NATS has pre-auth server panic via leafnode handling

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 allows hub/spoke topologies using "leafnode" connections by other nats-servers.

Problem Description

A client which can connect to the leafnode port can crash the nats-server with a certain malformed message pre-authentication.

Affected Versions

Any version before v2.12.6 or v2.11.15

Workarounds

1. Disable leafnode support if not needed.
2. Restrict network connections to your leafnode port, if plausible without compromising the service offered.

References

• This document is canonically: https://advisories.nats.io/CVE/secnote-2026-10.txt
• GHSA advisory: https://github.com/nats-io/nats-server/security/advisories/GHSA-vprv-35vv-q339
• MITRE CVE entry: https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2026-33218