Link: Free file hosting with undisclosed recipients

name: "Link: Free file hosting with undisclosed recipients"
description: "Detects messages containing links to free file hosting or subdomain services that are sent to undisclosed recipients or only CC/BCC recipients. The rule identifies suspicious distribution patterns where legitimate recipients are hidden, potentially indicating mass distribution of malicious content through file sharing platforms."
type: "rule"
severity: "medium"
source: |
  type.inbound
  // no previous threads
  and (
    length(body.previous_threads) == 0
    // If there is a previous thread, it is unrelated to current thread
    or any(body.previous_threads, .sender.email.email != mailbox.email.email)
  )
  
  // few links that aren't "social" links
  and 0 < length(filter(body.current_thread.links,
                        .href_url.domain.root_domain not in (
                          'x.com',
                          'facebook.com',
                          'twitter.com',
                          'instagram.com',
                          'youtube.com',
                          'linkedin.com'
                        )
                 )
  ) < 10
  
  // undisclosed recipients or all recipients cc'd
  and (
    any(recipients.to, strings.ilike(.display_name, "undisclosed?recipients"))
    or (length(recipients.cc) > 0 and length(recipients.to) == 0)
    or (length(recipients.bcc) > 0 and length(recipients.to) == 0)
    or (
      length(recipients.to) == 1
      and length(recipients.cc) == 0
      and length(recipients.bcc) == 0
      and all(recipients.to, .email.email == sender.email.email)
      and all(recipients.to, .email.email != mailbox.email.email)
    )
  )
  
  // links to free file hosts or free subdomain hosts
  and any(body.current_thread.links,
          (
            .href_url.domain.root_domain in $free_file_hosts
            or .href_url.domain.root_domain in $free_subdomain_hosts
          )
          and .visible
          and not (
            .href_url.domain.root_domain == "googleusercontent.com"
            and strings.istarts_with(.href_url.path, "/mail-sig")
          )
          and not .href_url.domain.domain in $tenant_domains
  )
  
  // negate listmailers & benign threads
  and not (
    any(headers.hops, any(.fields, .name == "List-Unsubscribe"))
    or any(ml.nlu_classifier(body.current_thread.text).intents,
           .name == "benign" and .confidence == "high"
    )
  )
  and not (
    (
      sender.email.domain.root_domain in ("twilio.com", "zendesk.com")
      or headers.return_path.domain.root_domain in ("twilio.com", "zendesk.com")
    )
    and coalesce(headers.auth_summary.dmarc.pass, false)
  )
  
  // unsolicited and passing auth, or failing/missing dmarc
  and (
    (
      coalesce(headers.auth_summary.dmarc.pass, false)
      and not profile.by_sender().solicited
    )
    or profile.by_sender_email().days_since.last_inbound > 365
    or (not coalesce(headers.auth_summary.dmarc.pass, false))
  )

attack_types:
  - "Credential Phishing"
  - "Malware/Ransomware"
tactics_and_techniques:
  - "Free file host"
  - "Free subdomain host"
  - "Evasion"
detection_methods:
  - "Header analysis"
  - "URL analysis"
  - "Sender analysis"
  - "Natural Language Understanding"
id: "b6281306-bf26-58e2-8445-0ef8d05d9820"