Business Email Compromise (BEC) attempt from unsolicited sender

name: "Business Email Compromise (BEC) attempt from unsolicited sender"
description: |
  Detects potential Business Email Compromise (BEC) attacks by analyzing text within the email body from unsolicited senders.
type: "rule"
severity: "medium"
source: |
  type.inbound
  and any(ml.nlu_classifier(body.current_thread.text).intents,
          .name in ("bec") and .confidence == "high"
  )
  and 
  // mismatched From and Reply-to
  (
    (
      length(headers.reply_to) > 0
      and all(headers.reply_to,
              .email.domain.root_domain != sender.email.domain.root_domain
      )
    )
    or not headers.auth_summary.dmarc.pass
    or not headers.auth_summary.spf.pass
  )
  
  // negate "via" senders via dmarc authentication or gmail autoforwards
  and not (
    strings.ilike(headers.return_path.local_part, "*+caf_=*")
    and strings.contains(sender.display_name, "via")
    and (headers.auth_summary.dmarc.pass)
  )
  and (
    not profile.by_sender().solicited
    or (
      profile.by_sender().any_messages_malicious_or_spam
      and not profile.by_sender().any_messages_benign
    )
  )

attack_types:
  - "BEC/Fraud"
tactics_and_techniques:
  - "Social engineering"
  - "Spoofing"
detection_methods:
  - "Content analysis"
  - "File analysis"
  - "Header analysis"
  - "Sender analysis"
id: "57eccc45-0408-55ee-ae33-8203e166254a"