This indicates an attack attempt against a buffer overflow vulnerability in RealNetworks RealPlayer.
The vulnerability is caused by an error when the vulnerable software handles a specifically crafted MP4 file with the "Sample Size" member of a 'stsz' structure having an overly long size value. An attacker can trick an unsuspecting user into opening a MP4 file and execute arbitrary code within the context of the application.
RealPlayer version v126.96.36.1995 and earlier
System Compromise: Remote attackers could gain control of vulnerable systems.
Apply the latest update from the vendor.