A system protects a computing device from potentially harmful code in a document by receiving a data structure representation of the document and adding dynamically one or more definitions of potentially harmful active content to an editable configuration file. Each definition identifies potentially harmful active content and specifies an action to be performed on that potentially harmful active content if that potentially harmful active content is found in the document. The editable configuration file is parsed to generate a data structure representation of the one or more definitions in the editable configuration file. The data structure representation of the document is compared with the data structure representation of the one or more definitions of potentially harmful active content to identify potentially harmful active content within the document. The document is modified to render harmless any identified potentially harmful active content before presenting the document to the computing device.