alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"ET INFO Mailtrack Email Activity Tracking M2"; flow:established,to_server; http.method; content:"GET"; http.host; content:"mailtrack.io"; bsize:12; http.uri; content:"/trace/link/"; startswith; fast_pattern; pcre:"/^(?:[a-z0-9]{40})\x3f/R"; content:"&url=http"; within:18; threshold:type limit, count 1, seconds 1800, track by_src; classtype:misc-activity; sid:2045063; rev:1; 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 2023_04_20;)