alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"ET MALWARE Cobalt Strike Activity"; flow:established,to_server; http.method; content:"GET"; http.uri; content:"/boxes?nid="; fast_pattern; startswith; http.cookie; content:"lu="; startswith; pcre:"/^(?:[A-Za-z0-9+/]{4})*(?:[A-Za-z0-9+/]{2}==|[A-Za-z0-9+/]{3}=|[A-Za-z0-9+/]{4})$/R"; http.header_names; content:!"Referer"; reference:md5,91b9c9db4916b7e416da074e29a36082; classtype:trojan-activity; sid:2032332; rev:1; metadata:attack_target Client_Endpoint, created_at 2021_03_25, deployment Perimeter, performance_impact Low, confidence Medium, signature_severity Major, tag Description_Generated_By_Proofpoint_Nexus, updated_at 2021_03_25;)