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:"|3c|head|3e 3c|title|3e|404|20|Not|20|Found|3c 2f|title|3e 3c 2f|head|3e 3c|body|3e|"; startswith; fast_pattern; content:"|3c|center|3e 3c|h1|3e|404|20|Not|20|Found|3c 2f|h1|3e 3c 2f|center|3e|"; distance:0; content:"|3c|hr|3e 3c|center|3e|nginx|2f|1|2e|24|2e|0|20 28|Ubuntu|29 3c 2f|center|3e|"; distance:0; content:"|0a 0a 0a 0a 0a 0a 0a 0a 0a 3c 2f|body|3e|"; endswith; reference:url,strikeready.com/blog/rattling-the-cage-of-a-sidewinder/; classtype:trojan-activity; sid:2063193; 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;)