CVE-2026-20963: Microsoft SharePoint Critical Vulnerability

What is CVE-2026-20963?

Deserialization of untrusted data in Microsoft Office SharePoint allows an unauthorized attacker to execute code over a network.

Technical Analysis

Weakness Classification

This vulnerability is classified as CWE-502 — Deserialization of Untrusted Data. The application deserializes untrusted data without sufficient validation, which can lead to remote code execution, denial of service, or other attacks.

Attack Vector Breakdown

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

Impact Analysis

This vulnerability is exploitable over the network, meaning any system reachable via the internet or internal network is potentially at risk without physical access. The attack complexity is low, requires no special privileges, and needs no user interaction — making it straightforward to exploit at scale. Successful exploitation can result in full confidentiality breach (complete data disclosure), complete integrity compromise (data modification or code execution), total availability loss (denial of service). The application deserializes untrusted data without sufficient validation, which can lead to remote code execution, denial of service, or other attacks.

Exploitation Probability (EPSS): 5.3% chance of exploitation in the next 30 days (90th percentile). While the base probability may appear low, inclusion in CISA KEV confirms real-world exploitation is already occurring.

Exploit Availability

Public exploit code or proof-of-concept is available for CVE-2026-20963. This significantly increases the risk of exploitation, as attackers can use or adapt existing exploit code rather than developing their own. A public exploit reference has been identified. Organizations should treat this as an urgent patching priority.

Is CVE-2026-20963 being exploited?

Yes. CVE-2026-20963 is listed in CISA's Known Exploited Vulnerabilities (KEV) catalog, restricted to CVEs with confirmed active exploitation in the wild. CISA added this vulnerability on 2026-03-18. Inclusion in KEV triggers the patching mandate under BOD 22-01 for U.S. federal civilian agencies, with a remediation deadline of 2026-03-21. Additionally, public exploit code is available, which lowers the barrier for attackers.

How to fix CVE-2026-20963

Apply the latest security update via Windows Update, WSUS, or Microsoft Update Catalog. For enterprise environments, use SCCM/Intune for managed deployment. If the patch cannot be applied immediately, check the MSRC advisory for workarounds such as registry-based mitigations or disabling affected features. Check the Microsoft Security Response Center page for the specific patch or update. CISA's Known Exploited Vulnerabilities catalog requires U.S. federal agencies to remediate this vulnerability by the due date shown below under Binding Operational Directive 22-01. Private organizations should treat KEV entries as priority-one patches because active exploitation has been confirmed in the wild.

Related Vulnerabilities

Other actively exploited vulnerabilities affecting SharePoint or sharing the same weakness (CWE-502):

• CVE-2025-49704 (High 8.8) [same product] — Improper control of generation of code ('code injection') in Microsoft Office…
• CVE-2025-49706 (Unknown N/A) [same product] — Microsoft SharePoint contains an improper authentication vulnerability that…
• CVE-2025-53770 (Unknown N/A) [same product] — Microsoft SharePoint Server on-premises contains a deserialization of…
• CVE-2023-21529 (High 8.8) [same vendor + same CWE] — Microsoft Exchange Server Remote Code Execution Vulnerability
• CVE-2026-45247 (Critical 9.8) [same CWE] — Mirasvit Full Page Cache Warmer for Magento 2 before version 1.11.12 contains…

Frequently asked questions

Is CVE-2026-20963 critical?

CVE-2026-20963 has a CVSS 3.1 base score of 9.8 — Critical severity. It should be treated as a priority patch, especially given the active exploitation confirmed by CISA. The availability of public exploit code further increases the urgency.

Is CVE-2026-20963 being actively exploited?

Yes — CISA added CVE-2026-20963 to the Known Exploited Vulnerabilities catalog on 2026-03-18. Inclusion in KEV means CISA has confirmed in-the-wild exploitation. Public exploit code has also been identified, making exploitation accessible to a broader range of threat actors.

How do I patch CVE-2026-20963?

Apply the security update from Microsoft for SharePoint. Federal agencies must remediate by 2026-03-21 under BOD 22-01. Refer to the Microsoft Security Response Center for the official patch.

What type of vulnerability is CVE-2026-20963?

CVE-2026-20963 is classified under CWE-502 (Deserialization of Untrusted Data). The application deserializes untrusted data without sufficient validation, which can lead to remote code execution, denial of service, or other attacks.

What is the CVSS score for CVE-2026-20963?

CVE-2026-20963 has a CVSS 3.1 base score of 9.8, classifying it as Critical severity. The attack vector is Network, attack complexity is Low, and no privileges are required.