CVE-2026-26114: SharePoint RCE Vulnerability

CVE-2026-26114 Overview

CVE-2026-26114 is an insecure deserialization vulnerability in Microsoft Office SharePoint that enables an authorized attacker to execute arbitrary code over a network. The vulnerability stems from improper handling of untrusted data during the deserialization process, which can be exploited by attackers with valid credentials to gain remote code execution capabilities on vulnerable SharePoint servers.

Critical Impact

Successful exploitation allows authenticated attackers to execute arbitrary code on SharePoint servers, potentially compromising sensitive corporate data, enabling lateral movement within enterprise networks, and facilitating persistent access to organizational resources.

Affected Products

• Microsoft Office SharePoint (specific version information pending vendor disclosure)

Discovery Timeline

• 2026-03-10 - CVE-2026-26114 published to NVD
• 2026-03-11 - Last updated in NVD database

Technical Details for CVE-2026-26114

Vulnerability Analysis

This vulnerability is classified under CWE-502 (Deserialization of Untrusted Data), a well-known vulnerability class that has historically affected enterprise applications like SharePoint. The flaw occurs when SharePoint processes serialized objects from untrusted sources without proper validation, allowing attackers to inject malicious payloads that execute during the deserialization process.

The network-based attack vector means exploitation can occur remotely, though it requires the attacker to have low-level privileges (authenticated access) to the SharePoint environment. No user interaction is required for successful exploitation, making this vulnerability particularly concerning in enterprise environments where many users have legitimate SharePoint access.

Root Cause

The root cause lies in SharePoint's failure to properly validate and sanitize serialized data before processing. When the application deserializes objects from untrusted sources, it instantiates them without sufficient type checking or content validation. This allows an attacker to craft malicious serialized payloads that, when deserialized, trigger code execution through gadget chains available in the .NET framework or SharePoint's dependencies.

Attack Vector

The attack leverages network access to SharePoint services. An attacker with valid credentials (low privilege required) can submit specially crafted serialized data to vulnerable SharePoint endpoints. The deserialization process reconstructs the malicious object, triggering the embedded payload and executing arbitrary code in the context of the SharePoint application pool identity.

Typical exploitation involves:

1. Identifying vulnerable deserialization endpoints in SharePoint
2. Crafting a malicious serialized payload using known .NET deserialization gadgets
3. Submitting the payload to the target endpoint while authenticated
4. Achieving code execution when SharePoint deserializes the malicious object

Due to the nature of this vulnerability, detailed exploitation code is not provided. For technical details, refer to the Microsoft Security Update CVE-2026-26114.

Detection Methods for CVE-2026-26114

Indicators of Compromise

• Unexpected process spawning from SharePoint application pools (e.g., w3wp.exe spawning cmd.exe or powershell.exe)
• Anomalous network connections originating from SharePoint servers to external or unusual internal destinations
• Large or malformed HTTP requests to SharePoint API endpoints containing Base64-encoded or binary serialized data
• Unusual SharePoint service account activity or privilege escalation attempts

Detection Strategies

• Monitor SharePoint IIS logs for suspicious POST requests to known deserialization endpoints
• Implement application-level logging to capture deserialization events and flag unexpected object types
• Deploy endpoint detection solutions to identify exploitation attempts through behavioral analysis
• Configure SIEM rules to correlate authentication events with subsequent suspicious SharePoint activity

Monitoring Recommendations

• Enable verbose logging on SharePoint servers and centralize logs for analysis
• Monitor for .NET deserialization-related exceptions in Windows Event Logs
• Implement network traffic inspection for serialized object patterns in HTTP traffic to SharePoint
• Establish baseline SharePoint behavior and alert on deviations in process creation or network activity

How to Mitigate CVE-2026-26114

Immediate Actions Required

• Apply the security patch from Microsoft immediately when available
• Review and restrict SharePoint access to only necessary users pending patch deployment
• Implement network segmentation to limit exposure of SharePoint servers
• Enable enhanced logging and monitoring on SharePoint infrastructure

Patch Information

Microsoft has released a security update addressing this vulnerability. Administrators should obtain the patch from the Microsoft Security Update CVE-2026-26114 advisory page.

Organizations should prioritize patching based on SharePoint's criticality to business operations and the authenticated nature of the attack vector. Testing patches in non-production environments before deployment is recommended to ensure compatibility.

Workarounds

• Implement Web Application Firewall (WAF) rules to inspect and filter serialized data in SharePoint requests
• Restrict network access to SharePoint servers using firewall rules to limit the attack surface
• Review and minimize user accounts with SharePoint access, following the principle of least privilege
• Consider temporarily disabling non-essential SharePoint features that may expose deserialization endpoints

# Example: Restrict SharePoint access at the network level (Windows Firewall)
# Adjust port numbers based on your SharePoint configuration
netsh advfirewall firewall add rule name="Restrict SharePoint Access" dir=in action=allow protocol=TCP localport=443 remoteip=10.0.0.0/8
netsh advfirewall firewall add rule name="Block External SharePoint" dir=in action=block protocol=TCP localport=443 remoteip=any