alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"ET HUNTING Suspicious User-Agent (NULL)"; flow:established,to_server; threshold:type limit, count 2, track by_src, seconds 300; http.user_agent; content:"NULL"; depth:4; classtype:trojan-activity; sid:2008488; rev:11; metadata:affected_product Any, attack_target Client_Endpoint, created_at 2010_07_30, deployment Perimeter, confidence Medium, signature_severity Major, tag User_Agent, updated_at 2020_10_17;)