Subject: nats-server leafnodes pre-auth server panic

NATS-advisory-ID: 2026-03
Aliases: CVE-2026-29785, GHSA-52jh-2xxh-pwh6
Date: 2026-03-09
Fixed-In: v2.11.14, v2.12.5

Background:

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

When configured to accept leafnode connections (for a hub/spoke topology
of multiple nats-servers), then the default configuration allows for
negotiating compression; a malicious remote NATS server can trigger a
server panic via that compression.


Problem Description:

If the nats-server has the "leafnode" configuration enabled (not
default), then anyone who can connect can crash the nats-server by
triggering a panic. This happens pre-authentication and requires that
compression be enabled (which it is, by default, when leafnodes are
used).

Context: a NATS server can form various clustering topologies, including
local clusters, and superclusters of clusters, but leafnodes allow for
separate administrative domains to link together with limited data
communication; eg, a server in a moving vehicle might use a local
leafnode for agents to connect to, and sync up to a central service as
and when available. The leafnode configuration here is where the central
server allows other NATS servers to connect into it, almost like regular
NATS clients. Documentation examples typically use port 7422 for
leafnode communications.


Affected versions:

NATS Server:
 * Version 2, prior to v2.11.14 or v2.12.5


Workarounds:

Disable compression on the leafnode port:

    leafnodes {
      port: 7422
      compression: off
    }


Solution:

Upgrade the NATS server to a fixed version.


Credits:

This was discovered internally by Synadia staff, using an AI analysis
tool.


References:

 * This document is canonically:
   <https://advisories.nats.io/CVE/secnote-2026-04.txt>
 * GHSA advisory:
   <https://github.com/nats-io/nats-server/security/advisories/GHSA-52jh-2xxh-pwh6>
 * MITRE CVE entry:
   <https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2026-29785>