Microsoft SharePoint Remote Code Execution (RCE) Vulnerability Advisory (CVE-2026-20963, CWE-502)

1. Apply Official Security Patches

Immediately apply the latest Cumulative Updates (CU) and security updates provided by Microsoft.

• Visit the Microsoft MSRC Security Update Guide to download and install the appropriate patches for your environment.

2. Temporary Mitigations (If patching cannot be applied immediately)

• Network Access Control: Restrict direct internet access to SharePoint Central Administration and web application ports. Implement firewall rules to allow access only from trusted internal IP ranges or via a secure VPN.
• WAF (Web Application Firewall) Rules: Deploy and enable WAF signatures designed to detect and block common .NET deserialization attack patterns (e.g., BinaryFormatter or ObjectStateFormatter payloads).
• Principle of Least Privilege: Ensure that SharePoint service accounts run with the minimum necessary privileges to limit the potential blast radius of a successful compromise.

This vulnerability (CVE-2026-20963) is a critical security flaw arising from the deserialization of untrusted data (CWE-502) within Microsoft SharePoint.

An attacker can exploit this vulnerability by sending a specially crafted serialized object payload to a vulnerable endpoint on the SharePoint server. When the SharePoint application deserializes this untrusted input without proper validation, it triggers the execution of arbitrary code within the context of the application service account.

With a CVSS 3.1 score of 9.8 (Critical), this vulnerability requires no privileges (PR:N) and no user interaction (UI:N) to be exploited remotely over the network. Furthermore, it is listed in CISA's Known Exploited Vulnerabilities (KEV) catalog, indicating active exploitation in the wild.