According to Microsoft: "This event occurs when more than 20 packets are filtered by a Proxy Server packet filter rule. 20 of these events in the packet filter log trigger event ID 120."
Build a great reporting interface using Splunk, one of the leaders in the Security Information and Event Management (SIEM) field, linking the collected Windows events to www.eventid.net.
Obtain enhanced visibility into Cisco ASA firewall logs using the free Firegen for Cisco ASA Splunk App. Take advantage of dashboards built to optimize the threat analysis process.