Subject: nats-server: MQTT hijacking via Client ID

NATS-advisory-ID: 2026-06
Aliases: CVE-2026-33215, GHSA-fcjp-h8cc-6879
Date: 2026-03-24
Fixed-In: 2.12.6, 2.11.15

Background:

NATS.io is a high performance open source pub-sub distributed communication
technology, built for the cloud, on-premise, IoT, and edge computing.

The nats-server provides an MQTT client interface.

Problem Description:

Sessions and Messages can by hijacked via MQTT Client ID malfeasance.

Affected Versions:

 nats-server: any version before v2.12.6 or v2.11.15

Workarounds:

None.

References:

 * This document is canonically:
   <https://advisories.nats.io/CVE/secnote-2026-06.txt>
 * GHSA advisory:
   <https://github.com/nats-io/nats-server/security/advisories/GHSA-fcjp-h8cc-6879>
 * MITRE CVE entry:
   <https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2026-33215>