alert udp $HOME_NET any -> $EXTERNAL_NET any (msg:"ET INFO kcptun Null Encryption Message Header"; flow:stateless,to_server; content:"|51 00 00 02|"; fast_pattern; offset:32; depth:4; content:"|01 02|"; within:20; content:"|03|"; within:4; reference:url,github.com/sspanel/kcptun; reference:url,github.com/nebulabox/kcptun; classtype:misc-activity; sid:2072396; rev:1; metadata:affected_product Any, attack_target Client_and_Server, tls_state plaintext, created_at 2026_10_08, deployment Perimeter, deployment Internal, performance_impact Low, confidence Medium, signature_severity Informational, tag kcptun, tag tunneling, updated_at 2026_10_08, mitre_tactic_id TA0011, mitre_tactic_name Command_And_Control, mitre_technique_id T1572, mitre_technique_name Protocol_Tunneling;)