http://secunia.com/advisories/23398/ 

Description:
Two vulnerabilities have been reported in Sun Java JRE (Java Runtime Environment), which can be exploited by malicious people to bypass certain security restrictions.

The vulnerabilities are caused due to unspecified errors in the Java Runtime Environment and may allow a malicious, untrusted applet to access data in other applets.

The vulnerabilities are reported in the following versions:
* JDK and JRE 5.0 Update 6 and prior
* SDK and JRE 1.4.2_12 and prior
* SDK and JRE 1.3.1_18 and prior

Secunia has constructed the Secunia Software Inspector, which you can use to check if your system is vulnerable:
http://secunia.com/software_inspector/

Solution:
Update to fixed versions.