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;
)