CVE-2026-102866 – Microsoft Office SharePoint Elevation of Privilege Vulnerability

Microsoft has identified a critical elevation of privilege vulnerability in Microsoft Office SharePoint that could allow an unauthorized attacker to gain access to another user’s account. The vulnerability exists because SharePoint does not properly verify cryptographic signatures. An attacker who obtains a valid authentication proof token could modify it to impersonate a legitimate user, potentially accessing or changing sensitive information. Although exploitation requires specific conditions, the vulnerability is particularly concerning because attackers do not need existing privileges or user interaction to launch an attack.

CVSS SCORE: 8.1

SEVERITY: Critical

Threat

This vulnerability presents a significant security risk to organizations running affected Microsoft SharePoint servers.

An unauthenticated attacker could exploit weaknesses in cryptographic signature verification to impersonate legitimate SharePoint users.

Successful exploitation could result in:

• Unauthorized Account Access: Attackers could authenticate as legitimate users without their permission.
• Sensitive Data Exposure: Confidential documents, internal communications, and business information could become accessible.
• Data Manipulation: Attackers could modify information accessible through the compromised account.
• Business Disruption: Unauthorized changes to critical SharePoint content could interfere with business operations.
• Security Trust Violations: Attackers could bypass authentication protections and operate under the identity of trusted users.

The vulnerability affects confidentiality, integrity, and availability, with all three receiving High impact ratings under CVSS.

Exploits

Microsoft classifies this vulnerability as Exploitation Less Likely.

At the time of publication, the vulnerability was not publicly disclosed, and no active exploitation had been reported.

The exploit code maturity is classified as Unproven, meaning there is no confirmed evidence of functional exploit code.

The existence of publicly available proof-of-concept (PoC) exploit code or zero-day exploitation cannot be confirmed.

However, the vulnerability has a network-based attack vector and does not require authentication or user interaction.

Exploitation is considered technically difficult because specific conditions, such as particular protocol settings or configurations, must be present before an attack can succeed.

Technical Summary

CVE-2026-102866 is an elevation of privilege vulnerability affecting Microsoft Office SharePoint.

The underlying issue is classified as CWE-347: Improper Verification of Cryptographic Signature.

Cryptographic signatures help applications verify the authenticity and integrity of security tokens and other trusted information.

In this vulnerability, improper verification of these signatures could allow an attacker to manipulate authentication information.

How the vulnerability works:

An unauthenticated attacker who obtains a valid proof token belonging to a targeted SharePoint user could modify that token and submit it to an affected SharePoint server.

Because of the cryptographic signature verification weakness, the server could accept the manipulated token.

If exploitation succeeds, the attacker could authenticate as the targeted user.

This could allow unauthorized access to SharePoint resources and potentially enable the attacker to read or modify content that the compromised account is authorized to access.

Technical characteristics:

• Attack Vector: Network
• Attack Complexity: High
• Privileges Required: None
• User Interaction: None
• Scope: Unchanged
• Confidentiality Impact: High
• Integrity Impact: High
• Availability Impact: High

The vulnerability does not require users to open malicious documents, click suspicious links, or perform other actions.

Microsoft also confirms that the Preview Pane is not an attack vector.

Although the attack complexity is high, successful exploitation could undermine SharePoint authentication and expose sensitive organizational information.

Exploitability

Affected Product:

Microsoft SharePoint Server Subscription Edition

Security Update: KB5002927

CVSS Base Score: 8.1

Exploitability Assessment: Exploitation Less Likely

An attacker could exploit this vulnerability remotely by obtaining and modifying a valid user proof token.

Successful exploitation could allow the attacker to authenticate as the targeted user and access or modify SharePoint content.

No existing privileges or user interaction are required.

However, exploitation requires specific conditions, making widespread attacks less likely.

The affected product is confirmed, but the exact affected build versions and fixed build number are not specified.

Business Impact

SharePoint is widely used by organizations to store documents, manage collaboration, and share business-critical information.

A vulnerability that allows attackers to impersonate legitimate users could create serious security and operational consequences.

Potential business impacts include:

• Confidential Information Exposure: Attackers could access sensitive business documents, internal reports, or other protected SharePoint content.
• Unauthorized Data Changes: Compromised user identities could allow attackers to modify documents and information.
• Operational Disruption: Unauthorized modifications could interrupt collaboration and affect critical business processes.
• Compliance Concerns: Unauthorized access to sensitive information could create regulatory and data protection risks.
• Loss of Trust: Successful account impersonation could weaken confidence in the security of organizational collaboration systems.

“When attackers can impersonate trusted SharePoint users, sensitive information can become accessible without anyone clicking a malicious link.”

Organizations using SharePoint for sensitive business operations should prioritize protecting affected systems against unauthorized authentication and privilege escalation.

Workaround

Microsoft has not identified any official mitigations or workarounds for this vulnerability.

Organizations should deploy security update KB5002927 for Microsoft SharePoint Server Subscription Edition.

If immediate patching is not possible, consider restricting network access to affected SharePoint servers and monitoring authentication activity for suspicious behavior.

These are general risk-reduction measures and are not confirmed fixes for the vulnerability.

The restart requirement is currently unknown.

Urgency

CVE-2026-102866 carries a Critical severity rating and a CVSS score of 8.1, making it an important security update for organizations operating affected SharePoint environments.

The vulnerability is particularly concerning because attackers can potentially exploit it remotely without authentication or user interaction.

Although Microsoft currently assesses exploitation as less likely, successful exploitation could enable unauthorized access to sensitive business information through legitimate user identities.

The requirement for specific exploitation conditions reduces the likelihood of widespread attacks but does not eliminate the potential impact.

Organizations should prioritize deploying KB5002927, particularly on SharePoint servers containing sensitive information or supporting critical business operations.

Delaying remediation could leave affected environments exposed to unauthorized account impersonation and potential data compromise.