I would like to know how to unzip a file in a folder on vRO server.
I'm able to zip files (with ZipWriter), but I don't know how to unzip, except by running a command "var command = new Command("unzip ...")" but I would like to avoid to do that for security reasons.
vRO does not provide a scripting API to work with zip files (apart from the ZipWriter class which you already found out). And I'm not aware of a vRO plug-in that provides such functionality.
There was indeed a zipReader scripting object as part of the LivecycleManager plug-in part of the LifecycleManager product we got years ago.
I remember requesting this functionality to be merged in the platform since we already got the zip writer but it never made it. This could be a low hanging fruit that would avoid having to use commands on the appliance or on external hosts.