WWW:Beyond the Basics

14 Java Security

14.3 Security Threats

14.3.4 DNS Weaknesses

A significant problem arises in JDK and browser implementation of the policy that an applet can open a TCP/IP connection back to the server it was loaded from. While this policy is sound, it is not uniformly enforced. The policy is enforced as follows:

The problem is in the second step: the applet can ask to connect to any hostname on the Internet, so it can control which DNS supplies the second list of IP addresses; information from this untrusted DNS server is used to make an access control decision. There is nothing that prevents an attacker from creating a DNS server that lies. Using the attacker's DNS server to provide a pair of addresses, the applet can connect to any host on the Internet. This attack is very dangerous if the browser is running behind a firewall, because the malicious applet can attack any machine behind the firewall. It can use a lot of known network security problems to break into other nearby machines.


FIGURE 14.3: A Three Party Attack

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Figure 14.3 presents an applet that exploits the DNS weakness to connect to arbitrary hosts on the Internet.

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Copyright © 1996 Virginia Polytechnic Institute & State University
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Vijay Sureshkumar <vijay@csgrad.cs.vt.edu>
Last modified: Sun Oct 20 21:52:09 1996