Case Study How Predictive Shielding in Defender Stopped GPO-Based Ransomware Before It Started

Case Study: How Predictive Shielding in Defender Stopped GPO-Based Ransomware Before It Started 🚀

Microsoft Defender successfully disrupted a human-operated ransomware incident targeting a large educational institution with thousands of devices. The attacker attempted to weaponize Group Policy Objects (GPOs) to tamper with security controls and distribute ransomware via scheduled tasks. Defender’s predictive shielding detected the attack before ransomware was deployed and proactively hardened against malicious GPO propagation across 700 devices. Defender blocked approximately 97% of the attacker’s attempted encryption activity, ensuring that zero machines were encrypted via the GPO path.

Incident Overview

The incident targeted a large educational institution with over a couple of thousand devices onboarded to Microsoft Defender. The infrastructure included 33 servers, 11 domain controllers, and 2 Entra Connect servers. The attacker began operating from an unmanaged device, with one Domain Admin account already compromised.

• Day 1: Reconnaissance activity using AD Explorer for Active Directory enumeration.
• Day 2: The attacker obtained credentials for multiple high privilege accounts through Kerberoasting and NTDS dump activity, then moved laterally. Defender initiated attack disruption against five compromised accounts, blocking thousands of authentication and access attempts, significantly slowing the attack.
• Day 5: The attacker transitioned to the impact phase, leveraging Group Policy as the primary distribution mechanism. Just before ransomware deployment, the attacker used GPO to propagate a tampering policy that disabled Defender protections.

Attack Details

The attacker’s first move was to create a malicious GPO designed to tamper with endpoint security controls, disabling key Defender protections including behavioral monitoring and real-time protection. This tampering attempt triggered a Defender tampering alert. In response, predictive shielding activated GPO hardening, temporarily pausing the propagation of new GPO policies, achieving protection for approximately 85% of devices against the tampering policy.

Approximately ten minutes later, the attacker attempted to proceed with ransomware payload distribution, placing three malicious files (run.bat, run.exe, and run.dll) under the SYSVOL share. A second malicious GPO was created to configure a scheduled task on targeted devices, executing the ransomware payload.

Thanks to the proactive measures taken by Microsoft Defender, zero machines were encrypted via the GPO path, and roughly 97% of devices the attacker attempted to encrypt were fully protected. The predictive shielding GPO hardening policy was applied to 700 devices, blocking the propagation of the malicious policy within approximately 3 hours.

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