RabbitMQ implements a loopback restriction that should verify the source address of an incoming connection against the address the server is bound to. In affected releases, the check mistakenly uses the address of the listener socket rather than the actual client source, allowing a connection that is forwarded through a trusted PROXY‑protocol endpoint to be treated as originating from localhost. This flaw enables a remote client to authenticate as the restricted guest user when traffic is routed through such a proxy, effectively bypassing the intended local‑only access limitation and providing unauthorized access to messaging resources.

RabbitMQ Server is affected. All versions prior to 3.13.15, 4.0.20, 4.1.11, and 4.2.6 are vulnerable. The flaw applies across the AMQP 0-9-1, AMQP 1.0, and Stream Protocol interfaces that the broker exposes.

With a CVSS score of 6.8 the vulnerability is considered moderate to high in severity, while the EPSS score of less than 1% indicates a very low probability of exploitation at the time of analysis. The issue is not listed in CISA's KEV catalog, suggesting there are no known or publicly available exploits. The attack vector requires the attacker to have the ability to send a PROXY‑protocol header that contains a loopback address to a RabbitMQ node whose listener is bound to localhost. If the broker is exposed to such traffic, an attacker can establish a guest session and read, write, or delete messages on any queues or exchanges that the guest account can access, compromising confidentiality, integrity, or availability of the messaging system.