Service abuse: Dropbox share from an unsolicited reply-to address

name: "Service abuse: Dropbox share from an unsolicited reply-to address"
description: "This rule detects Dropbox share notifications which contain a reply-to address or domain that has not been previously observed sending messages to or receiving messages from the recipient organization."
type: "rule"
severity: "medium"
source: |
  type.inbound
  
  // Legitimate Dropbox sending infratructure
  and sender.email.email == "no-reply@dropbox.com"
  and headers.auth_summary.spf.pass
  and headers.auth_summary.dmarc.pass
  and strings.ends_with(headers.auth_summary.spf.details.designator,
                        '.dropbox.com'
  )
  and strings.icontains(subject.subject, 'shared')
  and strings.icontains(subject.subject, 'with you')
  
  //
  // This rule makes use of a beta feature and is subject to change without notice
  // using the beta feature in custom rules is not suggested until it has been formally released
  //
  
  // reply-to address has never sent an email to the org
  and beta.profile.by_reply_to().prevalence == "new"
  
  // reply-to email address has never been sent an email by the org
  and not beta.profile.by_reply_to().solicited

  // do not match if the reply_to address has been observed as a reply_to address
  // of a message that has been classified as benign
  and not beta.profile.by_reply_to().any_messages_benign
  
tags:
 - "Attack surface reduction"
attack_types:
  - "Callback Phishing"
  - "BEC/Fraud"
tactics_and_techniques:
  - "Evasion"
  - "Social engineering"
detection_methods:
  - "Sender analysis"
  - "Header analysis"
  - "Content analysis"
id: "50a1499f-bb59-5ee0-b4f4-e3cc84a5c41e"