CVE-2026-33218

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

The application does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths. Typical impact: varies by context: data corruption, logic bypass, or denial of service.

CVE-2026-33218 has a CVSS score of 7.5 (High). 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-2026-33218? CVE-2026-33218 is a high-severity improper input validation 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 does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths.
2. How severe is CVE-2026-33218? CVE-2026-33218 has a CVSS score of 7.5 (High). 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-2026-33218?
   • 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-2026-33218? Yes. CVE-2026-33218 is fixed in 2.11.15, 2.12.6. Upgrade to this version or later.
5. Is CVE-2026-33218 exploitable, and should I be worried? Whether CVE-2026-33218 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-2026-33218 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-2026-33218?
   • 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