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NATS allows MQTT clients to bypass ACL checks

High severity GitHub Reviewed Published Mar 24, 2026 in nats-io/nats-server • Updated Mar 27, 2026

Package

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

Affected versions

> 0

Patched versions

None
gomod github.com/nats-io/nats-server/v2 (Go)
< 2.11.15
>= 2.12.0-RC.1, < 2.12.6
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 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.

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
Published by the National Vulnerability Database Mar 25, 2026
Last updated Mar 27, 2026

Severity

High

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
Low
User interaction
None
Scope
Unchanged
Confidentiality
Low
Integrity
High
Availability
None

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:L/UI:N/S:U/C:L/I:H/A:N

EPSS score

Exploit Prediction Scoring System (EPSS)

This score estimates the probability of this vulnerability being exploited within the next 30 days. Data provided by FIRST.
(8th percentile)

Weaknesses

Incorrect Authorization

The product performs an authorization check when an actor attempts to access a resource or perform an action, but it does not correctly perform the check. Learn more on MITRE.

CVE ID

CVE-2026-33217

GHSA ID

GHSA-jxxm-27vp-c3m5

Source code

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