11. A system, comprising: a computer readable storage medium storing program instructions for protecting a computing device from potentially harmful code in a document; a processor in communication with the computer readable storage medium to acquire and execute the program instructions, wherein the processor executes the program instructions to: receive a data structure representation of the document; add dynamically one or more definitions of potentially harmful active content to an editable configuration file, each definition identifying potentially harmful active content and specifying an action to be performed on that potentially harmful active content if that potentially harmful active content is found in the document; parse the editable configuration file to generate a data structure representation of the one or more definitions in the editable configuration file; compare the data structure representation of the document 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; and modify the document to render harmless any identified potentially harmful active content before presenting the document to the computing device.
15. The system of claim 11 , wherein the editable configuration file is an XML (extensible markup language) file.