NATS is vulnerable to pre-auth DoS through WebSockets client service

### 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 offers a WebSockets client service, used in deployments where browsers are the NATS clients. ### Problem Description A malicious client which can connect to the WebSockets port can cause unbounded memory use in the nats-server before authentication; this requires sending a corresponding amount of data. This is a milder variant of [NATS-advisory-ID 2026-02](https://advisories.nats.io/CVE/secnote-2026-02.txt) (aka CVE-2026-27571; GHSA-qrvq-68c2-7grw). That earlier issue was a compression bomb, this vulnerability is not. Attacks against this new issue thus require significant client bandwidth. ### Affected Versions Any version before v2.12.6 or v2.11.15 ### Workarounds Disable websockets if not required for project deployment.

CVSS 3.1

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

Severity from

GitHub (reviewed advisory)

Weakness

CWE-770

Also known as

BIT-nats-2026-33219, CVE-2026-33219, GO-2026-4831

• github.com/nats-io/nats-server/security/advisories/GHSA-8r68-gvr4-jh7j
• nvd.nist.gov/vuln/detail/CVE-2026-33219
• advisories.nats.io/CVE/secnote-2026-02.txt
• advisories.nats.io/CVE/secnote-2026-11.txt
• github.com/advisories/GHSA-qrvq-68c2-7grw
• github.com/nats-io/nats-server