CVE-2026-20963: SharePoint Deserialization RCE Vulnerability

CVE-2026-20963 Overview

CVE-2026-20963 is an insecure deserialization vulnerability affecting Microsoft Office SharePoint that enables an authorized attacker to execute arbitrary code over a network. This vulnerability stems from improper handling of untrusted data during deserialization operations, a common weakness pattern classified as CWE-502 (Deserialization of Untrusted Data).

SharePoint's widespread deployment in enterprise environments for document management and collaboration makes this vulnerability particularly concerning. Attackers with low-privilege authenticated access to a SharePoint environment can exploit this flaw to achieve remote code execution, potentially compromising sensitive business data and gaining a foothold for lateral movement within corporate networks.

Critical Impact

Authorized attackers can achieve remote code execution on SharePoint servers, potentially leading to complete system compromise, data exfiltration, and lateral movement within enterprise networks.

Affected Products

• Microsoft Office SharePoint

Discovery Timeline

• January 13, 2026 - CVE-2026-20963 published to NVD
• January 13, 2026 - Last updated in NVD database

Technical Details for CVE-2026-20963

Vulnerability Analysis

This vulnerability exploits SharePoint's handling of serialized objects during processing of user-supplied data. When SharePoint deserializes objects without proper validation, an attacker can craft malicious serialized payloads that, when processed by the server, execute arbitrary code in the context of the SharePoint application.

The attack requires only low-level authentication, meaning any authorized SharePoint user could potentially exploit this vulnerability. No user interaction is required beyond the initial authenticated session, and the attack can be executed remotely over the network. Successful exploitation results in complete compromise of confidentiality, integrity, and availability of the affected SharePoint server.

Root Cause

The vulnerability originates from CWE-502: Deserialization of Untrusted Data. SharePoint fails to adequately validate or sanitize serialized objects before deserializing them. This allows attackers to inject malicious object graphs that trigger code execution during the deserialization process. .NET deserialization vulnerabilities often leverage gadget chains—sequences of existing classes that, when deserialized together, result in unintended code execution.

Attack Vector

The attack is network-based and requires authenticated access to the SharePoint environment. An attacker with valid low-privilege credentials can submit specially crafted serialized data to vulnerable SharePoint endpoints. The malicious payload is processed during deserialization, triggering the execution of attacker-controlled code on the server.

The exploitation flow typically involves:

1. Authenticating to the SharePoint environment with low-privilege credentials
2. Identifying vulnerable endpoints that process serialized data
3. Crafting a malicious serialized object using known .NET deserialization gadget chains
4. Submitting the payload to the vulnerable endpoint
5. Achieving code execution as the SharePoint application service account

For detailed technical information about this vulnerability, refer to the Microsoft Security Update Guide.

Detection Methods for CVE-2026-20963

Indicators of Compromise

• Unusual serialized data patterns in SharePoint HTTP requests, particularly containing known .NET gadget chain signatures
• Unexpected process spawning from SharePoint worker processes (w3wp.exe)
• Anomalous network connections originating from SharePoint servers to external hosts
• Suspicious PowerShell or command-line activity executed under the SharePoint application pool identity

Detection Strategies

• Monitor SharePoint web server logs for requests containing suspicious serialized object patterns or unusually large payloads
• Implement endpoint detection rules to identify child process creation from SharePoint application pool processes
• Deploy network monitoring to detect command-and-control traffic originating from SharePoint infrastructure
• Enable enhanced logging for SharePoint Unified Logging Service (ULS) and correlate with security events

Monitoring Recommendations

• Configure SentinelOne to monitor SharePoint server processes for behavioral anomalies and exploitation attempts
• Implement application-layer firewall rules to inspect and block malicious serialized payloads
• Enable Windows Event logging for process creation events (Event ID 4688) on SharePoint servers
• Monitor for suspicious file system activity in SharePoint installation directories

How to Mitigate CVE-2026-20963

Immediate Actions Required

• Apply the latest Microsoft security updates for SharePoint as soon as they become available
• Review and restrict SharePoint user permissions to minimize the attack surface from authenticated users
• Implement network segmentation to isolate SharePoint servers from critical infrastructure
• Enable enhanced monitoring and alerting for SharePoint server activity

Patch Information

Microsoft has released security updates addressing this vulnerability. Organizations should review the Microsoft Security Update Guide for specific patch information and deployment guidance. Prioritize patching production SharePoint environments given the network-accessible attack vector and low authentication requirements.

Workarounds

• Restrict network access to SharePoint servers to trusted IP ranges only where possible
• Implement Web Application Firewall (WAF) rules to inspect and block suspicious serialized data patterns
• Review and remove unnecessary user accounts with SharePoint access to reduce potential attack vectors
• Consider temporarily disabling non-essential SharePoint features or APIs that may process serialized data

# Review SharePoint application pool identity permissions
Get-SPServiceApplicationPool | Select-Object Name, ProcessAccountName

# Enable enhanced ULS logging for security monitoring
Set-SPDiagnosticConfig -LogLocation "C:\SharePointLogs" -LogMaxDiskSpaceUsageEnabled $true

# Check for latest SharePoint updates
Get-SPFarm | Select-Object BuildVersion