Hello,
Please Migrate the SEP to the Latest version.
Here is an Article for the same -
Symantec Endpoint Protection client Release Update 6 is detecting a Denial of Service attack of type "UDP Flood Attack" from your DNS server.
http://www.symantec.com/docs/TECH132161
Release notes for Endpoint Protection and Network Access Control 11
Resolved a UDP flood attack false positive
Fix ID: 2058022
Symptom: After upgrading to Symantec Endpoint Protection 11.0 RU6, the client detects a UDP flood attack.
Solution: The UDP flood detection thresholds were modified to reduce the occurrence of false positive flood attacks.
Migrating to Symantec Endpoint Protection 11.0.7000 (RU7)
Hope this helps!!