CVE-2026-111235: OpenNMS REST authorization bypass via non-canonical HTTP method
An authorization bypass vulnerability exists in the REST API of multiple versions of OpenNMS Meridian and Horizon. The /rest/ Spring Security filter chains authorize requests with intercept-url rules that name the HTTP method as an uppercase token, compare the method case-sensitively, and have no terminal deny rule. A request that uses a non-canonical (for example lowercase) method token matches no rule and is processed without authentication. A remote, unauthenticated attacker can read the monitored inventory and can create, modify, or delete resources through the REST API, which includes injecting events to fabricate or suppress alarms. The /rest/alarms chain has the same missing deny rule and discloses whether an alarm exists through a difference in the response. The fix adds a terminal deny rule to each REST filter chain.
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 networks and do not make them directly accessible from the Internet.
Event History
Frequently Asked Questions
Which deployments are most exposed?
Any affected Meridian or Horizon instance whose REST API can be reached by an attacker is exposed, especially instances directly accessible from the Internet. The installation guidance states these products are intended to run within private organizational networks rather than be directly Internet-accessible.
What does an attacker need to exploit this issue?
An attacker needs network access to the affected REST API and can send a request with a non-canonical HTTP method token, such as a lowercase method. No authentication, privileges, or user interaction are required.
What can an unauthenticated attacker do through the affected API?
They can read monitored inventory and create, modify, or delete REST resources. This includes injecting events that can fabricate or suppress alarms; the alarms endpoint can also reveal whether an alarm exists based on response differences.
What should be done if an upgrade cannot be performed immediately?
Restrict access to the REST API to trusted private-network users and prevent direct Internet exposure. This reduces the opportunity for an unauthenticated remote attacker to reach the vulnerable filter chains.
Which releases contain the fix?
Upgrade Meridian to 2024.3.14, 2025.0.11, 2026.0.1, or newer. Upgrade Horizon to 36.0.5 or newer.