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

Moderate severity GitHub Reviewed Published Mar 24, 2026 in nats-io/nats-server • Updated Mar 24, 2026

Package

gomod github.com/nats-io/nats-server/v2 (Go)

Affected versions

< 2.11.15
>= 2.12.0-RC.1, < 2.12.6

Patched versions

2.11.15
2.12.6

Description

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 (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

@philpennock philpennock published to nats-io/nats-server Mar 24, 2026
Published to the GitHub Advisory Database Mar 24, 2026
Reviewed Mar 24, 2026
Last updated Mar 24, 2026

Severity

Moderate

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v3 base metrics

Attack vector
Network
Attack complexity
Low
Privileges required
None
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
Low

CVSS v3 base metrics

Attack vector: More severe the more the remote (logically and physically) an attacker can be in order to exploit the vulnerability.
Attack complexity: More severe for the least complex attacks.
Privileges required: More severe if no privileges are required.
User interaction: More severe when no user interaction is required.
Scope: More severe when a scope change occurs, e.g. one vulnerable component impacts resources in components beyond its security scope.
Confidentiality: More severe when loss of data confidentiality is highest, measuring the level of data access available to an unauthorized user.
Integrity: More severe when loss of data integrity is the highest, measuring the consequence of data modification possible by an unauthorized user.
Availability: More severe when the loss of impacted component availability is highest.
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

EPSS score

Weaknesses

Allocation of Resources Without Limits or Throttling

The product allocates a reusable resource or group of resources on behalf of an actor without imposing any intended restrictions on the size or number of resources that can be allocated. Learn more on MITRE.

CVE ID

CVE-2026-33219

GHSA ID

GHSA-8r68-gvr4-jh7j

Source code

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