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CVE-2026-58209 nats-io CVE debrief

CVE-2026-58209 is a vulnerability in NATS Server that allows MQTT retained message delivery and QoS1+ durable replay to deliver messages whose original topics matched a subscriber configured subscribe deny rule. This issue is fixed in versions 2.14.3 and 2.12.12. The vulnerability occurs because these delivery paths do not consistently recheck the concrete original topic before sending the MQTT PUBLISH to the subscriber. Users of NATS Server should review and update subscriber configured subscribe deny rules and monitor for unusual MQTT message delivery patterns.

Vendor
nats-io
Product
nats-server
CVSS
MEDIUM 4.3
CISA KEV
Not listed in stored evidence
Original CVE published
2026-07-08
Original CVE updated
2026-07-13
Advisory published
2026-07-08
Advisory updated
2026-07-13

Who should care

Users of NATS Server versions prior to 2.14.3 and 2.12.12 who utilize MQTT retained message delivery and QoS1+ durable replay should apply the patches to prevent unauthorized message delivery. Operators, platform administrators, vulnerability management teams, and security teams should review the vulnerability and apply mitigations as needed.

Technical summary

The NATS Server vulnerability CVE-2026-58209 allows MQTT retained message delivery and QoS1+ durable replay to deliver messages whose original topics matched a subscriber configured subscribe deny rule. This occurs because these delivery paths do not consistently recheck the concrete original topic before sending the MQTT PUBLISH to the subscriber. The issue is addressed in NATS Server versions 2.14.3 and 2.12.12. Users of NATS Server should apply patches to prevent unauthorized message delivery.

Defensive priority

Medium priority should be given to patching NATS Server instances to prevent potential unauthorized message delivery through MQTT retained message delivery and QoS1+ durable replay. Defenders should verify affected systems and apply patches or mitigations as needed.

Recommended defensive actions

  • Apply patches to upgrade NATS Server to version 2.14.3 or 2.12.12
  • Review and update subscriber configured subscribe deny rules
  • Monitor for unusual MQTT message delivery patterns
  • Confirm whether affected product deployments exist in managed environments and assign an owner for follow-up
  • Review compensating controls for exposed systems while remediation is scheduled and verified
  • Check relevant monitoring, detection, and logs for exposed assets that need extra review
  • Track exceptions, retest remediated assets, and close the item only after evidence is documented

Evidence notes

The CVE record was published on 2026-07-08T20:16:53.800Z and was last modified on 2026-07-13T13:53:02.150Z. The NVD entry is currently Analyzed. Evidence is limited to public sources and may not reflect the full scope or impact of the vulnerability. Defenders should verify affected systems and apply patches or mitigations as needed.

Sources and references

Verified primary and authoritative sources

  • CVE-2026-58209 CVE Program record

    Publisher, destination, and source semantics verified

    URL: https://www.cve.org/CVERecord?id=CVE-2026-58209

    CVE Program - Official CVE Program record with source-provided CVE metadata.

  • CVE-2026-58209 NVD vulnerability detail

    Publisher, destination, and source semantics verified

    URL: https://nvd.nist.gov/vuln/detail/CVE-2026-58209

    NIST National Vulnerability Database - Official NIST NVD detail page and source-specific vulnerability assessment.

Supplemental references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.