This indicates an attack attempt to exploit a Buffer Overflow vulnerability in Sophos Anti-Virus and Endpoint Protection.
The vulnerability is due to an error when the vulnerable software handles a maliciously crafted encrypted PDF file. A remote attacker may be able to exploit this to execute arbitrary code within the context of the application, via a crafted PDF file.
Sophos Threat Detection Engine 3.36.2 and prior
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the most recent upgrade or patch from the vendor.