Advanced Interface Casting in Java

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Advanced Interface Casting
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Here is the core of Cargill's second discovery:
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interface Inter { void f(); } class Secure implements Inter { private void f(); } class Dummy implements Inter { public void f(); static void attack() { Inter inter[2] = {new Dummy(), new Secure() }; for(int j=0; j<2; ++j) inter[j].f(); } }
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The first call, inter[0].f(), is legal since Dummy's f method is public. The next time around the loop, inter[1].f() is illegal since Secure's f method is private. In this case, Java was too smart for its own good. In order to improve performance, it only checked for legality the first time through the loop. Theoretically, what was legal the first time would be legal the next time. (See 2.) Though this is often a correct assumption, it broke down for the code just shown. This trick allows an attacker to call private methods in violation of Java's security rules. Had Netscape not improved the structure of their system after the previous bug was reported, this bug would have once again allowed the class-loader attack to work. However, because Netscape had protected their system against private method attacks, this flaw was not easy to exploit.
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... Preface -- 1 -- 2 -- 3 -- 4 -- 5 -- 6 -- 7 -- 8 -- 9 -- A -- B -- C -- Refs Front -- Contents -- Help
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Copyright 1999 Gary McGraw and Edward Felten. All rights reserved. Published by John Wiley & Sons, Inc.
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Attack Applets: Exploiting Holes in the Security Model
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CHAPTER SECTIONS: 1 / 2 / 3 / 4 / 5 / 6 / 7 / 8 / 9 / 10 / 11 / 12 / 13 / 14 / 15 / 16 / 17 / 18 / 19 / 20
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Section 8 -- Applets Running Wild
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The next security problem is the Princeton class-loader attack. This was the most widely publicized of all Java security breaches. The problem was caused by mistakes in the way the Java system integrated separate pieces of code. By corrupting this integration or linking process, an attacker could break through Java's security and do anything at all. To help better understand this issue, the following section looks more closely at how Java manages the dynamic-linking process.
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A Java program is composed of several separate pieces called classes. Each class is stored in a separate file, and the Java system uses a just-in-time strategy to load each class only when it is first needed. Just-intime loading allows Java applets to start running quickly, without waiting for the entire applet to be pulled across the Net. It does have one drawback, however: A running applet is usually incomplete. When an applet is built from several code pieces, the system has to be clever enough to make sure that the right pieces are attached in the right places. A Java class file contains a series of instructions telling the Java system how the class should behave. The instructions sometimes reference other classes by name. Since classes are stored separately, the Java system translates each name into the identity of another class. This may involve loading the mentioned class across the Net. The core Java system does not do this translation itself, but outsources it to Java objects called Class Loaders. Outsourcing in this way allows programmers to create their own class loaders, extending Java's linking mechanism.
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The interaction between a Class Loader and the core elements of Java is simple. When Java needs to determine which class corresponds to which name, the following steps are followed: 1. Java calls the Class Loader's loadClass method, passing it the name to look up. 2. The Class Loader consults its internal dictionary (which can include a list of built-in classes) to see whether a class with that name already exists. If one exists, that class is returned. 3. If the Class Loader does not have a class with the requested name, it tries to find one. Usually, it does this by fetching the byte code for the class across the Net. 4. After getting the byte code for the class, the Class Loader calls a special method called defineClass to turn the byte code into a usable class. 5. When defineClass is finished, the Class Loader returns the resulting class to Java. The Class Loader's loadClass method returns the class that corresponds to the name being looked up. There are usually several class loaders in operation. When Java needs to translate a name, it asks the Class Loader that originally loaded the class referencing the name. Thus, each Class Loader is responsible for maintaining and defining its own part of the namespace.
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