alert tls $EXTERNAL_NET any -> $HOME_NET any (msg:"ET MALWARE Observed Malicious SSL Cert (ELF/Rekoobe CnC)"; flow:from_server,established; tls.cert_subject; content:"CN=kooktijd.acc.dynapps.be"; nocase; fast_pattern; endswith; tls.cert_issuer; content:"C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3"; reference:url,intezer.com/blog-linux-rekoobe-operating-with-new-undetected-malware-samples/; classtype:domain-c2; sid:2029307; rev:2; metadata:affected_product Linux, attack_target Client_and_Server, created_at 2020_01_22, deployment Perimeter, malware_family Rekoobe, performance_impact Low, confidence High, signature_severity Major, tag SSL_Malicious_Cert, tag Description_Generated_By_Proofpoint_Nexus, updated_at 2020_10_27, mitre_tactic_id TA0011, mitre_tactic_name Command_And_Control, mitre_technique_id T1071, mitre_technique_name Application_Layer_Protocol;)