Systems and methods for controlling the ability of scripts to access objects defined at the client. The client stores an access control data structure having multiple entries. Each entry is associated with one or more objects for which access is to be controlled. The entries include a source identifier field containing one or more URLs and a permission identifier field containing data defining a permission. When the client receives a script from a script source, such as a web site, the client begins execution of the script until the script requests access to an object. The client identifies a relevant entry of the access control data structure, which is an entry that is associated with the requested object and lists the URL of the script source in the source identifier field. The client applies the permission defined by the permission identifier of the relevant entry to the request, thereby granting or denying access to the script. Thus, the client permits selected remote entities to exercise control over objects and, in turn, over components of the client, while preventing unauthorized scripts from accessing the objects.