Microsoft SharePoint Weak Authentication Bypass Vulnerability (CVE-2026-55040) Security Advisory

1. Apply Official Security Patches

Immediately install the latest Cumulative Updates (CU) and security patches provided by Microsoft.

• Microsoft MSRC Update Guide

2. Temporary Mitigations (If patching is delayed)

• Network Access Control: Restrict direct internet access to SharePoint servers (typically TCP ports 80 and 443). Limit access to trusted internal IP ranges or require VPN authentication.
• WAF (Web Application Firewall) Rules: Deploy WAF signatures to detect and block anomalous JWT token structures and unauthorized authentication bypass attempts targeting SharePoint endpoints.
• IIS Log Monitoring: Audit IIS web server logs on SharePoint hosts for unusual POST/GET requests to authentication endpoints and spikes in 401/403 response codes.

CVE-2026-55040 is a critical authentication bypass vulnerability in Microsoft SharePoint Server, stemming from weak authentication validation (CWE-1390) within the JWT (JSON Web Token) handling mechanism.

A remote, unauthenticated attacker can exploit this vulnerability by sending crafted network requests to bypass security controls without valid credentials. With a CVSS score of 9.1 (Critical), low attack complexity, and no user interaction required, this flaw poses a severe risk. This vulnerability is actively exploited in the wild and is listed in CISA's Known Exploited Vulnerabilities (KEV) catalog, with an EPSS score of 0.94, indicating an extremely high threat level.