This indicates an attack attempt against a Directory Traversal vulnerability in Symantec Messaging Gateway.
The vulnerability is due to insufficient sanitizing of user supplied inputs when handling a crafted HTTP packet. It allows a remote attacker to execute malicious codes against affected machine via crafted requests.
Symantec Messaging Gateway 10.6.1 and prior
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the latest update from the vendor