Description

A JEXL expression sandbox bypass exists in multiple versions of OpenNMS Meridian and Horizon. A low-privileged authenticated user can submit a crafted expression to the Measurements REST API that escapes the sandbox and loads arbitrary Java classes on the server. This can potentially allow an attacker to gain access to confidential information and compromise integrity. The solution is to upgrade to Meridian 2024.3.12, 2025.0.9 and Horizon 36.0.3 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.

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "5.4",
  "pubDate": "2026-08-13T05:17:22.750Z",
  "pubdate": "2026-08-13T05:17:22.750Z",
  "executiveSummary": "A critical JEXL expression sandbox bypass vulnerability has been identified in multiple versions of OpenNMS Meridian and Horizon. This security defect arises from insufficient input validation and inadequate sandboxing within the application logic, allowing a low-privileged authenticated adversary to supply maliciously crafted expressions that successfully break out of the intended execution constraints. The primary impact of this flaw includes the unauthorized loading and execution of arbitrary Java classes on the underlying server host, potentially leading to a complete compromise of system integrity and the unauthorized disclosure of confidential information. Affected products include OpenNMS Meridian and OpenNMS Horizon across multiple versions. Successful exploitation requires low-privileged authentication and the ability to interact with the application interface, specifically through the Measurements REST API. While the software installation guidelines mandate deployment within restricted private networks without direct internet accessibility, internal threat actors or compromised credentials remain viable vectors for initiating attacks against the vulnerable endpoints.",
  "technicalDetails": "The vulnerability exists within the expression evaluation mechanism utilized by the Measurements REST API in OpenNMS Meridian and Horizon. The root cause is an insufficient sandbox implementation that fails to adequately restrict Java Expression Language (JEXL) evaluations. By submitting a specially crafted JEXL expression via the vulnerable API endpoint, an authenticated user with low privileges can bypass the restrictive boundaries of the security sandbox.\nThe attack flow proceeds as follows: First, the attacker authenticates to the application using low-privileged credentials. Second, the attacker formulates a malicious payload encapsulated within a JEXL expression designed to target the evaluation engine. Third, the attacker submits this payload to the Measurements REST API. Fourth, the application processes the input without proper sanitization or boundary enforcement, causing the JEXL engine to evaluate the expression outside of the intended sandbox context.\nUpon successful sandbox escape, the payload executes within the context of the running Java Virtual Machine, allowing the attacker to load arbitrary Java classes. This capability facilitates advanced post-exploitation activities, including unauthorized access to sensitive internal data structures, confidential configuration information, and potential lateral movement or remote code execution depending on the underlying security context of the service process.\nThe vulnerable component is the JEXL evaluation subsystem tied to the Measurements REST API. Exploitation requires network access to the API endpoints and valid low-privileged authentication credentials. The affected software versions comprise OpenNMS Meridian and OpenNMS Horizon. Remediation requires applying the official vendor-supplied upgrades to secure the evaluation context and enforce rigorous sandbox restrictions."
}