ET MALWARE Observed Malicious SSL Cert (APT32 CnC)

alert tls $EXTERNAL_NET any -> $HOME_NET any (
    msg:"ET MALWARE Observed Malicious SSL Cert (APT32 CnC)";
    flow:from_server,established;
    tls.cert_subject;
    content:"CN=office.windown-update.com"; nocase; endswith;
    tls.cert_issuer;
    content:"C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3";
    reference:url,unit42.paloaltonetworks.com/tracking-oceanlotus-new-downloader-kerrdown/;
    classtype:domain-c2;
    sid:2026871; rev:3;
    metadata:affected_product Windows_XP_Vista_7_8_10_Server_32_64_Bit, attack_target Client_and_Server, created_at 2019_02_01, deployment Perimeter, malware_family KerrDown, performance_impact Low, confidence High, signature_severity Major, tag SSL_Malicious_Cert, tag APT32, tag Description_Generated_By_Proofpoint_Nexus, updated_at 2020_09_16, mitre_tactic_id TA0011, mitre_tactic_name Command_And_Control, mitre_technique_id T1071, mitre_technique_name Application_Layer_Protocol;
)