alert http $EXTERNAL_NET any -> $HOME_NET any (msg:"ET ATTACK_RESPONSE TA399 Fake 404 WebPage Inbound"; flow:established,to_client; http.response_body; content:"404|2e 20|That|3f|s|20|an|20|error|2e|"; content:"That|3f|s|20|all|20|we|20|know|2e 3c 2f|pre|3e 3c 2f|body|3e|"; endswith; fast_pattern; reference:url,strikeready.com/blog/rattling-the-cage-of-a-sidewinder/; classtype:trojan-activity; sid:2063194; rev:1; metadata:attack_target Client_Endpoint, tls_state plaintext, created_at 2025_06_26, deployment Perimeter, confidence Medium, signature_severity Major, tag Sidewinder, tag TA399, tag Description_Generated_By_Proofpoint_Nexus, updated_at 2025_06_26, mitre_tactic_id TA0011, mitre_tactic_name Command_And_Control, mitre_technique_id T1105, mitre_technique_name Ingress_Tool_Transfer; target:dest_ip;)