Learning Java, 4th Edition

Application and User-Level Security

There’s a fine line between having enough power to do something useful and having all the power to do anything you want. Java provides the foundation for a secure environment in which untrusted code can be quarantined, managed, and safely executed. However, unless you are content with keeping that code in a little black box and running it just for its own benefit, you will have to grant it access to at least some system resources so that it can be useful. Every kind of access carries with it certain risks and benefits. For example, in the web browser environment, the advantages of granting an untrusted (unknown) applet access to your windowing system are that it can display information and let you interact in a useful way. The associated risks are that the applet may instead display something worthless, annoying, or offensive.

At one extreme, the simple act of running an application gives it a resource—computation time—that it may put to good use or burn frivolously. It’s difficult to prevent an untrusted application from wasting your time or even attempting a “denial of service” attack. At the other extreme, a powerful, trusted application may justifiably deserve access to all sorts of system resources (e.g., the filesystem, process creation, network interfaces); a malicious application could wreak havoc with these resources. The message here is that important and sometimes complex security issues have to be addressed.

In some situations, it may ...