CVE-2026-19596: The Opennms Group Horizon

Medium severity, CVSS 5.9. EPSS: 0.2% chance of exploitation in the next 30 days.

An XML External Entity (XXE) vulnerability exists in the XML collector of multiple versions of OpenNMS Meridian and Horizon. When OpenNMS collects XML from a source whose response is attacker-controlled (for example a compromised monitored host or an HTTP man-in-the-middle position), the collector's XML parser resolves external entities and external DTDs. This allows an attacker to read files accessible to the OpenNMS service account, including database credentials, and to induce out-of-band requests. The solution is to upgrade to Meridian 2024.3.13, 2025.0.10 and Horizon 36.0.4 or newer. Meridian and Horizon installation instructions state that they are intended for installation within an organization's private networks and should not be directly accessible from the Internet.

Affected products

• The Opennms Group Horizon: from 36.0.0, before 36.0.4 (fixed in 36.0.4)
• The Opennms Group Meridian: from 2024.1.0, before 2024.3.13 (fixed in 2024.3.13); from 2025.0.0, before 2025.0.10 (fixed in 2025.0.10)

Published 2026-09-10. Last modified 2026-09-18.