CVE-2026-58253 in nats-server
Summary
by MITRE • 07/08/2026
NATS Server is a high-performance server for NATS.io, the cloud and edge native messaging system. Prior to 2.14.0, 2.12.7, and 2.11.16, when no_auth_user was configured, a parser fast path intended for ordinary client connections could also apply to route or leafnode listeners, allowing an unauthenticated peer to bypass inter-server CONNECT authentication and operate with the privileges associated with that connection type. This issue is fixed in versions 2.14.0, 2.12.7, and 2.11.16.
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Analysis
by VulDB Data Team • 07/08/2026
The NATS Server vulnerability represents a critical authorization bypass flaw affecting versions prior to 2.14.0, 2.12.7, and 2.11.16 within the NATS.io messaging ecosystem. This issue stems from improper handling of authentication checks in specific connection listener types, creating a pathway for unauthenticated attackers to escalate their privileges within the messaging infrastructure. The vulnerability specifically impacts systems configured with the no_auth_user setting, which is designed to provide default user access for clients without explicit authentication. However, the implementation flaw allows malicious actors to exploit this configuration through route or leafnode listeners where the intended parser fast path logic was incorrectly applied.
The technical implementation of this vulnerability involves the server's connection handling logic that processes different types of connections with varying authentication requirements. Route connections and leafnode connections require specific inter-server authentication protocols that differ fundamentally from standard client connections. The parser fast path mechanism, which is optimized for regular client connections to improve performance, was inadvertently extended to include these inter-server connection types. This incorrect application allowed unauthenticated peers to bypass the necessary authentication checks required for route and leafnode connections, effectively granting them privileges equivalent to those of legitimate inter-server connections.
The operational impact of this vulnerability extends beyond simple unauthorized access, as it enables attackers to manipulate the messaging topology and potentially compromise the entire NATS network. An attacker exploiting this vulnerability could establish unauthorized routes between servers, intercept or modify messages intended for specific subscribers, or even disrupt service availability by creating malicious connection paths. The privilege escalation aspect is particularly concerning because route and leafnode connections typically operate with elevated permissions within the NATS infrastructure, allowing attackers to perform operations that would normally be restricted to authenticated system components.
Security implications align with CWE-284 which addresses insufficient access control in software systems, and this vulnerability demonstrates how improper access control implementation can create persistent security weaknesses. The issue also maps to ATT&CK technique T1078.004 which covers legitimate credentials, as attackers could potentially use this vulnerability to operate under the assumed identity of legitimate inter-server connections. Organizations running affected NATS Server versions face significant risk exposure, particularly in environments where the no_auth_user configuration is used for operational convenience but security requirements demand strict authentication controls.
The recommended mitigation strategy involves immediate deployment of patched versions 2.14.0, 2.12.7, and 2.11.16 across all NATS Server instances. Additionally, system administrators should review their configuration files to ensure that no_auth_user is not used in production environments where route or leafnode connections are active, as this combination creates a dangerous security posture. Network segmentation and monitoring of inter-server connections should be implemented to detect any unauthorized connection attempts. Organizations should also consider implementing additional authentication mechanisms such as TLS mutual authentication for route and leafnode connections, and regularly audit their NATS Server configurations to ensure that default user settings do not inadvertently create security vulnerabilities in complex messaging topologies.