Network Security Issues

Applets don't need to show any visible area. You've already seen applets that only played sounds, but didn't have any pictures. Now imagine an applet that was completely hidden, with no visible appearance at all, but that when loaded, checked the client's IP address, scanned for other machines in the same subnet, possibly behind a firewall, and then sent that information out to a third host on the Internet. Or imagine an applet that scanned for open ports on the client machine. See the problem? These aren't the only problems either, but you get the idea.

To prevent these sorts of attacks Java defines a number of different security levels for applets loaded from the Internet. As a general rule applets are only allowed to communicate with the host from which they were downloaded (the code base). They cannot make connections to arbitrary hosts on the Internet. Applications, however, are allowed to connect to arbitrary hosts.

There is some level of user control--some browsers allow the user to prevent an applet from making any network connections or to allow it unrestricted access--but most of the itme this is the case. This is always the case with Netscape.

If you're uncertain of how much network access you'll have, you can use these methods from java.lang.SecurityManager to check:

 public void checkConnect(String host, int port)
 public void checkConnect(String host, int port, Object context)
 public void checkListen(int port)
 public void checkAccept(String hostname, int port)
 public void checkMulticast(InetAddress maddr)
 public void checkMulticast(InetAddress maddr, byte ttl)

Each of these methods throws a SecurityException (which is a runtime exception so it doesn't need to be declared) if the requested operation is not permitted. For example, to check whether you're allow to open a socket to port 80 of you would write:

try {
  SecurityManager sm = SecurityManager.getSecurityManager();
  if (sm != null) sm.checkConnect("", 80);
  // open the socket...
catch (SecurityException ex) {
  System.err.println("Sorry. I'm not allowed to connect to that host.");

checkConnect() tests whether a socket connection is allowed. checkListen() tests whether binding to a particular port is allowed. checkAccept() tests whether you can accept a connection from a particular remote host and port. checkMulticast() tests whether multicasting is allowed.

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Copyright 1997, 1999 Elliotte Rusty Harold
Last Modified December 2, 1999