alert tls $EXTERNAL_NET any -> $HOME_NET any (msg:"ET MALWARE TA421/YTTRIUM/APT29 TLS Certificate M1"; flow:established,to_client; tls.cert_subject; content:"C=IL, O=StartCom Ltd., CN="; fast_pattern; startswith; isdataat:!20,relative; content:"CN=*"; endswith; reference:url,community.riskiq.com/article/541a465f; classtype:command-and-control; sid:2033632; rev:3; metadata:attack_target Client_and_Server, created_at 2021_07_31, deployment Perimeter, performance_impact Low, confidence High, signature_severity Major, tag SSL_Malicious_Cert, tag Description_Generated_By_Proofpoint_Nexus, updated_at 2025_09_17;)