According to Reston, Vir.-based iDefense and Danish security vendor Secunia, the bug in Java 2 Runtime Environment (JRE), Standard Edition could let attackers bypass the Java security "sandbox" and all security restrictions within Java applets on Web sites.
JRE is the plug-in software that establishes a connection between the browser and the Java platform, and makes it possible for Web browsers to run Java applets stashed on Web sites.
Hackers using the exploit could essentially can complete control of the compromised computer, said iDefense, letting them "access, download, upload, or execute files as well as access the network."
iDefense confirmed that the vulnerability exists on J2SE 1.4.2_01 and 1.4.2_04, and may also be within earlier versions as well.
Because the bug exists in Java, it's not limited to one browser. "Various browsers such as Internet Explorer, Mozilla, and Firefox on both Windows and Unix platforms can be exploited if they are running a vulnerable Java Virtual Machine," said iDefense in its online advisory.
Pynnonen brought the problem to Sun's attention in late April, 2004, but Sun has only now posted an update -- J2SE 1.4.2_06 -- on its Web site.
Secunia rated the vulnerability as "Highly critical," and urged users to update Java 2 immediately.