This indicates an attack attempt against an Arbitrary File Upload vulnerability in Symantec Web Gateway.
The vulnerability is caused by an error when the vulnerable software handles a malicious HTTP request with a crafted URI. It allows a remote attacker to upload an arbitrary file via sending a crafted URI to a vulnerable application.
Symantec Web Gateway 5.0.1
Symantec Web Gateway 5.0
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the most recent upgrades or patches from the vendor.