Rapid7 Analysis: Microsoft SharePoint JWT Token Authentication Bypass (CVE-2026-55040)

Rapid7 has published a technical analysis and proof-of-concept exploit for CVE-2026-55040, a critical authentication bypass vulnerability in Microsoft SharePoint. The flaw stems from multiple weaknesses in the JWT token validation pipeline, specifically allowing attackers to disable signature requirements and forge valid tokens using the server's own Security Token Service certificate.

Unauthenticated remote attackers can leverage this issue to impersonate any site user or administrator on SharePoint Server Subscription Edition. Rapid7’s research confirms that the vulnerability is actively exploitable through a public script that demonstrates how to craft malicious Bearer tokens to bypass standard security controls.