Embodiments are directed to managing sandboxed application extensions. A first request that include includes information that identifies an extension may be provided to an extension server. Information included in a first response may be employed to instantiate an extension shell that corresponds to the extension and embed it in the hosting application such that the extension shell is associated with a first security policy included in the first response. A second request may be provided to the extension server based on information included in the extension shell such that the second request includes an identifier associated with an extension body that corresponds to the extension shell. The first security policy or the second security policy may be employed to enable one or more of the extension shell or the extension body to just access the hosting application or just access the extension server.