CVE-2026-4349 identifies an improper authentication vulnerability in Duende IdentityServer version 4, specifically within an unspecified function of the /connect/authorize endpoint related to token renewal. The vulnerability arises from improper handling or validation of the id_token_hint argument, which is used during token renewal flows to authenticate or validate the user session. By manipulating this parameter, an attacker can bypass intended authentication mechanisms, potentially gaining unauthorized access or renewing tokens without proper validation. The attack can be initiated remotely without requiring user interaction or prior authentication, but it is characterized by high complexity and difficult exploitability, indicating that successful exploitation demands advanced skills or specific conditions. The vulnerability has a CVSS 4.0 base score of 6.3 (medium severity), reflecting limited impact on confidentiality, integrity, and availability due to low scope and vector complexity. The vendor Duende was contacted but did not respond, and no patches or known exploits are currently available. This vulnerability affects only IdentityServer version 4, a widely used OpenID Connect and OAuth 2.0 framework for .NET applications, often deployed in enterprise environments for identity and access management.

Organizations using Duende IdentityServer 4 for authentication and token management face risks of unauthorized access or token misuse due to this vulnerability. Attackers exploiting the improper authentication flaw could potentially renew tokens or bypass authentication checks, leading to unauthorized access to protected resources or services. Although the exploit complexity is high and no known exploits exist, the impact on confidentiality and integrity could be significant if exploited, especially in environments relying heavily on IdentityServer for secure authentication. This could lead to data breaches, privilege escalation, or unauthorized transactions. The vulnerability does not directly affect availability but could undermine trust in authentication processes. Enterprises in sectors such as finance, healthcare, government, and technology that deploy IdentityServer 4 are particularly at risk, as compromised authentication can lead to regulatory violations and reputational damage.

To mitigate CVE-2026-4349, organizations should first monitor for any official patches or updates from Duende and apply them promptly once available. In the absence of vendor patches, organizations should conduct a thorough review of their IdentityServer 4 deployment, focusing on the /connect/authorize endpoint and the handling of the id_token_hint parameter. Implement additional validation and sanitization of input parameters to prevent manipulation. Employ strict access controls and monitoring on token renewal endpoints to detect anomalous requests. Consider deploying Web Application Firewalls (WAFs) with custom rules to block suspicious token renewal attempts. Additionally, enforce multi-factor authentication (MFA) and session management best practices to reduce the risk of token misuse. Regularly audit authentication logs for unusual activity and prepare incident response plans specific to authentication bypass scenarios. Finally, evaluate upgrading to newer versions or alternative identity providers if Duende IdentityServer 4 remains unsupported.