CVE-2025-27533: Apache ActiveMQ DoS Vulnerability

CVE-2025-27533 Overview

CVE-2025-27533 is a Memory Allocation with Excessive Size Value vulnerability (CWE-789) affecting Apache ActiveMQ, a popular open-source message broker. The vulnerability exists in the unmarshalling process of OpenWire commands, where the size value of buffers is not properly validated. This improper validation allows attackers to trigger excessive memory allocation, which can be exploited to cause a denial of service (DoS) by depleting process memory and disrupting applications and services that rely on the availability of the ActiveMQ broker.

Critical Impact

Successful exploitation of this vulnerability can lead to complete denial of service of the ActiveMQ broker by exhausting process memory, affecting all dependent applications and services when mutual TLS connections are not in use.

Affected Products

• Apache ActiveMQ versions 6.0.0 to before 6.1.6
• Apache ActiveMQ versions 5.18.0 to before 5.18.7
• Apache ActiveMQ versions 5.17.0 to before 5.17.7
• Apache ActiveMQ versions before 5.16.8
• Note: ActiveMQ 5.19.0 is not affected

Discovery Timeline

• 2025-05-07 - CVE-2025-27533 published to NVD
• 2025-11-03 - Last updated in NVD database

Technical Details for CVE-2025-27533

Vulnerability Analysis

This vulnerability is classified as Memory Allocation with Excessive Size Value (CWE-789), a flaw that occurs when software allocates memory based on an untrusted size value without adequate bounds checking. In the context of Apache ActiveMQ, the vulnerability manifests during the unmarshalling process of OpenWire protocol commands.

The OpenWire protocol is ActiveMQ's native wire protocol used for communication between clients and brokers. When processing incoming commands, the broker must deserialize (unmarshal) data structures that include size-prefixed buffers. The vulnerability arises because the code responsible for unmarshalling these commands fails to properly validate buffer size values before allocating memory.

An attacker can craft malicious OpenWire commands containing extremely large size values for buffers. When the broker attempts to process these commands, it will attempt to allocate the specified amount of memory, potentially exhausting available process memory and causing a denial of service condition.

Root Cause

The root cause is insufficient input validation in the OpenWire command unmarshalling logic. Specifically, when parsing size-prefixed data structures, the deserialization code accepts attacker-controlled size values without verifying they fall within reasonable bounds. This allows an attacker to specify arbitrarily large buffer sizes, triggering excessive memory allocation that can exhaust the JVM heap or system memory.

Attack Vector

The attack can be executed remotely over the network by any client that can establish a connection to the ActiveMQ broker's OpenWire transport connector. The attacker sends specially crafted OpenWire protocol messages containing oversized buffer size values. When the broker processes these messages during unmarshalling, it attempts to allocate the specified memory amount, leading to memory exhaustion.

The vulnerability is particularly concerning in environments where mutual TLS (mTLS) is not configured, as any network-reachable attacker can establish connections and send malicious commands. The attack requires no authentication and can be executed with relatively low complexity, though some conditions must be met for successful exploitation.

The attack flow involves:

1. Attacker establishes a connection to the ActiveMQ broker's OpenWire port (default: 61616)
2. Attacker sends crafted OpenWire commands with malicious buffer size values
3. The broker's unmarshalling code attempts to allocate memory based on the attacker-specified size
4. Repeated requests exhaust available memory, causing the broker to crash or become unresponsive
5. Applications and services dependent on the broker experience denial of service

Detection Methods for CVE-2025-27533

Indicators of Compromise

• Unusual memory consumption patterns on ActiveMQ broker hosts, particularly rapid memory growth
• ActiveMQ broker crashes or OutOfMemoryError exceptions in broker logs
• Anomalous network traffic patterns on OpenWire ports (default TCP/61616) with malformed or oversized packets
• Connection attempts from unexpected or unauthorized IP addresses to the broker
• Repeated broker restarts or service failures without apparent configuration changes

Detection Strategies

• Monitor JVM heap usage and garbage collection metrics for ActiveMQ processes; sudden spikes may indicate exploitation attempts
• Implement network intrusion detection rules to identify malformed OpenWire protocol traffic with abnormally large size fields
• Configure alerting on ActiveMQ broker process crashes, restarts, or memory-related exceptions
• Review connection logs for unusual patterns such as many short-lived connections or connections from untrusted sources
• Deploy application-level logging to capture and analyze incoming OpenWire command sizes

Monitoring Recommendations

• Set up continuous monitoring of ActiveMQ broker memory utilization with threshold-based alerting
• Implement network flow analysis to detect unusual traffic volumes or patterns on OpenWire ports
• Configure centralized logging to aggregate and correlate ActiveMQ broker events across the environment
• Establish baseline metrics for normal broker operation to facilitate anomaly detection
• Regularly review security logs and audit trails for signs of reconnaissance or exploitation attempts

How to Mitigate CVE-2025-27533

Immediate Actions Required

• Upgrade Apache ActiveMQ to a patched version immediately: 6.1.6+, 5.19.0+, 5.18.7+, 5.17.7+, or 5.16.8+
• If immediate patching is not possible, implement mutual TLS (mTLS) to restrict broker access to authenticated clients only
• Review network firewall rules to limit access to ActiveMQ OpenWire ports to only trusted hosts and networks
• Enable enhanced monitoring on ActiveMQ brokers to detect potential exploitation attempts
• Develop and test an incident response plan for broker availability incidents

Patch Information

Apache has released security patches addressing this vulnerability in the following versions:

• Apache ActiveMQ 6.1.6 and later
• Apache ActiveMQ 5.19.0 and later
• Apache ActiveMQ 5.18.7 and later
• Apache ActiveMQ 5.17.7 and later
• Apache ActiveMQ 5.16.8 and later

Users should upgrade to one of these patched versions based on their current version branch. Refer to the Apache Security Mailing List for official guidance. Additional information is available from the Openwall OSS Security Update and the Debian LTS Announcement.

Workarounds

• Implement mutual TLS (mTLS) authentication on the broker to ensure only authenticated clients can connect
• Restrict network access to the OpenWire transport connector using firewall rules or network segmentation
• Consider temporarily disabling the OpenWire transport connector if not required and alternative protocols are available
• Deploy a reverse proxy or application gateway with request validation capabilities in front of the broker
• Implement rate limiting on connections to the OpenWire port to slow down potential exploitation attempts

# Example: Configure mutual TLS in activemq.xml
# Add the following to your transportConnector configuration:
# <transportConnector name="openwire" 
#   uri="ssl://0.0.0.0:61617?needClientAuth=true"/>
#
# Ensure SSL context is properly configured with:
# - Broker keystore containing server certificate
# - Truststore containing authorized client certificates
# - Proper file permissions on keystore/truststore files