CVE-2026-111252: OpenNMS denial of service via XML entity expansion
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 Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
OpenNMS Meridianto a version that resolves this vulnerability.Fixed in 2024.3.14 - Upgrade
Upgrade
OpenNMS Meridianto a version that resolves this vulnerability.Fixed in 2025.0.11 - Upgrade
Upgrade
OpenNMS Meridianto a version that resolves this vulnerability.Fixed in 2026.0.1 - Upgrade
Upgrade
OpenNMS Horizonto a version that resolves this vulnerability.Fixed in 36.0.5 - Compensating control
Install OpenNMS Meridian and Horizon within the organization's private network and do not expose them directly to the Internet.
Event History
Frequently Asked Questions
Who can exploit this issue in practice?
An authenticated OpenNMS user with provisioning access can exploit it by supplying malicious XML through a requisition or foreign-source import. The import can reference a remote requisition URL that the server fetches and parses.
What is the operational impact of a successful exploit?
Malicious internal XML entities can expand until server memory is exhausted, causing a denial of service. Both deeply nested entity definitions and repeated references to one large entity can trigger the condition.
Which releases contain the fix?
Upgrade to Meridian 2024.3.14, 2025.0.11, or 2026.0.1 or later, or Horizon 36.0.5 or later. The fix limits entity expansions across XML parsing and rejects DOCTYPE declarations in requisition and foreign-source XML.
What can be done while an upgrade is pending?
Restrict provisioning access to trusted users and prevent untrusted requisition or foreign-source XML, including remote requisition URLs, from being imported. OpenNMS is intended for private-network deployment and should not be directly accessible from the Internet.