CVE-2026-33248: NATS has mTLS verify_and_map authentication bypass via incorrect Subject DN matching
Background
NATS.io is a high performance open source pub-sub distributed communication technology, built for the cloud, on-premise, IoT, and edge computing.
One authentication model supported is mTLS, deriving the NATS client identity from properties of the TLS Client Certificate.
Problem Description
When using mTLS for client identity, with verifyandmap to derive a NATS identity from the client certificate's Subject DN, certain patterns of RDN would not be correctly enforced, allowing for authentication bypass.
This does require a valid certificate from a CA already trusted for client certificates, and DN naming patterns which the NATS maintainers consider highly unlikely.
So this is an unlikely attack. Nonetheless, administrators who have been very sophisticated in their DN construction patterns might conceivably be impacted.
Affected Versions
Fixed in nats-server 2.12.6 & 2.11.15
Workarounds
Developers should review their CA issuing practices.
Other sources
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, when using mTLS for client identity, with verifyandmap to derive a NATS identity from the client certificate's Subject DN, certain patterns of RDN would not be correctly enforced, allowing for authentication bypass. This does require a valid certificate from a CA already trusted for client certificates, and DN naming patterns which the NATS maintainers consider highly unlikely. So this is an unlikely attack. Nonetheless, administrators who have been very sophisticated in their DN construction patterns might conceivably be impacted. Versions 2.11.15 and 2.12.6 contain a fix. As a workaround, developers should review their CA issuing practices.
— MITRE
Affected Software
Event History
Frequently Asked Questions
What is the severity of CVE-2026-33248?
CVE-2026-33248 has a medium severity rating due to potential risks in mTLS authentication.
How do I fix CVE-2026-33248?
To fix CVE-2026-33248, upgrade to NATS server version 2.12.6 or later, or 2.11.15 if you are using an older version.
What version of NATS server is affected by CVE-2026-33248?
CVE-2026-33248 affects NATS server versions from 2.12.0-RC.1 up to 2.11.15.
What is the impact of CVE-2026-33248 on NATS users?
The impact of CVE-2026-33248 can lead to unauthorized access or misuse of mTLS authentication if not addressed.
Is there any additional information available about CVE-2026-33248?
Additional information about CVE-2026-33248 can be found in the advisories on GitHub related to NATS server vulnerabilities.