CVE-2026-111252: OpenNMS denial of service via XML entity expansion

Severity: medium (CVSS 6.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.2% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-10-15 · updated 2026-10-16
Weakness: CWE-400

Description

A denial of service vulnerability exists in the XML parsing of multiple versions of OpenNMS Meridian and Horizon. OpenNMS parsed XML without an effective limit on entity expansion. A document with a DOCTYPE that declares internal entities can expand to a very large value and exhaust the server memory. An authenticated user with provisioning access can supply such a document through a requisition or foreign-source import, which includes a remote requisition URL that the server fetches and parses. Both deeply nested entities and a single large entity referenced many times cause the memory exhaustion. The fix caps the number of entity expansions for all XML parsing and rejects any DOCTYPE in requisition and foreign-source XML. The solution is to upgrade to Meridian 2024.3.14, 2025.0.11, 2026.0.1 and Horizon 36.0.5 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 36.0.5 — fixed in 36.0.5
The OpenNMS Group Meridian: from 2024.1.0 before 2024.3.14; from 2025.0.0 before 2025.0.11; from 2026.0.0 before 2026.0.1 — fixed in 2024.3.14, 2025.0.11, 2026.0.1

How to fix

Upgrade to a fixed version: The OpenNMS Group Horizon 36.0.5; The OpenNMS Group Meridian 2024.3.14, 2025.0.11, 2026.0.1. Check the vendor advisory for mitigations.

References

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