alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"ET MALWARE CastleBot CnC Checkin - Task Complete"; flow:established,to_server; urilen:<30; http.method; content:"GET"; http.uri; content:"/service/tasks/complete/id/"; startswith; fast_pattern; pcre:"/^\d{1,3}$/R"; http.user_agent; bsize:<26; pcre:"/^(?:[a-zA-Z0-9]{20,25})$/"; reference:md5,1a3896a986195c277f1547706dbdea08; reference:md5,f75bb2393f80bf30c4788c82f3a7dadf; reference:url,ibm.com/think/x-force/dissecting-castlebot-maas-operation; classtype:trojan-activity; sid:2064207; rev:1; metadata:affected_product Windows_XP_Vista_7_8_10_Server_32_64_Bit, attack_target Client_Endpoint, tls_state TLSDecrypt, created_at 2025_08_28, deployment Perimeter, deployment SSLDecrypt, malware_family CastleBot, confidence High, signature_severity Critical, tag c2, tag Description_Generated_By_Proofpoint_Nexus, updated_at 2025_08_28, mitre_tactic_id TA0011, mitre_tactic_name Command_And_Control, mitre_technique_id T1071, mitre_technique_name Application_Layer_Protocol; target:src_ip;)