JWT Authentication Bypass in Microsoft SharePoint (CVE-2026-55040)

Rapid7's article examines the vulnerability CVE-2026-55040 in Microsoft SharePoint, which is related to a bypass of JWT token validation in the authentication mechanism. The issue stems from improper token signature validation in the JSON Web Token processing chain, allowing a remote attacker to bypass authentication.

Exploitation of the vulnerability enables an unauthenticated attacker to spoof a user identifier and perform actions on the user's behalf, including accessing SharePoint data and functionality. The article also notes that this authentication bypass could be used as the first stage of an attack chain leading to further system compromise, including potential code execution on the server.

Article: https://www.rapid7.com/blog/post/ra-microsoft-sharepoint-jwt-token-authentication-bypass-cve-2026-55040/