In the first problem, an attack applet launches network security attacks on other machines. This is something that an attacker could already do before Java came along. The twist is that by embedding the attack into an applet, the bad guy makes the attack come from the machine of an innocent bystander. This kind of confusion is reason enough to use Java as the penetration vehicle, but the culprit has an even better reason for using Java.
Many corporate networks protect themselves from Internet intrusion through the use of a firewall.
If your firewall is well configured, it prevents the mischievous cracker from making direct use of the network to probe the defenses of your machines. The firewall does this by blocking certain types of network traffic from entering the corporate network. A firewall stands between your internal network and the rest of the Internet. It screens every packet of data coming across the network, allowing only certain kinds of packets through. A firewall is only as effective as the policy it implements.