Defending Against Active Exploits: Urgent Guidance for Microsoft SharePoint CVE-2026-20963

Introduction

The Cybersecurity and Infrastructure Security Agency (CISA) has added a critical security flaw, CVE-2026-20963, to its Known Exploited Vulnerabilities (KEV) Catalog. This vulnerability affects Microsoft SharePoint and involves the deserialization of untrusted data.

Why does this matter to your team? CISA only adds vulnerabilities to this list when there is concrete evidence of active exploitation in the wild. This is not a theoretical risk; attackers are likely already scanning for unpatched systems or using this flaw to establish a foothold in networks. For defenders, this moves CVE-2026-20963 to the top of the priority list, requiring immediate action to secure collaboration environments.

Technical Analysis

Vulnerability: CVE-2026-20963 Affected Product: Microsoft SharePoint Server Vulnerability Type: Deserialization of Untrusted Data

Deserialization vulnerabilities are particularly dangerous because they allow an attacker to manipulate serialized data (objects converted into a format for storage or transfer) that the application then trusts. By crafting a malicious serialized object and sending it to the SharePoint server, an attacker can trigger arbitrary code execution under the context of the SharePoint application pool.

This type of flaw is a frequent pathway for ransomware gangs and advanced persistent threats (APTs) to move laterally through a network after gaining initial access, or to exfiltrate sensitive data stored in SharePoint document libraries.

Severity: Critical CISA Requirement: Federal Civilian Executive Branch (FCEB) agencies must remediate this vulnerability by the due date mandated in Binding Operational Directive (BOD) 22-01. Private sector organizations are strongly urged to follow this timeline as well.

Patch Details: Microsoft has released security updates to address this flaw. Administrators should review the latest Security Update Guide for the specific Cumulative Update (CU) or Security Update applicable to their SharePoint version.

Defensive Monitoring

To determine if your environment is currently vulnerable or if exploitation attempts are occurring, use the following scripts and queries.

1. Verify SharePoint Version (PowerShell)

Use this PowerShell script to check the build version of your SharePoint farm. You will need to compare the resulting build number against the security bulletin released by Microsoft to confirm if the patch for CVE-2026-20963 has been applied.

# Check SharePoint Farm Build Version
Add-PSSnapin Microsoft.SharePoint.PowerShell -ErrorAction SilentlyContinue

$farm = Get-SPFarm
$buildVersion = $farm.BuildVersion

Write-Host "Current SharePoint Farm Build Version: $buildVersion" -ForegroundColor Cyan

# Note: Compare this version against the fixed version in the Microsoft Security Update Bulletin.
# If your build is lower, the system is vulnerable.

2. Hunt for Potential Exploitation Activity (KQL for Microsoft Sentinel)

Deserialization attacks often result in the w3wp.exe (IIS worker process) spawning unusual child processes, such as PowerShell or CMD. Use this KQL query to hunt for suspicious process execution patterns indicative of a web shell or remote code execution.

DeviceProcessEvents
| where Timestamp >= ago(7d)
// Focus on IIS worker processes used by SharePoint
| where InitiatingProcessFileName =~ "w3wp.exe"
// Look for suspicious command shells or scripts
| where ProcessFileName in~ ("powershell.exe", "cmd.exe", "cscript.exe", "wscript.exe", "bash.exe")
// Filter out potential false positives from trusted maintenance paths
| where not(InitiatingProcessCommandLine contains "Microsoft\\Exchange Server" or InitiatingProcessCommandLine contains "AppData\\Local\\Temp")
| project Timestamp, DeviceName, AccountName, InitiatingProcessCommandLine, ProcessCommandLine, FileName
| order by Timestamp desc

Remediation

1. Patch Immediately: Apply the latest security updates provided by Microsoft for SharePoint Server immediately. Ensure you apply the update to all SharePoint servers in your farm, including Front-end and Application servers.

2. Validate the Patch: After installation, run the PowerShell script provided above to verify the build version matches the fixed version specified in the security advisory.

3. Review IIS Logs: Conduct a retrospective analysis of your IIS logs for the past 30 days. Look for unusual POST requests to SharePoint pages, particularly those containing large payloads or encoded data that might indicate attempted deserialization attacks.

4. Restrict External Access: Ensure that your SharePoint servers are not directly exposed to the public internet unless absolutely necessary. Use a Web Application Firewall (WAF) to inspect incoming traffic and block known attack signatures associated with deserialization.

5. Backup and Recovery: Ensure that recent backups of your SharePoint content databases and configurations are available and tested. In the event of a successful compromise, having a clean restore point is critical.

Related Resources

Security Arsenal Penetration Testing Services AlertMonitor Platform Book a SOC Assessment vulnerability-management Intel Hub