GHSA-8r68-gvr4-jh7j

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

Source: GitHub Advisory Database (reviewed) Moderate CVE-2026-33219

Packages

Ecosystem	Package	Affected	Fixed in
Go	github.com/nats-io/nats-server/v2	< 2.11.15	2.11.15
Go	github.com/nats-io/nats-server/v2	>= 2.12.0-RC.1, < 2.12.6	2.12.6
Go	github.com/nats-io/nats-server	all versions	no fix listed

Details

As written in the advisory (Markdown source, shown as plain text).

### 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.

References

External links from the advisory. They lead to third-party sites.