This indicates an attack attempt against a buffer-overflow vulnerability in Sun Microsystems' Java Web Start.
The vulnerability is caused by a boundary error when the vulnerable software handles a customized splash screen GIF image. It allows a remote attacker to execute arbitrary code.
Sun JDK and JRE 6 Update 10 and earlier
Sun JDK and JRE 5.0 Update 16 and earlier
Sun SDK and JRE 1.4.2_18 and earlier
Sun SDK and JRE 1.3.1_23 and earlier
Update to the latest versions:
JDK and JRE 6 Update 11
JDK and JRE 5.0 Update 17
SDK and JRE 1.4.2_19
SDK and JRE 1.3.1_24