alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"ET MALWARE Cobalt Strike Beacon Activity (GET)"; flow:established,to_server; http.method; content:"GET"; http.uri; content:".css?open="; fast_pattern; http.cookie; content:"lu="; startswith; pcre:"/^(?:[A-Za-z0-9+/]{4})*(?:[A-Za-z0-9+/]{2}==|[A-Za-z0-9+/]{3}=|[A-Za-z0-9+/]{4})$/R"; http.header_names; content:!"Referer"; reference:md5,c49f5361c91208f95a3f4d8a9cccf5cc; reference:url,twitter.com/_brettfitz/status/1404438059962208256; classtype:trojan-activity; sid:2033145; rev:1; metadata:attack_target Client_Endpoint, created_at 2021_06_15, deployment Perimeter, malware_family Cobalt_Strike, confidence Medium, signature_severity Major, updated_at 2021_06_15, mitre_tactic_id TA0011, mitre_tactic_name Command_And_Control, mitre_technique_id T1001, mitre_technique_name Data_Obfuscation;)