This indicates an attack attempt against a Security Bypass vulnerability in Oracle Java JRE.
The vulnerability is caused by a design weakness error when the vulnerability software is loading a serialized class via the applet tag object attribute. It allows a remote attacker to execute arbitrary Java code via an unsigned Java applet.
Oracle Java Development Kit (JDK) 7 update 10
Oracle Java Development Kit (JDK) 7 update 11
Oracle Java Runtime Environment (JRE) 7 update 11
Oracle Java Runtime Environment (JRE) 7 update 10
System Compromise: Remote attackers can execute arbitrary Java Code within the context of the vulnerable systems.
Apply the most recent upgrade or patch from the vendor.