CVE-2025-29953: Apache ActiveMQ NMS OpenWire RCE Flaw

CVE-2025-29953 Overview

CVE-2025-29953 is a critical Insecure Deserialization vulnerability affecting the Apache ActiveMQ NMS OpenWire Client. This issue allows malicious servers to exploit unbounded deserialization in client connections, potentially leading to arbitrary code execution on systems running the vulnerable client. Version 2.1.0 introduced an allow/denylist feature to restrict deserialization, but researchers discovered this protection mechanism could be bypassed.

The vulnerability is particularly concerning because it affects client-side connections to potentially untrusted servers. When a .NET application using the vulnerable ActiveMQ NMS OpenWire Client connects to a malicious server, that server can craft malicious serialized responses that, when deserialized by the client, can execute arbitrary code with the privileges of the client application.

Critical Impact

Successful exploitation allows remote attackers to achieve arbitrary code execution on client systems connecting to malicious servers, potentially leading to complete system compromise.

Affected Products

• Apache ActiveMQ NMS OpenWire Client versions prior to 2.1.1
• .NET applications utilizing the vulnerable NMS OpenWire Client library
• Systems connecting to untrusted ActiveMQ servers

Discovery Timeline

• 2025-04-18 - CVE CVE-2025-29953 published to NVD
• 2025-07-09 - Last updated in NVD database

Technical Details for CVE-2025-29953

Vulnerability Analysis

This Insecure Deserialization vulnerability (CWE-502) exists in the Apache ActiveMQ NMS OpenWire Client's handling of server responses. The client performs unbounded deserialization of data received from servers without adequate validation, creating an exploitable condition when connecting to untrusted endpoints.

The vulnerability is accessible over the network without requiring authentication or user interaction. When exploited, attackers can achieve complete compromise of confidentiality, integrity, and availability on affected client systems.

Notably, while version 2.1.0 attempted to address this class of vulnerability by implementing allow/denylist restrictions on deserialization, the implementation contained flaws that allowed attackers to bypass these protections. The Apache Software Foundation has acknowledged the broader issue with .NET binary serialization, noting that Microsoft has deprecated this feature starting with .NET 9 and recommending migration away from binary serialization entirely.

Root Cause

The root cause is the use of .NET binary serialization (BinaryFormatter) for deserializing data received from servers without proper validation or type restrictions. BinaryFormatter is inherently unsafe when processing untrusted data, as it can instantiate arbitrary types and execute code during the deserialization process. The allow/denylist bypass in version 2.1.0 indicates that even type-filtering approaches are insufficient to fully secure .NET binary serialization.

Attack Vector

The attack vector requires a vulnerable client to establish a connection to a malicious server controlled by the attacker. The attack flow is as follows:

1. An attacker sets up a rogue ActiveMQ server or compromises an existing server
2. A victim's application using the vulnerable NMS OpenWire Client connects to the malicious server
3. The server responds with specially crafted serialized objects containing malicious payloads
4. The client deserializes the response without proper validation
5. During deserialization, the malicious payload triggers arbitrary code execution

This is a client-side vulnerability, meaning the attacker must convince or trick the target into connecting to a malicious server, or the attacker must perform a man-in-the-middle attack to intercept and modify legitimate server responses.

Detection Methods for CVE-2025-29953

Indicators of Compromise

• Unexpected network connections to unknown ActiveMQ brokers from client applications
• Anomalous process spawning from .NET applications using the NMS OpenWire Client
• Unusual deserialization exceptions in application logs followed by suspicious activity
• Evidence of BinaryFormatter usage with external data sources in code or runtime analysis

Detection Strategies

• Monitor for applications using Apache ActiveMQ NMS OpenWire Client versions prior to 2.1.1
• Implement network monitoring to detect connections to unexpected or untrusted ActiveMQ endpoints
• Deploy endpoint detection solutions capable of identifying deserialization-based exploitation attempts
• Review application dependencies for vulnerable versions of the Apache.NMS.ActiveMQ NuGet package

Monitoring Recommendations

• Enable verbose logging for ActiveMQ client connections to identify connections to unauthorized servers
• Implement allowlisting for trusted ActiveMQ broker endpoints in network security controls
• Monitor for child process creation from .NET applications that typically don't spawn processes
• Configure SIEM rules to correlate ActiveMQ connection events with suspicious subsequent activity

How to Mitigate CVE-2025-29953

Immediate Actions Required

• Upgrade Apache ActiveMQ NMS OpenWire Client to version 2.1.1 or later immediately
• Audit all applications using the NMS OpenWire Client library to identify vulnerable deployments
• Restrict client connections to trusted, known-good ActiveMQ servers only
• Consider disabling or removing applications that connect to untrusted ActiveMQ endpoints until patched

Patch Information

Apache has released version 2.1.1 of the ActiveMQ NMS OpenWire Client that addresses this deserialization vulnerability. Users should update their dependencies immediately via NuGet or their preferred package management solution.

For additional details, refer to the Apache Thread Discussion and the Openwall OSS-Security Posting.

Long-term, the Apache project recommends migrating away from .NET binary serialization entirely, in alignment with Microsoft's deprecation of BinaryFormatter in .NET 9. Consider adopting safer serialization alternatives such as System.Text.Json or protocol buffers.

Workarounds

• Configure network-level controls to restrict which ActiveMQ brokers clients can connect to
• Implement certificate pinning or mutual TLS to ensure clients only connect to trusted servers
• If possible, disable binary message serialization features in client applications
• Deploy application-level firewalls to inspect and filter ActiveMQ protocol traffic

# Example: Restrict outbound ActiveMQ connections to trusted servers using firewall rules
# Replace TRUSTED_BROKER_IP with your authorized ActiveMQ server addresses
iptables -A OUTPUT -p tcp --dport 61616 -d TRUSTED_BROKER_IP -j ACCEPT
iptables -A OUTPUT -p tcp --dport 61616 -j DROP