Specify the 'Define file shares for downloading definition updates' (File Shares Sources) This policy setting allows you to configure UNC file share sources for downloading definition updates. Sources will be contacted in the order specified. The value of this setting should be entered as a pipe-separated string enumerating the definition update sources. For example: "{\\unc1 | \\unc2 }". The list is empty by default. If you enable this setting, the specified sources will be contacted for definition updates. Once definition updates have been successfully downloaded from one specified source, the remaining sources in the list will not be contacted. If you disable or do not configure this setting, the list will remain empty by default and no sources will be contacted. Counter Measure: Configure this setting depending on your organization's requirements. Potential Impact: Standard definition update sources (such as Microsoft Updates) will not be contacted.

(1) GPO: Computer Configuration\Administrative Templates\Windows Components\Windows Defender\Signature Updates\Define file shares for downloading definition updates (2) REG: HKEY_LOCAL_MACHINE\Software\Policies\Microsoft\Windows Defender\Signature Updates\DefinitionUpdateFileSharesSources