Java Plugin arbitrary package access vulnerability


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.


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)

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 contain methods for modifying private fields of other 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.


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


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