Microsoft SharePoint Cryptographic Signature Verification Flaw CVE-2026-102866 Enables Token Forgery and Account Takeover

The Microsoft Security Response Center (MSRC) has addressed a high-severity elevation of privilege and authentication bypass vulnerability tracked as CVE-2026-102866 (CVSS v3.1 score: 8.1, Critical) impacting on-premises Microsoft SharePoint Server. The vulnerability allows an attacker who possesses or intercepts a valid user token to modify its identity claims and impersonate any corporate user—including Site Collection and Farm Administrators—without tripping cryptographic validation checks.

Root Cause: Signature Verification Failure in Claims Validation (CWE-347)

Microsoft SharePoint Server relies on the Windows Identity Foundation (WIF) and internal Security Token Service (STS) modules to process SAML 1.1/2.0 and JWT authentication assertions across web applications. When an authenticated user navigates SharePoint sites, their identity token contains a cryptographic signature envelope ensuring that claims (such as User Principal Name, Primary SID, and security group memberships) have not been altered in transit.

Under affected versions, SharePoint's token parser contains an implementation flaw in the signature validation routine. Specifically, when handling multi-part assertions where the signature block is positioned before the claims payload, the parser verifies the digest of the header envelope but neglects to cross-validate the hash of the claims payload against the signed signature value:

// Conceptual representation of vulnerable token validation sequence
public ClaimsPrincipal ValidateToken(string rawToken)
{
    var tokenXml = ParseXml(rawToken);
    var signature = tokenXml.SelectSingleNode("//ds:Signature");
    
    // VULNERABILITY: Verifies signature structure against header digest only!
    if (!CryptoHelper.VerifyEnvelope(signature, GetTrustedCertificate()))
    {
        throw new SecurityException("Invalid signature");
    }
    
    // Claims extracted from payload without asserting payload hash match
    return ExtractClaimsPrincipal(tokenXml.SelectSingleNode("//saml:AttributeStatement"));
}

Attack Surface & Exploitation Walkthrough

An attacker operating on the internal network or possessing low-privileged credentials (such as a temporary contractor or compromised workstation) can capture their own legitimate proof token. By modifying the PrimarySid claim to match the Active Directory SID of the SharePoint Farm Administrator or an executive user and submitting the crafted token to the SharePoint OAuth/STS endpoint, the attacker is instantly granted root-level administrative access to all documents, intranet portals, and database repositories.

Version Matrix & Update Packages

Defensive Checklist for SharePoint Administrators

• Apply Cumulative Security Updates: Deploy the October 2026 Security Updates across all SharePoint Front-End (WFE) and Application servers. Run the SharePoint Products Configuration Wizard (psconfig) on every farm node following installation.
• Enforce ASP.NET Request Validation: Ensure request validation and anti-tampering flags are enabled across all IIS virtual directories hosting SharePoint web applications.
• Enable Certificate Revocation Checking: Verify that local SharePoint STS token signing certificates are securely stored in the Windows Certificate Store with mandatory CRL checking enabled.