Case study: How predictive shielding in Defender stopped GPO-based ransomware before it started

Summary

Microsoft Defender disrupted a human-operated ransomware campaign targeting a large educational institution where attackers abused Group Policy Objects (GPOs) to disable security controls and distribute encryption payloads at scale. Defender's predictive shielding detected the attack before ransomware deployment and proactively hardened 700 devices, blocking ~97% of encryption attempts with zero successful GPO-based encryptions. The case study details the attacker's multi-day progression from initial access through lateral movement, credential theft, and attempted impact via GPO weaponization.

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Share Link copied to clipboard! Content types Research Products and services Microsoft Defender Topics Actionable threat insightsDetection and protection success stories Summary Microsoft Defender disrupted a human operated ransomware incident targeting a large educational institution with more than a couple of thousand 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 ~97% of the attacker’s attempted encryption activity in total, and zero machines were encrypted via the GPO path. The growing threat: GPO abuse in ransomware operations Modern ransomware operators have evolved well beyond simple payload delivery. Today’s attackers understand enterprise infrastructure intimately. They actively exploit the administrative mechanisms that organizations depend on to both neutralize security products and distribute ransomware at scale. Group Policy Objects (GPOs) have become a favored tool for exactly this purpose. GPOs are a built-in, trusted mechanism for pushing configuration changes across domain-joined devices. Attackers have learned to abuse them: pushing tampering configurations to disable security tools, deploying scheduled tasks that distribute and execute ransomware, and achieving wide organizational impact without needing to touch each machine individually. In this blog, we examine a real incident where an attacker weaponized GPOs in exactly this way, and how Defender’s predictive shielding responded by catching the attack before the ransomware was even deployed. The incident The target was a large educational institution with approximately more than a couple of thousand devices onboarded to Microsoft Defender and the full Defender suite deployed. The infrastructure included 33 servers, 11 domain controllers, and 2 Entra Connect servers. Attack chain overview The attacker’s progression through the environment was methodical: Initial Access and Privilege Escalation: The attacker began operating from an unmanaged device. At this stage, one Domain Admin account had already been compromised. Due to limited visibility, the initial access vector and the method used to obtain Domain Admin privileges remain unknown. Day 1: Reconnaissance: The attacker began reconnaissance activity using AD Explorer for Active Directory enumeration and brute force techniques to map the environment. Defender generated alerts in response to these activities. Day 2: Credential Access and Lateral Movement: The attacker obtained credentials for multiple high privilege accounts, with Kerberoasting and NTDS dump activity observed leading up to this point. During this phase, the attacker also created multiple local accounts on compromised systems to establish additional persistent access. Using some of the acquired credentials, the attacker then began moving laterally within the network. During these activities, Defender initiated attack disruption against five compromised accounts. This action caused the attacker’s lateral movement attempts to be blocked at scale, resulting in thousands of blocked authentication and access attempts and a significant slowdown of the attack. With attack disruption in place, the attacker’s progress was significantly constrained at this stage, limiting lateral movement and preventing rapid escalation. Without this intervention, the customer would have faced a far more severe outcome. Day 5: Defense Evasion and Impact: While some accounts were disrupted and blocked, the attacker was still able to leverage additional privileged accounts still in their hands. Using these accounts, the attacker transitioned to the impact phase and leveraged Group Policy as the primary distribution mechanism.Just prior to the ransomware deployment, the attacker used GPO to propagate a tampering policy that disabled Defender protections. Ransomware payload was then distributed via GPO, while in parallel, the attacker executed additional remote ransomware operations, delivering the payload over SMB using multiple compromised accounts. A second round of attack disruption was initiated by Defender as a reaction to this new stage, this time alongside predictive shielding. More than a dozen compromised entities were disrupted, together with GPO hardening, ultimately neutralizing the attack and preventing the attacker from making any further progress. Deep dive: How the attacker weaponized group policy and how predictive shielding stopped the attack. Step 1: Tampering with security controls The attacker’s first move was to create a malicious GPO designed to tamper with endpoint security controls. The policy disabled key Defender protections, including behavioral monitoring and real-time protection, with the goal of weakening defenses ahead of ransomware deployment. This tampering attempt triggered a Defender tampering alert. In response, predictive shielding activated GPO hardening, temporarily pausing the propagation of new GPO policies, across all MDE onboarded devices reachable from the attacker’s standpoint – achieved protection of ~85% of devices against the tampering policy. Step 2: Ransomware distribution via scheduled tasks Approximately ten minutes after creating the tampering GPO, without being aware of Defender’s GPO Hardening policy being deployed and activated, the attacker attempted to proceed with the next stage of the attack: ransomware payload distribution[EF2] . The attacker placed three malicious files: run.bat, run.exe and run.dll under the SYSVOL share. These files were responsible for deploying the ransomware payload. A second malicious GPO was created to configure a scheduled task on targeted devices. The scheduled task copied the payload files locally and executed them using the following chain: cmd /c start run.bat → cmd /c c:\users\…\run.exe → rundll32 c:\users\…\run.dll Encryptor This approach is effective because each device pulls the payload to itself through the scheduled task. The attacker sets the GPO once, and the devices do the rest. It’s a self-service distribution model that leverages the infrastructure the organization depends on. Because GPO hardening had already been applied during the tampering stage, by the time the attacker created the ransomware GPO ten minutes later, the environment was already hardened. The system recognized that GPO tampering is a precursor to ransomware distribution and acted preemptively. The system didn’t wait for ransomware to appear. It acted on what the attacker was about to do. The results The numbers speak for themselves: Zero machines were encrypted via the GPO path. Roughly 97% of devices the attacker attempted to encrypt were fully protected by Defender. A limited number of devices experienced encryption during concurrent ransomware activity over SMB; however, attack disruption successfully contained the incident and stopped further impact. 700 devices applied the predictive shielding GPO hardening policy, reflecting the attacker’s broad targeting scope, and blocking the propagation of the malicious policy set by the attacker within approximately 3 hours. The hardening dilemma: Why threat actors love operational mechanisms Enterprise environments rely on administrative mechanisms such as Group Policy, scheduled tasks, and remote management tools to manage and automate operations at scale. These capabilities are highly privileged and widely trusted, making them a natural part of everyday IT workflows. Because they are designed for legitimate administration and automation, attackers increasingly target them as a low-friction way to disable defenses and distribute malware using the same tools administrators use every day. This creates a fundamental asymmetry. Defenders must keep these mechanisms open for legitimate use, while atta

Indicators of Compromise

• malware — run.bat
• malware — run.exe
• malware — run.dll
• mitre_attack — T1484.001
• mitre_attack — T1562.001
• mitre_attack — T1021.002