Hi,
An attack signature is a unique arrangement of information that can be used to identify an attacker's attempt to exploit a known operating system or application vulnerability. When Intrusion Detection detects an attack signature, it displays a Security Alert.
http://www.symantec.com/security_response/attacksignatures/detail.jsp?asid=28747
It looks as per logs traffic has been blocked through Chrome application.
Make sure using latest supported version of Chrome, Udate operating system to the latest service pack & windows patches.