alert tls $EXTERNAL_NET any -> $HOME_NET any (msg:"ET POLICY Observed SSL Cert (DoH Service)"; flow:from_server,established; tls.cert_subject; content:"CN=www.rubyfish.cn"; nocase; fast_pattern; endswith; tls.cert_issuer; content:"C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3"; classtype:policy-violation; sid:2029051; rev:3; metadata:created_at 2019_11_21, deployment Perimeter, performance_impact Low, confidence High, signature_severity Minor, tag DNS_over_HTTPS, tag Description_Generated_By_Proofpoint_Nexus, updated_at 2020_10_22, mitre_tactic_id TA0011, mitre_tactic_name Command_And_Control, mitre_technique_id T1572, mitre_technique_name Protocol_Tunneling;)