alert tls $EXTERNAL_NET any -> $HOME_NET any (msg:"ET MALWARE Observed Malicious SSL Cert (Baka Skimmer Staging CnC)"; flow:from_server,established; tls.cert_subject; content:"CN=b-metric.com"; nocase; fast_pattern; endswith; tls.cert_issuer; content:"C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3"; classtype:domain-c2; sid:2030844; rev:1; metadata:attack_target Client_and_Server, created_at 2020_09_05, deployment Perimeter, performance_impact Low, confidence High, signature_severity Major, tag SSL_Malicious_Cert, tag Description_Generated_By_Proofpoint_Nexus, updated_at 2020_09_05, mitre_tactic_id TA0042, mitre_tactic_name Resource_Development, mitre_technique_id T1587, mitre_technique_name Develop_Capabilities;)