Microsoft Defender AV must use advanced protection against ransomware.

This policy setting provides an extra layer of protection against ransomware. It uses both client and cloud heuristics to determine whether a file resembles ransomware. This rule does not block files that have one or more of the following characteristics: - The file is found to be unharmful in the Microsoft cloud. - The file is a valid signed file. - The file is prevalent enough to not be considered as ransomware. - The rule tends to err on the side of caution to prevent ransomware.

Verify the policy value for Computer Configuration >> Administrative Templates >> Windows Components >> Microsoft Defender Antivirus >> Microsoft Defender Exploit Guard >> Attack Surface Reduction >> Configure Attack Surface Reduction rules is set to "Enabled". Procedure: Use the Windows Registry Editor to navigate to the following key: HKLM\Software\Policies\Microsoft\Windows Defender\Windows Defender Exploit Guard\ASR\Rules Criteria: If the value "c1db55ab-c21a-4637-bb3f-a12568109d35" is REG_SZ = 1, this is not a finding. If the value is other than 1, this is a finding.

Set the policy value for Computer Configuration >> Administrative Templates >> Windows Components >> Microsoft Defender Antivirus >> Microsoft Defender Exploit Guard >> Attack Surface Reduction >> Configure Attack Surface Reduction rules to "Enabled". Under the policy option "Set the state for each ASR rule:", then click "Show". Enter GUID "c1db55ab-c21a-4637-bb3f-a12568109d35" in the "Value Name" column. Enter "1" in the "Value" column.