Create service_abuse_sendgrid_free_email_provider.yml

detection-rules/service_abuse_sendgrid_free_email_provider.yml

@@ -0,0 +1,26 @@
+name: "Service abuse: Free provider with SendGrid routing"
+description: "Message From header includes a free email provider domain but is routed through SendGrid infrastructure, indicating potential service abuse for delivery evasion."
+type: "rule"
+severity: "medium"
+source: |
+  type.inbound
+  and sender.email.domain.domain in $free_email_providers
+  and any(headers.domains, .root_domain == "sendgrid.net")
+  and not any(ml.nlu_classifier(body.current_thread.text).intents,
+              .name == "benign"
+  )
+  and not any(ml.nlu_classifier(body.current_thread.text).topics,
+              .name == "Bounce Back and Delivery Failure Notifications"
+  )
+
+tags:
+  - "Attack surface reduction"
+attack_types:
+  - "Credential Phishing"
+tactics_and_techniques:
+  - "Free email provider"
+  - "Evasion"
+detection_methods:
+  - "Header analysis"
+  - "Sender analysis"
+id: "3079cacb-2ab9-533a-bced-59545bd54d63"