CVE-2026-33248 in nats-server
Summary
by MITRE • 03/25/2026
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 `verify_and_map` 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.
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Analysis
by VulDB Data Team • 04/01/2026
The vulnerability identified as CVE-2026-33248 affects NATS-Server versions prior to 2.11.15 and 2.12.6, representing a critical authentication bypass flaw within the mTLS client identity verification mechanism. This issue specifically impacts systems utilizing the `verify_and_map` functionality to derive NATS identities from client certificate Subject Distinguished Names, creating a pathway for unauthorized access when certain RDN patterns are improperly handled. The vulnerability resides in the certificate validation logic that processes client certificates for authentication purposes, particularly when the server attempts to map certificate attributes to NATS user identities through the Subject DN structure.
The technical flaw manifests in the improper enforcement of Distinguished Name patterns during the certificate verification process, where specific sequences of Relative Distinguished Names within the certificate's Subject field fail to be correctly validated. This misconfiguration allows attackers with valid certificates from trusted CAs to potentially bypass intended authentication controls by crafting certificate subject names that exploit the parsing inconsistencies in the server's verification logic. The vulnerability is classified under CWE-287, Authentication Bypass Through Modification of Authentication-Related Data, as it enables unauthorized access through manipulation of certificate attributes that should be strictly validated.
The operational impact of this vulnerability is significant for organizations relying on mTLS authentication with certificate-based identity mapping, particularly those implementing sophisticated DN naming conventions for their certificate authorities. While the attack vector requires a valid certificate from a trusted CA and specific DN patterns that NATS maintainers consider highly unlikely, the potential for exploitation exists in environments where administrators have implemented complex certificate naming structures. The vulnerability affects systems where client certificates are used for identity mapping, potentially allowing attackers to assume unauthorized identities within the NATS messaging system, leading to possible data exposure, message interception, or system compromise.
Organizations should prioritize upgrading to NATS-Server versions 2.11.15 or 2.12.6 to remediate this vulnerability, as these releases contain the necessary patches to properly enforce Distinguished Name validation patterns. The workaround suggested by developers involves reviewing CA issuing practices to ensure that certificate naming patterns do not create opportunities for exploitation, particularly avoiding complex or unconventional DN structures that might trigger the parsing inconsistencies. This vulnerability aligns with ATT&CK technique T1550.001, Use of Valid Credentials, as it enables attackers to leverage legitimate certificates to bypass authentication controls, and T1078.004, Valid Accounts, by allowing unauthorized access through legitimate certificate-based authentication mechanisms. The fix implemented in the patched versions addresses the core issue by strengthening the validation logic for Distinguished Name patterns, ensuring that all RDN sequences are properly enforced during certificate verification processes and preventing the bypass conditions that previously existed in the authentication flow.