alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"ET INFO Mailtrack .io Email Activity Tracking M1"; flow:established,to_server; urilen:66; http.method; content:"GET"; http.host; bsize:12; content:"mailtrack.io"; http.uri; content:"/trace/mail/"; startswith; fast_pattern; pcre:"/^(?:[a-z0-9]{40})\x2epng\x3fu\x3d(?:0-9]{7})$/R"; threshold:type limit, count 1, seconds 1800, track by_src; classtype:misc-activity; sid:2045062; rev:2; metadata:attack_target Client_Endpoint, created_at 2023_04_20, deployment Perimeter, deployment SSLDecrypt, performance_impact Moderate, confidence High, signature_severity Informational, tag Description_Generated_By_Proofpoint_Nexus, updated_at 2024_03_26;)