[Full-Disclosure] Sun Java Plugin arbitrary package access vulnerability

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To: full-disclosure@netsys.com
Date: Tue, 23 Nov 2004 03:39:38 +0200

OVERVIEW
========

Sun Microsystem's Java Plugin connects the Java technology to web
browsers and allows the use of Java Applets. Java Plugin technology is
available for numerous platforms and supports major web browsers.

A vulnerability in Java Plugin allows an attacker to create an Applet
which can disable Java's security restrictions and break out of the
Java sandbox. The attack can be launched when a victim views a web page
created by the attacker. Further user interaction is not required as
Java Applets are normally loaded and started automatically.

Such Applet can then take any action which the user could: browse,
read, or modify files, upload more programs to the victim system and
run them, or send out data from the system. Java is a cross-platform
language so the same exploit could run on various OS'es and
architectures.

DETAILS
=======

There is a number of private Java packages in the Java VM, meant to be
used only by the VM internally. Java Applets can't normally access
these packages because of security concerns. Attempting to access
them normally results in an AccessControlException.

The problem is that JavaScript code can bypass the access control by
using so called reflection API. The following piece of example
JavaScript acquires a reference to a supposedly restricted, private
class "sun.text.Utility":

 [script language=javascript]
 var c=document.applets[0].getClass().forName('sun.text.Utility');
 alert('got Class object: '+c)
 [/script]

This isn't possible by a normal Java Applet, and shouldn't be for
JavaScript either. The JavaScript code could now instantiate the class
or pass it to an Applet that could use it.

An attacker can't do much with the utility class in this example, but
could use other private classes to exploit the vulnerability. Some of
them allow e.g. direct access to memory or methods for modifying
private fields of Java objects. The latter allows an attacker
to simply turn off the Java security manager, after which there is no
sandbox restricting what the Applet can do.

VULNERABLE VERSIONS
===================

The Java Plugin versions 1.4.2_04 and 1.4.2_05 were tested on Windows
and Linux. Web browsers tested were Microsoft Internet Explorer,
Mozilla Firefox and Opera. It should be noted that Opera uses a
different way of connecting JavaScript and Java which caused the test
exploit not to work on Opera. However the problem itself (access to
private packages) was demonstrated on Opera too, so it may be
vulnerable to a variation of the exploit.

SOLUTION
========

Sun Microsystems was informed on April 29, 2004 and has fixed the
problem in J2SE 1.4.2_06, available at

  http://java.sun.com/j2se/1.4.2/download.html

CREDITS
=======

The vulnerability was discovered and researched by Jouko Pynnonen,
Finland.

-- 
Jouko Pynnönen          Web: http://iki.fi/jouko/
jouko@iki.fi
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