A critical deserialization vulnerability in Microsoft 365 Copilot allows an authorized attacker to achieve remote code execution, posing a severe risk to data integrity and system security.

This flaw involves the deserialization of untrusted data, which allows an authorized user to execute code remotely. The attack vector is network-based and does not require user interaction, meaning the impact is immediate upon successful exploitation.

The vulnerability carries a CVSS score of 9.9, indicating a critical severity level. Successful exploitation grants an attacker the ability to execute arbitrary code, which could lead to complete system compromise, unauthorized access to sensitive corporate data, and significant operational disruption.

Immediate Action: No manual action is required on the client side, as this is an official patch managed server-side by Microsoft as of July 23, 2026.

Proactive Monitoring: Security teams should review access logs for anomalous activity or unexpected execution patterns associated with Copilot service accounts.

Compensating Controls: Ensure robust network segmentation and adhere to the principle of least privilege for all service-level accounts to limit the potential blast radius of a successful compromise.

Public Exploit Available: No.

Given the critical CVSS score of 9.9, this vulnerability represents a significant risk to organizational infrastructure. Administrators should confirm that their environments are operating on the latest service versions and continue to monitor for any unusual administrative access patterns.