CVE-2026-19596 in Horizon
Summary
by MITRE • 09/11/2026
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.
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Analysis
by VulDB Data Team • 09/11/2026
The identified vulnerability represents a critical XML External Entity injection flaw located within the XML collector component of OpenNMS Meridian and Horizon software versions prior to 2024.3.13, 2025.0.10, and 36.0.4 respectively. This security weakness stems from the improper configuration or lack of validation in the underlying XML parsing library used by the collector. When the system ingests data formatted as Extensible Markup Language, it fails to adequately restrict the resolution of external entities and Document Type Definitions. In a standard secure implementation, parsers should be configured to disable DTD processing entirely or strictly limit entity expansion to prevent abuse. However, in this vulnerable state, the parser processes any external references embedded within the XML payload provided by upstream sources. This architectural oversight allows an adversary who can influence the content of the collected data to inject malicious entity definitions that trigger unintended side effects during the parsing process.
The operational impact of this vulnerability is severe due to its potential for both local file disclosure and remote code execution via out-of-band channels. Because the XML collector typically runs with elevated privileges associated with the OpenNMS service account, any files read by the parser are accessible using those same permissions. An attacker can craft a malicious XML payload containing an external entity that points to sensitive system resources such as configuration files, log data, or critically, database credentials stored on disk. By triggering the resolution of this entity, the application reads the file contents and may inadvertently expose them within error messages or response bodies depending on how the parser handles failures. Furthermore, the vulnerability supports out-of-band network interactions where the attacker can specify a remote Uniform Resource Locator as an external entity. When resolved, the server initiates HTTP requests to domains controlled by the adversary, effectively allowing for data exfiltration through DNS queries or HTTP traffic even if direct response retrieval is blocked by firewalls.
From a threat modeling perspective, this vulnerability aligns with Common Weakness Enumeration identifier CWE-611 which describes Improper Restriction of XML External Entity Reference. The exploitation technique maps directly to the MITRE ATT&CK framework under T1059 Command and Scripting Interpreter for parsing logic abuse or potentially T1078 Valid Accounts if the service account credentials are compromised via file reading. Additionally, the ability to induce out-of-band requests corresponds to techniques used in data exfiltration where attackers leverage legitimate application functionality to communicate with external command and control infrastructure without triggering traditional intrusion detection systems that monitor for direct malicious payloads. The risk is further amplified by the fact that OpenNMS often monitors network devices which may be compromised, thereby providing a vector through which an attacker can feed malicious XML into the monitoring system itself.
Mitigation strategies must prioritize immediate software updates to address the root cause within the parsing engine. Organizations running affected versions of Meridian or Horizon should upgrade to version 2024.3.13, 2025.0.10, or 36.0.4 and newer where these security controls have been implemented. In scenarios where immediate patching is not feasible due to operational constraints, administrators can implement compensating controls at the network perimeter by restricting outbound traffic from the OpenNMS server to only known trusted domains using strict firewall rules. This limits the effectiveness of out-of-band attacks even if an injection occurs. Additionally, input validation should be enforced on any custom integrations that feed XML data into the collector to ensure no unexpected entity definitions are processed. It is also crucial to review and harden the permissions of the OpenNMS service account so that it operates with minimal privileges necessary for its function, thereby reducing the scope of files an attacker could access if a successful exploitation occurs.
The vendor documentation explicitly states that Meridian and Horizon installations are intended for deployment within private organizational networks rather than being directly exposed to the public internet. This design assumption implies that trust boundaries may be less rigidly enforced compared to publicly facing services, making internal network segmentation vital. If these systems must interact with untrusted or potentially compromised sources such as third-party devices or external APIs, additional sanitization layers should be placed between those data sources and the OpenNMS collector. Regular security assessments including static code analysis focused on XML processing libraries and dynamic testing using XXE-specific payloads are recommended to verify that no other components within the application share similar vulnerabilities. Maintaining an inventory of all third-party dependencies used for XML parsing ensures that future updates can be applied promptly as new CVEs emerge in related open-source projects like Apache Xerces or libxml2 which may underpin these enterprise monitoring tools.