How severe is CVE-2026-33217?

CVE-2026-33217 has a CVSS score of 7.1/10, rated HIGH. Immediate patching is strongly recommended.

Which packages are affected by CVE-2026-33217?

CVE-2026-33217 affects the following packages: github.com/nats-io/nats-server/v2 (Go), github.com/nats-io/nats-server/v2 (Go), github.com/nats-io/nats-server (Go). Ecosystems affected: Go.

How do I fix CVE-2026-33217?

Update github.com/nats-io/nats-server/v2 to 2.11.15 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-33217 is resolved across your whole dependency graph.

How do I detect CVE-2026-33217 in my Go dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/nats-io/nats-server/v2, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2026-33217 if there is no patch (or I can't update yet)?

Put an independent control in front of the weakness: restrict the affected endpoint or interface to trusted networks, require an additional authentication factor or proxy-level check, and invalidate existing sessions and credentials in case the flaw has already been used.

Is CVE-2026-33217 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-33217 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2026-33217?

CVE-2026-33217 has an EPSS (Exploit Prediction Scoring System) score of 0.3%, placing it in the 18th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2026-33217?

CVE-2026-33217 is classified as CWE-863 (CWE-863), CWE-425 (CWE-425). These weakness types describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2026-33217 published, and has it been updated?

CVE-2026-33217 was published on March 25, 2026 and was last updated on October 28, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐹

🐹 Go

Not in CISA KEV

HIGH severity

CVE-2026-33217 — nats-server

HIGH

CVE-2026-33217 is a high-severity (CVSS 7.1) CWE-863 vulnerability in github.com/nats-io/nats-server/v2. A fix is available for github.com/nats-io/nats-server/v2 — see the affected versions and patch details below.

NATS allows MQTT clients to bypass ACL checks

Also known asBIT-nats-2026-33217GHSA-jxxm-27vp-c3m5GO-2026-4834

Published

Mar 25, 2026

Updated

Oct 28, 2026

Affected

3 pkgs

Patched

2 / 3

Exploits

None indexed

Exploitation data as of Nov 3, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

No confirmed exploitation observed yet

CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.

Exploitation and automatability from CISA’s SSVC triage for CVE-2026-33217.

EPSS Exploitation Probability

via FIRST.org ↗

0.3%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs18th percentile — riskier than 18% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

Where this sits among everything scored

Real-World Exposure

3 pkgs affected

🐹github.com/nats-io/nats-server/v2🐹github.com/nats-io/nats-server/v2🐹github.com/nats-io/nats-server

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Go packages — download data is not available via public APIs for these ecosystems.

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 provides an MQTT client interface.

Problem Description

When using ACLs on message subjects, these ACLs were not applied in the $MQTT.> namespace, allowing MQTT clients to bypass ACL checks for MQTT subjects.

Affected Versions

Any version before v2.12.6 or v2.11.15

Workarounds

None.

Affected Packages

3 total 2 fixed

Ecosystem	Package	Vulnerable range	Fix
🐹Go	`github.com/nats-io/nats-server/v2`	all versions	2.11.15`go get github.com/nats-io/nats-server/v2@v2.11.15`
🐹Go	`github.com/nats-io/nats-server/v2`	≥ 2.12.0-RC.1&&< 2.12.6	2.12.6`go get github.com/nats-io/nats-server/v2@v2.12.6`
🐹Go	`github.com/nats-io/nats-server`	all versions	No fix

Affected Products

1 product · 2 configurations

Application

nats-serverlinuxfoundation

≥ 2.12.0 && < 2.12.6

range

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/nats-io/nats-server/v2, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update github.com/nats-io/nats-server/v2 to 2.11.15 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-33217 is resolved across your whole dependency graph.
Workarounds
Put an independent control in front of the weakness: restrict the affected endpoint or interface to trusted networks, require an additional authentication factor or proxy-level check, and invalidate existing sessions and credentials in case the flaw has already been used.

Fixing This On Your OS

If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.

Red HatImportant

Workaround published by Red Hat

Mitigation for this issue is either not available or the currently available options do not meet the Red Hat Product Security criteria comprising ease of use and deployment, applicability to widespread installation base or stability.

Source: Red Hat security advisory for CVE-2026-33217 (CC BY 4.0)

Product	Fixed in	Advisory
Multicluster Global Hub 1.4.9	`multicluster-globalhub/multicluster-globalhub-grafana-rhel9:1779579439`	RHSA-2026:22347
Multicluster Global Hub 1.6.5	`multicluster-globalhub/multicluster-globalhub-grafana-rhel9:1780167118`	RHSA-2026:23345
Red Hat multicluster global hub 1.5.3	`multicluster-globalhub/multicluster-globalhub-grafana-rhel9:1778867753`	RHSA-2026:21769

Frequently Asked Questions

### 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 provides an MQTT client interface. ### Problem Description When using ACLs on message subjects, these ACLs were not applied in the `$MQTT.>` namespace, allowing MQTT clients to bypass ACL checks for MQTT subjects. ### Affected Versions Any version before v2.12.6 or v2.11.15 ### Workarounds None.