Executive brief
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.
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