What’s Vulnerable › Vulnerabilities

CVE-2026-58209: NATS Server: MQTT retained and QoS replay bypass subscribe deny filters

• Severity: medium (CVSS 4.3 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.3% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-07-08 · updated 2026-10-25
• Weakness: CWE-863

Description

NATS Server is a high-performance server for NATS.io, the cloud and edge native messaging system. Prior to 2.14.3 and 2.12.12, MQTT retained message delivery and QoS1+ durable replay could deliver messages whose original topics matched a subscriber configured subscribe deny rule because these delivery paths did not consistently recheck the concrete original topic before sending the MQTT PUBLISH to the subscriber. This issue is fixed in versions 2.14.3 and 2.12.12.

Affected products

• nats-io nats-server: before 2.12.12; from 2.14.0 before 2.14.3 — fixed in 2.12.12, 2.14.3

How to fix

Upgrade to a fixed version: nats-io nats-server 2.12.12, 2.14.3. Check the vendor advisory for mitigations.

References

• https://github.com/nats-io/nats-server/commit/181b1f51f40b9954c57e9d478e051fb257679356
• https://github.com/nats-io/nats-server/commit/1c429b6fdc5afd4188cc5faf1127f6334896cd87
• https://github.com/nats-io/nats-server/releases/tag/v2.12.12
• https://github.com/nats-io/nats-server/releases/tag/v2.14.3
• https://github.com/nats-io/nats-server/security/advisories/GHSA-7qmq-8cc4-hxwg

Get alerted when CVE-2026-58209 or the next vulnerability affects your stack