alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"ET MALWARE CastleLoader User-Agent Observed"; flow:established,to_server; http.user_agent; bsize:20; content:"400yBAXUg5BGosOCwsmv"; reference:md5,835ab1cf597812b0e6464c2e8f100678; classtype:trojan-activity; sid:2064003; rev:1; metadata:affected_product Windows_XP_Vista_7_8_10_Server_32_64_Bit, attack_target Client_Endpoint, tls_state plaintext, created_at 2025_08_13, deployment Perimeter, malware_family CastleLoader, confidence High, signature_severity Major, tag c2, tag Description_Generated_By_Proofpoint_Nexus, updated_at 2025_08_13, mitre_tactic_id TA0011, mitre_tactic_name Command_And_Control, mitre_technique_id T1071, mitre_technique_name Application_Layer_Protocol; target:src_ip;)