CVE-2025-27371 identifies an authentication bypass vulnerability rooted in ambiguities within the JSON Web Token (JWT) Profile for OAuth 2.0 Client Authentication as specified in IETF RFC 7523 and related RFCs (7521, 7522, 9101, and 9126). OAuth 2.0 is a widely adopted authorization framework used to grant limited access to resources on behalf of users or clients. The JWT Profile for OAuth 2.0 Client Authentication allows clients to authenticate to authorization servers using JWTs. However, this vulnerability arises due to unclear or ambiguous definitions of the 'audience' (aud) claim values in these JWTs. The audience claim is intended to specify the recipient(s) that the JWT is intended for, typically the authorization server. Ambiguities in this claim can lead to scenarios where an attacker crafts a JWT with manipulated audience values that bypass the intended authentication checks, effectively allowing unauthorized clients to gain access or escalate privileges. This weakness is classified under CWE-305 (Authentication Bypass by Primary Weakness), indicating that the core issue is a fundamental flaw in the authentication mechanism rather than an implementation error. The affected specifications include RFC 7523 and other related RFCs that extend or complement OAuth 2.0 client authentication methods, such as the JWT Assertion Profile (RFC 7521), SAML Assertion Profile (RFC 7522), JWT Authorization Response (RFC 9101), and Pushed Authorization Requests (RFC 9126). No patches or fixes have been published yet, and no known exploits are currently reported in the wild. The vulnerability's medium severity reflects the potential for unauthorized access but also the complexity involved in exploiting the ambiguity in audience claims. Organizations relying on OAuth 2.0 client authentication mechanisms as defined by these RFCs may be at risk if their implementations do not enforce strict validation of audience claims or if they accept JWTs from untrusted sources.

For European organizations, the impact of this vulnerability can be significant, especially for those heavily reliant on OAuth 2.0 for securing APIs, cloud services, and internal applications. An attacker exploiting this vulnerability could bypass client authentication, potentially gaining unauthorized access to sensitive data, performing actions on behalf of legitimate clients, or escalating privileges within identity and access management systems. This could lead to data breaches, unauthorized transactions, or disruption of services. Sectors such as finance, healthcare, government, and critical infrastructure, which often use OAuth 2.0-based authentication for secure access to resources, may face increased risk. Additionally, organizations using federated identity systems or third-party identity providers that implement these RFCs could be indirectly affected. The ambiguity in audience claims could also undermine trust in authentication tokens, complicating incident response and forensic investigations. Given the widespread adoption of OAuth 2.0 and JWTs in modern European digital services, the vulnerability poses a moderate but tangible threat to confidentiality and integrity of systems and data.

1. Strict Audience Validation: Implementers should enforce strict validation of the 'aud' claim in JWTs, ensuring that tokens are accepted only if the audience exactly matches the expected authorization server identifier. Avoid accepting tokens with ambiguous or multiple audience values. 2. Use Updated Specifications and Libraries: Monitor updates from IETF and OAuth working groups for clarifications or revisions to RFC 7523 and related RFCs. Adopt updated libraries and frameworks that incorporate fixes or enhanced validation logic. 3. Token Issuer Whitelisting: Restrict accepted JWT issuers to trusted entities only, reducing the risk of accepting forged tokens with manipulated audience claims. 4. Implement Additional Authentication Layers: Where feasible, complement JWT-based client authentication with additional factors or mechanisms such as mutual TLS or client certificates to reduce reliance on JWT audience claims alone. 5. Continuous Monitoring and Anomaly Detection: Deploy monitoring to detect unusual authentication patterns or token usage that may indicate exploitation attempts. 6. Security Reviews and Penetration Testing: Conduct thorough security assessments of OAuth 2.0 implementations focusing on JWT validation logic, particularly audience claim processing. 7. Incident Response Preparedness: Prepare response plans for potential authentication bypass incidents, including token revocation and user notification procedures. These measures go beyond generic advice by focusing on the specific ambiguity in audience claims and the nuances of OAuth 2.0 client authentication.