This indicates an attack attempt to exploit a Buffer Overflow Vulnerability in TightVNC.
The vulnerability is due to the application improper handling of messages. A remote attacker could exploit this to execute arbitrary code within the context of the application, via a crafted message.
TightVNC Viewer prior to 2.8.63
System Compromise: Remote attackers can gain control of vulnerable systems.
TightVNC has not released an advisory or patch regarding this vulnerability.