alert tls $EXTERNAL_NET any -> $HOME_NET any (msg:"ET MALWARE Observed Malicious SSL Cert (ServHelper CnC)"; flow:from_server,established; tls.cert_subject; content:"CN=kiparis.xyz"; nocase; fast_pattern; endswith; tls.cert_issuer; content:"C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3"; classtype:domain-c2; sid:2029611; rev:3; metadata:attack_target Client_and_Server, created_at 2020_03_11, deployment Perimeter, malware_family ServHelper, performance_impact Low, confidence High, signature_severity Major, tag SSL_Malicious_Cert, tag Description_Generated_By_Proofpoint_Nexus, updated_at 2020_11_05, mitre_tactic_id TA0011, mitre_tactic_name Command_And_Control, mitre_technique_id T1071, mitre_technique_name Application_Layer_Protocol;)