This indicates an attack attempt to exploit a Directory Traversal vulnerability in Symantec Messaging Gateway.
The vulnerability is due to insufficient sanitizing of user supplied inputs in the application when handling crafted HTTP requests. A remote attacker can exploit this to gain unauthorized access to sensitive information via a crafted request.
Symantec Messaging Gateway 9.5.x
Information Disclosure: Remote attackers can gain sensitive information from vulnerable systems.
Refer to the vendor's website for suggested workaround.