CVE-2025-29953 is a critical vulnerability identified in the Apache ActiveMQ NMS OpenWire Client, specifically affecting versions prior to 2.1.1. The vulnerability arises from the deserialization of untrusted data, a classic CWE-502 weakness, where the client processes serialized data received from potentially untrusted ActiveMQ servers. The core issue is that the client performs unbounded deserialization of binary data without sufficient validation or restriction, allowing a malicious server to craft responses that, when deserialized by the client, can trigger arbitrary code execution. This means an attacker controlling or impersonating an ActiveMQ server could exploit this flaw to execute malicious code on the client machine without requiring any user interaction or authentication. Version 2.1.0 introduced an allow/denylist mechanism intended to restrict deserialization to safe classes; however, this control was found to be bypassable, leaving clients vulnerable. The definitive fix was implemented in version 2.1.1, which properly addresses the deserialization controls. Additionally, the vulnerability is tied to the use of .NET binary serialization, a technology deprecated starting with .NET 9 due to inherent security risks. The Apache project is considering removing this serialization approach entirely from the NMS API to prevent similar issues in the future. The CVSS v3.1 base score is 9.8, reflecting the vulnerability's critical nature, with an attack vector of network (AV:N), low attack complexity (AC:L), no privileges required (PR:N), no user interaction (UI:N), and high impact on confidentiality, integrity, and availability (C:H/I:H/A:H). No known exploits are currently reported in the wild, but the severity and ease of exploitation make this a high-priority issue for affected users to address promptly.

For European organizations using Apache ActiveMQ NMS OpenWire Client, particularly in environments where clients connect to external or untrusted ActiveMQ servers, this vulnerability poses a severe risk. Successful exploitation could lead to full compromise of client systems, enabling attackers to execute arbitrary code, potentially leading to data breaches, disruption of messaging infrastructure, and lateral movement within corporate networks. This is especially critical for sectors relying on messaging middleware for real-time communications, such as financial services, telecommunications, manufacturing automation, and critical infrastructure. The ability to exploit this vulnerability remotely without authentication or user interaction amplifies the threat, increasing the likelihood of widespread impact. Furthermore, organizations using legacy versions or relying on .NET binary serialization without migration plans remain exposed. The compromise of messaging clients could undermine trust in enterprise messaging systems, disrupt business processes, and cause significant operational and reputational damage.

1. Immediate upgrade to Apache ActiveMQ NMS OpenWire Client version 2.1.1 or later, which contains the fix for this vulnerability. 2. Audit all ActiveMQ client deployments to identify and remediate any instances running vulnerable versions. 3. Restrict client connections to trusted ActiveMQ servers only, implementing network-level controls such as firewall rules, VPNs, or zero-trust network segmentation to limit exposure to untrusted servers. 4. Disable or avoid using .NET binary serialization in client applications; migrate to safer serialization frameworks such as JSON or Protocol Buffers where feasible. 5. Implement application-layer allow/denylist controls for deserialization explicitly, ensuring that only known safe classes are deserialized, and validate deserialization inputs rigorously. 6. Monitor network traffic and logs for unusual or unexpected ActiveMQ server responses that could indicate exploitation attempts. 7. Engage in proactive threat hunting focused on messaging infrastructure to detect early signs of compromise. 8. Plan for long-term removal of binary serialization dependencies in line with .NET deprecation guidance to reduce future risk.