Search notes:

Registry: HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Google\Chrome\ExtensionInstallBlacklist

This registry can be used by an adminstrator to prevent a user from installing a specific extension into Chrome. If unallowed, the user trying to install an extension gets the message Extesnion (extension ID “xxxxxxx”) is blocked by the administrator.
In order to unallow all extensions, the value * can be used. This setting can then be overridden with a list of allowed extensions in ExtensioninstallWhitelist

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