The vulnerability arises in NATS Server’s handling of MQTT retained messages and QoS1+ durable replay. Because the server does not re‑evaluate the concrete original topic before sending a PUBLISH to a subscriber, messages whose original topics match a subscriber’s deny rule can still be delivered. This bypass undermines the intended filtering and allows messages that should be blocked to reach recipients. The flaw is an Access Control Weakness (CWE-863) that results in unintended message delivery.

The impacted product is nats‑io NATS Server. Versions older than 2.14.3 and 2.12.12 are vulnerable. Upgrading to 2.14.3, 2.12.12, or any later release resolves the issue.

With a CVSS score of 4.3 the vulnerability is of moderate severity. The EPSS score is less than 1%, indicating a very low exploitation probability, and the vulnerability is not listed in CISA's KEV catalog. The likely attack vector is remote, via network access to the broker, by publishing a retained MQTT message or triggering a QoS replay that contains a topic matching a subscriber’s deny rule. The exploit does not provide direct code execution or data exfiltration, but it allows an attacker to bypass configured access controls and deliver unsolicited messages to subscribers.