WARNING:

Users of 6.x should upgrade to 6.2.4 or later as the fix was missed in previous 6.x releases.

See the  following for more details:
https://activemq.apache.org/security-advisories.data/CVE-2026-40046-announcement.txt
https://www.cve.org/CVERecord?id=CVE-2026-40046

Original Report:

Apache ActiveMQ does not properly validate the remaining length field which may lead to an overflow during the decoding of malformed packets. When this integer overflow occurs, ActiveMQ may incorrectly compute the total Remaining Length and subsequently misinterpret the payload as multiple MQTT control packets which makes the broker susceptible to unexpected behavior when interacting with non-compliant clients. This behavior violates the MQTT v3.1.1 specification, which restricts Remaining Length to a maximum of 4 bytes. The scenario occurs on established connections after the authentication process. Brokers that are not enabling mqtt transport connectors are not impacted.

This issue affects Apache ActiveMQ: before 5.19.2, 6.0.0 to 6.1.8, and 6.2.0

Users are recommended to upgrade to version 5.19.2, 6.1.9, or 6.2.1, which fixes the issue.

Apache ActiveMQ no valida correctamente el campo de longitud restante, lo que puede llevar a un desbordamiento durante la decodificación de paquetes malformados. Cuando ocurre este desbordamiento de entero, ActiveMQ puede calcular incorrectamente la longitud restante total y posteriormente malinterpretar la carga útil como múltiples paquetes de control MQTT, lo que hace que el broker sea susceptible a un comportamiento inesperado al interactuar con clientes no conformes. Este comportamiento viola la especificación MQTT v3.1.1, que restringe la longitud restante a un máximo de 4 bytes. El escenario ocurre en conexiones establecidas después del proceso de autenticación. Los brokers que no están habilitando conectores de transporte mqtt no se ven afectados.

Este problema afecta a Apache ActiveMQ: antes de 5.19.2, 6.0.0 a 6.1.8, y 6.2.0

Se recomienda a los usuarios actualizar a la versión 5.19.2, 6.1.9, o 6.2.1, que corrige el problema.

Integer Overflow or Wraparound vulnerability in Apache ActiveMQ, Apache ActiveMQ All, Apache ActiveMQ MQTT.

The fix for "CVE-2025-66168: MQTT control packet remaining length field is not properly validated" was only applied to 5.19.2 (and future 5.19.x) releases but was missed for all 6.0.0+ versions.

This issue affects 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.

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.