CVE-2026-40046: Apache ActiveMQ Buffer Overflow Vulnerability

CVE-2026-40046 Overview

An Integer Overflow or Wraparound vulnerability (CWE-190) has been identified in Apache ActiveMQ affecting versions 6.0.0 through 6.2.3. This vulnerability stems from an incomplete fix for CVE-2025-66168, where the MQTT control packet remaining length field validation patch was applied only to version 5.19.2 and future 5.19.x releases but was inadvertently omitted from all 6.0.0+ versions.

Critical Impact

Integer overflow vulnerabilities in message broker systems like Apache ActiveMQ can potentially lead to memory corruption, denial of service, or in severe cases, remote code execution through maliciously crafted MQTT packets.

Affected Products

• Apache ActiveMQ: from 6.0.0 before 6.2.4
• Apache ActiveMQ All: from 6.0.0 before 6.2.4
• Apache ActiveMQ MQTT: from 6.0.0 before 6.2.4

Discovery Timeline

• 2026-04-09 - CVE-2026-40046 published to NVD
• 2026-04-09 - Last updated in NVD database

Technical Details for CVE-2026-40046

Vulnerability Analysis

The vulnerability exists within the MQTT protocol handling component of Apache ActiveMQ. When processing MQTT control packets, the broker fails to properly validate the remaining length field, which uses a variable-length encoding scheme as defined in the MQTT specification. Without proper bounds checking, an attacker can supply a maliciously crafted value that causes an integer overflow during length calculations.

This issue is particularly significant because it represents a regression in security controls—the fix that was implemented for CVE-2025-66168 was not backported to the 6.x branch, leaving all Apache ActiveMQ deployments running version 6.0.0 through 6.2.3 exposed to this attack vector.

Root Cause

The root cause is an incomplete patch deployment across version branches. The MQTT remaining length field uses a variable-length encoding that can represent values from 0 to 268,435,455 bytes. When the validation logic is missing or improperly implemented, specially crafted length values can trigger an integer wraparound condition during memory allocation or buffer size calculations.

Attack Vector

An attacker with network access to the MQTT broker port can exploit this vulnerability by sending specially crafted MQTT control packets with malformed remaining length fields. The MQTT protocol typically operates on TCP port 1883 (unencrypted) or 8883 (TLS). The attack does not require authentication if the broker accepts anonymous connections, which is a common configuration in development or improperly secured production environments.

The exploitation involves:

1. Establishing a TCP connection to the ActiveMQ MQTT listener
2. Sending a malformed MQTT CONNECT or other control packet with an integer overflow-inducing remaining length value
3. Triggering memory corruption or denial of service when the broker processes the malformed packet

For detailed technical information about the exploitation mechanism, refer to the Apache ActiveMQ Security Advisory.

Detection Methods for CVE-2026-40046

Indicators of Compromise

• Unusual MQTT connection patterns with malformed packet headers arriving at the broker
• ActiveMQ broker crashes or unexpected restarts without clear operational cause
• Memory utilization anomalies in the ActiveMQ Java process
• Log entries indicating MQTT protocol parsing errors or buffer-related exceptions

Detection Strategies

• Deploy network intrusion detection rules to identify MQTT packets with abnormally large or malformed remaining length fields
• Monitor ActiveMQ broker logs for MQTT parsing exceptions or unexpected disconnections
• Implement application performance monitoring to detect memory allocation anomalies
• Use SentinelOne Singularity Platform to detect exploitation attempts and memory corruption indicators

Monitoring Recommendations

• Enable detailed MQTT protocol logging in ActiveMQ configuration
• Configure alerts for ActiveMQ service restarts or crashes
• Monitor network traffic on MQTT ports (1883, 8883) for anomalous patterns
• Implement heap dump analysis for investigating potential exploitation attempts

How to Mitigate CVE-2026-40046

Immediate Actions Required

• Upgrade Apache ActiveMQ to version 6.2.4 or later immediately
• If upgrade is not immediately possible, consider temporarily disabling the MQTT transport connector
• Restrict network access to MQTT ports using firewall rules to trusted clients only
• Review and implement authentication requirements for MQTT connections

Patch Information

Users are recommended to upgrade to version 6.2.4 or a 5.19.x version starting with 5.19.2 or later (currently latest is 5.19.5), which fixes the issue. Patch information and release notes are available through the Apache ActiveMQ Security Advisory and the Apache Mailing List Thread.

Workarounds

• Disable the MQTT transport connector if not required by commenting out or removing the MQTT transportConnector entry in activemq.xml
• Implement network-level filtering to restrict MQTT access to known trusted IP addresses
• Deploy a reverse proxy or API gateway with protocol validation capabilities in front of the MQTT endpoint
• Consider using the 5.19.x branch (5.19.2+) as an alternative if the 6.x branch features are not required

# Disable MQTT transport connector in activemq.xml
# Comment out or remove the following line in the transportConnectors section:
# <transportConnector name="mqtt" uri="mqtt://0.0.0.0:1883"/>

# Restrict MQTT port access via iptables (Linux)
iptables -A INPUT -p tcp --dport 1883 -s 192.168.1.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 1883 -j DROP