CVE-2026-33219

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.

The application allocates resources such as memory, threads, or file descriptors based on untrusted input without enforcing a cap. Typical impact: resource exhaustion leading to denial of service.

CVE-2026-33219 has a CVSS score of 5.3 (Medium). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (2.11.15, 2.12.6); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-33219? CVE-2026-33219 is a medium-severity allocation of resources without limits or throttling vulnerability in github.com/nats-io/nats-server/v2 (go), affecting versions < 2.11.15. It is fixed in 2.11.15, 2.12.6. The application allocates resources such as memory, threads, or file descriptors based on untrusted input without enforcing a cap.
2. How severe is CVE-2026-33219? CVE-2026-33219 has a CVSS score of 5.3 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which packages are affected by CVE-2026-33219?
   • github.com/nats-io/nats-server/v2 (go) (versions < 2.11.15)
   • github.com/nats-io/nats-server (go) (versions > 0)
4. Is there a fix for CVE-2026-33219? Yes. CVE-2026-33219 is fixed in 2.11.15, 2.12.6. Upgrade to this version or later.
5. Is CVE-2026-33219 exploitable, and should I be worried? Whether CVE-2026-33219 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-33219 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-33219?
   • Upgrade github.com/nats-io/nats-server/v2 to 2.11.15 or later
   • Upgrade github.com/nats-io/nats-server/v2 to 2.12.6 or later