JavaScript: The Definitive Guide, Fourth Edition

JavaScript Security

Any time that programs (such as JavaScript scripts, Visual Basic programs, or Microsoft Word macros) are included within shared documents, particularly documents that are transmitted over the Internet or by email, there is a potential for viruses or other malicious programs. The designers of JavaScript were aware of these security issues and took care not to give JavaScript programs the power to perform damaging acts. As described previously, for example, client-side JavaScript programs cannot read local files or perform networking operations.

Because of the complexity of the web-browser environment, however, a number of security problems did arise in early browser versions. In Netscape 2, for example, it was possible to write JavaScript code that could automatically steal the email address of any visitor to a page containing the code and then automatically send email in the visitor’s name, without the visitor’s knowledge or approval. This, and a number of other security holes, have been fixed. Although there is no guarantee that other security holes will not be found, most knowledgeable users are comfortable letting modern browsers run the JavaScript code found in web pages. Chapter 21 contains a complete discussion of security in client-side JavaScript.