alert tls $EXTERNAL_NET any -> $HOME_NET any (msg:"ET POLICY Lets Encrypt Free SSL Cert Observed"; flow:established,to_client; tls.cert_subject; content:"O=Let|27|s Encrypt"; reference:url,letsencrypt.org/about/; classtype:policy-violation; sid:2022218; rev:4; metadata:attack_target Client_Endpoint, created_at 2015_12_04, deployment Perimeter, confidence High, signature_severity Informational, tag SSL_Malicious_Cert, updated_at 2024_04_23;)