alert tcp $HOME_NET any -> $EXTERNAL_NET 81 (msg:"ET DELETED Unknown Malware Keepalive"; flow:established,to_server; content:"keepalive"; nocase; depth:9; pcre:"/keepalive([0-9]{4}|\x7c[0-9]{4})/i"; threshold:type limit, track by_src, count 1, seconds 60; classtype:trojan-activity; sid:2012409; rev:3; metadata:created_at 2011_03_02, signature_severity Unknown, tag Description_Generated_By_Proofpoint_Nexus, updated_at 2019_07_26;)