What’s Vulnerable › Vulnerabilities

CVE-2026-33219: NATS is vulnerable to pre-auth DoS through WebSockets client service

• Severity: medium (CVSS 5.3 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.5% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-03-25 · updated 2026-09-09
• Weakness: CWE-770

Description

NATS-Server is a High-Performance server for NATS.io, a cloud and edge native messaging system. Prior to versions 2.11.15 and 2.12.6, 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 CVE-2026-27571. That earlier issue was a compression bomb, this vulnerability is not. Attacks against this new issue thus require significant client bandwidth. Versions 2.11.15 and 2.12.6 contain a fix. As a workaround, disable websockets if not required for project deployment.

Affected products

• nats-io nats-server: before 2.11.15; from 2.12.0 before 2.12.6 — fixed in 2.11.15, 2.12.6

How to fix

Upgrade to a fixed version: nats-io nats-server 2.11.15, 2.12.6. Check the vendor advisory for mitigations.

References

• https://advisories.nats.io/CVE/secnote-2026-02.txt
• https://advisories.nats.io/CVE/secnote-2026-11.txt
• https://github.com/advisories/GHSA-qrvq-68c2-7grw
• https://github.com/nats-io/nats-server/security/advisories/GHSA-8r68-gvr4-jh7j
• https://access.redhat.com/errata/RHSA-2026:21769
• https://access.redhat.com/errata/RHSA-2026:22347
• https://access.redhat.com/errata/RHSA-2026:23345
• https://access.redhat.com/security/cve/CVE-2026-33219
• https://bugzilla.redhat.com/show_bug.cgi?id=2451445
• https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-33219.json

Get alerted when CVE-2026-33219 or the next vulnerability affects your stack