CVE-2026-20963 is a deserialization vulnerability classified under CWE-502 affecting Microsoft SharePoint Enterprise Server 2016 version 16.0.0. The flaw arises from improper handling of untrusted serialized data, allowing an attacker with authorized access and network connectivity to remotely execute arbitrary code on the server. The vulnerability does not require user interaction and has a low attack complexity, but it does require the attacker to have some level of privileges (PR:L). The CVSS v3.1 base score is 8.8, indicating a high severity due to its impact on confidentiality, integrity, and availability (all rated high). The vulnerability could allow attackers to compromise SharePoint servers, potentially leading to data breaches, service disruption, or further lateral movement within an enterprise network. Although no known exploits are currently in the wild, the nature of deserialization vulnerabilities makes them attractive targets for attackers. SharePoint, being a widely used collaboration platform in enterprises, represents a critical attack surface. The vulnerability was reserved in December 2025 and published in January 2026, with no patch links currently available, suggesting that organizations must monitor for updates closely. The flaw emphasizes the risks associated with deserializing untrusted input without proper validation or sandboxing.

For European organizations, this vulnerability poses a significant risk to the confidentiality, integrity, and availability of sensitive data managed through SharePoint Enterprise Server 2016. Exploitation could lead to unauthorized code execution, enabling attackers to steal intellectual property, disrupt business operations, or establish persistent footholds for further attacks. Given SharePoint's role in document management and collaboration, a successful attack could compromise critical workflows and regulatory compliance, especially under GDPR. The network-based attack vector means that attackers could exploit this vulnerability remotely, increasing the risk of widespread impact across interconnected systems. Organizations in sectors such as finance, government, healthcare, and critical infrastructure are particularly vulnerable due to their reliance on SharePoint for secure document sharing and collaboration. The lack of public exploits currently provides a window for proactive defense, but the high severity score and ease of exploitation necessitate urgent attention.

1. Immediately restrict network access to SharePoint Enterprise Server 2016 instances to trusted internal networks and VPNs only, minimizing exposure to potential attackers. 2. Implement strict role-based access controls (RBAC) to limit the number of users with privileges sufficient to exploit this vulnerability. 3. Monitor SharePoint server logs and network traffic for unusual deserialization activity or anomalous behavior indicative of exploitation attempts. 4. Employ application-layer firewalls or web application firewalls (WAFs) with custom rules to detect and block suspicious serialized payloads targeting SharePoint. 5. Regularly review and update security policies to enforce the principle of least privilege and ensure timely application of patches once Microsoft releases an official fix. 6. Conduct internal penetration testing and vulnerability scanning focused on deserialization attack vectors to identify and remediate weaknesses. 7. Educate system administrators and security teams on the risks of deserialization vulnerabilities and the importance of secure coding and configuration practices. 8. Consider isolating SharePoint servers in segmented network zones to contain potential breaches and limit lateral movement.