CVE-2026-55040: CVE-2026-55040: Microsoft SharePoint Server Security Feature Bypass Vulnerability

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CVE-2026-55040: Microsoft SharePoint Server Security Feature Bypass Vulnerability

Vulnerability ID: CVE-2026-55040
CVSS Score: 9.1
Published: 2026-07-14

CVE-2026-55040 is a critical security feature bypass vulnerability in Microsoft SharePoint Server arising from a weak authentication mechanism (CWE-1390). An unauthenticated remote attacker can exploit this security flaw over a network to bypass authentication validation routines, gaining unauthorized access to the application and complete control over sensitive enterprise data assets without any user interaction.

TL;DR

Unauthenticated remote security feature bypass in Microsoft SharePoint Server allows full authentication bypass due to weak token validation logic.

Technical Details

CWE ID: CWE-1390 (Weak Authentication)
Attack Vector: Network
CVSS Base Score: 9.1 (Critical)
EPSS Score: 0.00668
Impact: Confidentiality: High, Integrity: High, Availability: None
Exploit Status: None (No public exploit or PoC available)
CISA KEV Status: Not listed

Affected Systems

Microsoft SharePoint Enterprise Server 2016
Microsoft SharePoint Server 2019
Microsoft SharePoint Server Subscription Edition
SharePoint Enterprise Server 2016: >= 16.0.0, < 16.0.5561.1001 (Fixed in: 16.0.5561.1001)
SharePoint Server 2019: >= 16.0.0, < 16.0.10417.20175 (Fixed in: 16.0.10417.20175)
SharePoint Server Subscription Edition: >= 16.0.0, < 16.0.19725.20434 (Fixed in: 16.0.19725.20434)

Mitigation Strategies

Apply official cumulative updates released by Microsoft.
Restrict exposure of vulnerable SharePoint virtual directories to external networks.
Implement IP-based restriction policies on administrative and authentication endpoints.

Remediation Steps:

Verify the current build number of the SharePoint Server deployment to confirm vulnerability status.
Download the appropriate cumulative update package corresponding to the SharePoint version.
Apply the patch during a scheduled maintenance window.
Run the SharePoint Products Configuration Wizard to finalize database schema changes.
Verify the installed patch level to ensure it meets or exceeds the secure thresholds.