What Went Wrong: A Name Alone in Java

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What Went Wrong: A Name Alone
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Unfortunately, the creators of Java were not always so careful. In Java 1.0.2, Netscape 2.02, and the first beta version of Internet Explorer, the types of interfaces and exceptions were compared by name rather than by (name, namespace) pair as required. This led to a set of attacks that could break Java's security system, achieving full system penetration. The attacks worked as described earlier. The attacker wrote two applets defining different classes with the same name C. One applet would create an object of class C and pass it to the other applet, which would operate on its C. This leads to a classic type-confusion situation, which can be exploited by methods seen several times in this chapter.
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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 9 -- Casting Caution to the Wind
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Software consultant Tom Cargill has discovered two security flaws related to the way in which Java handles interface types. Both flaws involve a rare case in which Java fails to check whether a method is private. Both also use type-casting operations on Java's interface types. By exploiting these flaws, an attacker can call private methods normally prohibited by Java's security rules. Since some of the securitycritical values inside the Java system are protected by private methods, a complete security breach using this attack is possible.
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Simple Interface Casting
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The core of Cargill's first discovery is shown in the following code: interface Inter { void f(); } class Secure implements Inter { private void f(); } class Dummy extends Secure implements Inter { public void f(); Dummy() { Secure s = new Secure(); Inter i = (Inter) s; i.f(); // should be illegal
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This code allows the private f method of class Secure to be called illegally. The Java interpreter fails to determine if f is private when i.f() is called. The Princeton team figured out how to use this flaw to achieve full system penetration. This was done by exploiting the fix to the class loader bug. The class loader bug was fixed by splitting the critical defineClass method into a private method and a public method. The private method, defineClass0, did the work. The public method checked the initialized flag and called defineClass0 only if the flag was true. Since the private defineClass0 method couldn't be called directly by an applet, this was supposed to fix the class loader bug. Unfortunately, a variant of the interface-casting trick shown here allows an applet to call the private defineClass0 method directly, bypassing the check. This meant that the attack could create a class loader by exploiting the Verifier bug. The initialized flag would be false, but that wouldn't matter. A programmer could bypass the flag-check by exploiting the interface-casting trick to call the private defineClass0 method directly. By using this trick, an attacker could gain full system penetration under Netscape Navigator 2.02.
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Netscape fixed this problem in two ways. First, it fixed the flaw in its Java Virtual Machine that allowed the interface-casting trick to work. Second, Netscape began storing and checking the initialized flag inside the Java Virtual Machine, rather than in programmer-generated Java code. Netscape eliminated the dangerous defineClass0 operation by integrating everything into the VM's implementation of defineClass. This change took effect in Navigator 3.0beta3. In reaction to the interface-casting bug, Netscape changed its Java implementation to protect itself more generally against an attacker who had the ability to call private methods. By going beyond a simple bug fix to improve the structure of the system, Netscape practiced good security engineering. Its decision paid off when the next bug was discovered.
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