When web applications become rich client applications, that provide advanced and even critical service, security becomes a key aspect. Active page content, the engine of new web applications, can be also a source of attack from malicious parties. Today there are few developers well trained on this aspect, because web applications never before used such mechanics to delivery their services.

This topic is really big, so, in this chapter, we will cover the key aspects linked to the Javascript security and so we will talk on the client-side security and on some client-server interaction issues.