RAPID 7 disclosed CVE-2026-55040, a serious authentication bypass vulnerability in Microsoft SharePoint, on July 14, 2026. The vulnerability allows unauthenticated attackers to forge valid JSON Web Tokens (JWT), impersonating SharePoint users or administrators due to weaknesses in the JWT token validation process.
Analysis identified four main weaknesses in the security system, including disabled cryptographic signature verification, problematic actor token validation, acceptance of unregistered certificate issuers, and non-cryptographic checks during signature verification. A proof-of-concept (PoC) script is available to demonstrate bypassing the authentication. The blog also promotes an upcoming webinar on this security research.