Fake thread with suspicious indicators

name: "Fake thread with suspicious indicators"
description: "Fake thread contains suspicious indicators, which can lead to BEC, credential phishing, and other undesirable outcomes."
type: "rule"
severity: "medium"
source: |
  type.inbound
  // fake thread check
  and (length(headers.references) == 0 or headers.in_reply_to is null)
  and (
    subject.is_reply
    or subject.is_forward
    // fake thread, but no indication in the subject line
    // current_thread pulls the recent thread, but the full body contains the fake "original" email
    or (
      not (subject.is_reply or subject.is_forward)
      and any([body.current_thread.text, body.html.display_text, body.plain.raw],
              3 of (
                strings.icontains(., "from:"),
                strings.icontains(., "to:"),
                strings.icontains(., "sent:"),
                strings.icontains(., "date:"),
                strings.icontains(., "cc:"),
                strings.icontains(., "subject:")
              )
      )
      and length(body.current_thread.text) + 100 < length(coalesce(body.html.display_text,
                                                                   body.plain.raw
                                                          )
      )
    )
  )
  
  // negating bouncebacks
  and not any(attachments,
              .content_type in ("message/delivery-status", "message/rfc822")
  )
  // negating Google Calendar invites
  and (
    (
      headers.return_path.domain.domain is not null
      and headers.return_path.domain.domain != 'calendar-server.bounces.google.com'
    )
    or headers.return_path.domain.domain is null
  )
  // not mimecast secure message from internal source
  and not (
    strings.istarts_with(headers.message_id, '<Mimecast.')
    and strings.iends_with(headers.message_id, '.mimecast.lan>')
    and headers.hops[0].received.server.raw == "relay.mimecast.com"
    and strings.icontains(headers.hops[0].received.source.raw, 'mimecast.lan')
  )
  
  // and not solicited
  and not profile.by_sender().solicited
  and 4 of (
    // language attempting to engage
    (
      any(ml.nlu_classifier(body.current_thread.text).entities,
          .name == "request"
      )
      and any(ml.nlu_classifier(body.current_thread.text).entities,
              .name == "financial"
      )
    ),
  
    // invoicing language
    (
      any(ml.nlu_classifier(body.current_thread.text).tags, .name == "invoice")
      or any(ml.nlu_classifier(body.current_thread.text).entities,
             .text == "invoice"
      )
    ),
  
    // urgency request
    any(ml.nlu_classifier(body.current_thread.text).entities, .name == "urgency"),
  
    // cred_theft detection
    any(ml.nlu_classifier(body.current_thread.text).intents,
        .name == "cred_theft" and .confidence in~ ("medium", "high")
    ),
  
    // commonly abused sender TLD
    strings.ilike(sender.email.domain.tld, "*.jp"),
  
    // headers traverse abused TLD
    any(headers.domains, strings.ilike(.tld, "*.jp")),
  
    // known suspicious pattern in the URL path
    any(body.links, regex.match(.href_url.path, '\/[a-z]{3}\d[a-z]')),
  
    // link display text is in all caps
    any(body.links, regex.match(.display_text, '[A-Z ]+')),
  
    // link display text contains invisible characters (U+200F)
    any(body.links, strings.contains(.display_text, "\u{200F}")),
  
    // Low reputation link with display text ending in a document extension
    any(body.links,
        .href_url.domain.root_domain not in $tranco_1m
        and .href_url.domain.valid
        and .href_url.domain.root_domain not in $org_domains
        and .href_url.domain.root_domain not in $high_trust_sender_root_domains
        and (
          any($file_extensions_macros, strings.ends_with(..display_text, .))
          or strings.ends_with(.display_text, 'pdf')
        )
    ),
  
    // display name contains an email
    regex.contains(sender.display_name, '[a-z0-9]+@[a-z]+'),
  
    // Sender domain is empty
    sender.email.domain.domain == "",
  
    // sender domain matches no body domains
    all(body.links,
        .href_url.domain.root_domain != sender.email.domain.root_domain
    ),
  
    // body contains name of VIP
    (
      any($org_vips, strings.icontains(body.html.inner_text, .display_name))
      or any($org_vips, strings.icontains(body.plain.raw, .display_name))
    ),
  
    // new body domain
    any(body.links, network.whois(.href_url.domain).days_old < 30),
  
    // new sender domain
    network.whois(sender.email.domain).days_old < 30,
  
    // new sender
    profile.by_sender().days_known < 7,
  
    // excessive whitespace
    (
      regex.icontains(body.html.raw, '((<br\s*/?>\s*){20,}|\n{20,})')
      or regex.icontains(body.html.raw, '(<p[^>]*>\s*<br\s*/?>\s*</p>\s*){30,}')
      or regex.icontains(body.html.raw,
                         '(<p class=".*?"><span style=".*?"><o:p>&nbsp;</o:p></span></p>\s*){30,}'
      )
      or regex.icontains(body.html.raw, '(<p>&nbsp;</p>\s*){7,}')
      or regex.icontains(body.html.raw, '(<p>&nbsp;</p><br>\s*){7,}')
      or regex.icontains(body.html.raw, '(<p[^>]*>\s*&nbsp;<br>\s*</p>\s*){5,}')
      or regex.icontains(body.html.raw, '(<p[^>]*>&nbsp;</p>\s*){7,}')
    ),
  
    // body contains recipient SLD
    any(recipients.to,
        strings.icontains(body.current_thread.text, .email.domain.sld)
    )
  )
  
  // negate highly trusted sender domains unless they fail DMARC authentication
  and (
    (
      sender.email.domain.root_domain in $high_trust_sender_root_domains
      and not headers.auth_summary.dmarc.pass
    )
    or sender.email.domain.root_domain not in $high_trust_sender_root_domains
  )
  and not profile.by_sender().any_messages_benign
tags:
  - "Attack surface reduction"
attack_types:
  - "BEC/Fraud"
  - "Credential Phishing"
  - "Spam"
tactics_and_techniques:
  - "Evasion"
  - "Social engineering"
detection_methods:
  - "Content analysis"
  - "Header analysis"
  - "Natural Language Understanding"
  - "Sender analysis"
id: "c2e18a57-1f52-544f-bb6d-a578e286cf89"