junglewise Threat Intelligence has no report of CVE-2026-4637 being exploited in the wild.

How severe is CVE-2026-4637?

Informational severity, published 24 September 2026.

Which products and versions are affected?

Paessler PRTG Network Monitor before 26.2.120.1449.

Is there a fix for CVE-2026-4637?

A patch was published on 2026-06-03: Fixed in version 26.2.120.1449 Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

How is CVE-2026-4637 attacked?

It is attacked over the network, in a web application.

Am I affected by CVE-2026-4637?

Check whether you run Paessler PRTG Network Monitor at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

CVE-2026-4637: Paessler PRTG Network Monitor reflected XSS in error responses

CVE-2026-4637 · Severity: info · Published 2026-09-24

Paessler PRTG Network Monitor is a network monitoring and management system used by companies to track infrastructure health. The product contains a reflected cross-site scripting vulnerability in its 403 error pages that allows unauthenticated attackers to inject and execute malicious JavaScript in a victim's browser. Since session cookies lack the HttpOnly protection flag, a successful attack enables an attacker to steal the session cookie and impersonate the logged-in user, potentially gaining unauthorized access to monitoring data and system controls.

Technologies: Paessler PRTG Network Monitor. Vendors: Paessler.

Quick answers

Is CVE-2026-4637 exploited in the wild?: No. Junglewise Threat Intelligence has no report of CVE-2026-4637 being exploited in the wild.
How severe is CVE-2026-4637?: Informational severity, published 24 September 2026.
Which products and versions are affected?: Paessler PRTG Network Monitor before 26.2.120.1449.
Is there a fix for CVE-2026-4637?: A patch was published on 2026-06-03: Fixed in version 26.2.120.1449 Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-4637 attacked?: It is attacked over the network, in a web application.
Am I affected by CVE-2026-4637?: Check whether you run Paessler PRTG Network Monitor at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

Technical details

The vulnerability exists in error page handling where the requested URL path is echoed into HTTP 403 responses without proper sanitization when a request targets a non-existent .htm file. An unauthenticated remote attacker can craft a malicious URL with JavaScript payload in the path (e.g., https://target/<script>alert(1)</script>/welcome.htm) and trick a victim with an active PRTG session into clicking it. The unescaped JavaScript executes in the victim's browser context where the session cookie is accessible because HttpOnly flag is not set, enabling session hijacking.

Affected products

Paessler PRTG Network Monitor before 26.2.120.1449

Timeline

2026-07-09: disclosed
2026-06-03: patched: Fixed in version 26.2.120.1449
2026-09-24: advisory

References

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