CVE-2026-39304

Denial of Service via Out of Memory vulnerability in Apache ActiveMQ Client, Apache ActiveMQ Broker, Apache ActiveMQ.

ActiveMQ NIO SSL transports do not correctly handle TLSv1.3 handshake KeyUpdates triggered by clients. This makes it possible for a client to rapidly trigger updates which causes the broker to exhaust all its memory in the SSL engine leading to DoS.

Note: TLS versions before TLSv1.3 (such as TLSv1.2) are broken but are not vulnerable to OOM. Previous TLS versions require a full handshake renegotiation which causes a connection to hang but not OOM. This is fixed as well.
This issue affects Apache ActiveMQ Client: before 5.19.4, from 6.0.0 before 6.2.4; Apache ActiveMQ Broker: before 5.19.4, from 6.0.0 before 6.2.4; Apache ActiveMQ: before 5.19.4, from 6.0.0 before 6.2.4.

Users are recommended to upgrade to version 6.2.4 or 5.19.5, which fixes the issue.

Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service. Typical impact: denial of service.

CVE-2026-39304 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (5.19.4, 6.2.4); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-39304? CVE-2026-39304 is a high-severity uncontrolled resource consumption vulnerability in org.apache.activemq:activemq-client (maven), affecting versions < 5.19.4. It is fixed in 5.19.4, 6.2.4. Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service.
2. How severe is CVE-2026-39304? CVE-2026-39304 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which packages are affected by CVE-2026-39304?
   • org.apache.activemq:activemq-client (maven) (versions < 5.19.4)
   • org.apache.activemq:activemq-broker (maven) (versions < 5.19.4)
   • org.apache.activemq:activemq-all (maven) (versions < 5.19.4)
   • org.apache.activemq:apache-activemq (maven) (versions < 5.19.4)
4. Is there a fix for CVE-2026-39304? Yes. CVE-2026-39304 is fixed in 5.19.4, 6.2.4. Upgrade to this version or later.
5. Is CVE-2026-39304 exploitable, and should I be worried? Whether CVE-2026-39304 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-39304 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-39304?
   • Upgrade org.apache.activemq:activemq-client to 5.19.4 or later
   • Upgrade org.apache.activemq:activemq-client to 6.2.4 or later
   • Upgrade org.apache.activemq:activemq-broker to 5.19.4 or later
   • Upgrade org.apache.activemq:activemq-broker to 6.2.4 or later
   • Upgrade org.apache.activemq:activemq-all to 5.19.4 or later
   • Upgrade org.apache.activemq:activemq-all to 6.2.4 or later
   • Upgrade org.apache.activemq:apache-activemq to 5.19.4 or later
   • Upgrade org.apache.activemq:apache-activemq to 6.2.4 or later