1. A method comprising: receiving, via a processor of a computing device, a document for use in a sandbox environment; passing the document to a parser that finds one or more references in the document to other resources to yield a list of one or more references; passing the list of the references to a verifier that verifies that a resource associated with each reference in the list of the one or more references exists to yield a list of verified references; and modifying the sandbox environment based on the list of verified references to yield a modified sandbox environment to include the verified resources.