Link: PDF and financial display text to free file host

name: "Link: PDF and financial display text to free file host"
description: "Detects messages containing a single link with PDF-named display text containing financial phrases that redirects to a free file hosting service, sent without previous message threads."
type: "rule"
severity: "high"
source: |
  type.inbound
  and length(body.links) < 20
  
  // the message does not contain previous threads
  and length(body.previous_threads) == 0
  
  // no PDF attachments
  and length(filter(attachments, .file_type == "pdf")) == 0
  // there is only a single link to the free file host
  and length(filter(body.links,
                    .href_url.domain.domain in $free_file_hosts
                    or .href_url.domain.root_domain in $free_file_hosts
                    or .href_url.domain.domain in $self_service_creation_platform_domains
                    or .href_url.domain.root_domain in $self_service_creation_platform_domains
                    or .href_url.domain.domain in $url_shorteners
                    or .href_url.domain.root_domain in $url_shorteners
                    or .href_url.domain.root_domain == "dynamics.com"
             )
  ) == 1
  // there are few distinct domains in the message
  and length(distinct(body.links, .href_url.domain.root_domain)) <= 3
  
  // the display_text ends in .pdf and goes to a free file host
  and any(body.links,
          strings.iends_with(.display_text, '.pdf')
          and (
            .href_url.domain.domain in $free_file_hosts
            or .href_url.domain.root_domain in $free_file_hosts
            or .href_url.domain.domain in $self_service_creation_platform_domains
            or .href_url.domain.root_domain in $self_service_creation_platform_domains
            or .href_url.domain.domain in $url_shorteners
            or .href_url.domain.root_domain in $url_shorteners
            or .href_url.domain.root_domain == "dynamics.com"
          )
          // the display text is financial related (remittance, invoice, etc)
          and (
            strings.icontains(.display_text, 'payment')
            or regex.icontains(.display_text, 'pay\b')
            or strings.icontains(.display_text, 'remit')
            or strings.icontains(.display_text, 'receipt')
            or strings.icontains(.display_text, 'Distribution')
            or strings.icontains(.display_text, 'payoff')
            or strings.icontains(.display_text, 'Wire Instructions')
            or regex.icontains(.display_text, 'ACH\b')
            or regex.icontains(.display_text, 'EFT\b')
            or strings.istarts_with(.display_text, 'INV')
            or strings.istarts_with(.display_text, 'View RFQ')
            or strings.istarts_with(.display_text, 'Contract')
  
            // the display text is the subject
            or (.display_text =~ subject.base and length(.display_text) > 0)
          )
  
          // negate links which make use of google icons inside of a bounding box
          // filter down to the link with the same display text
          and not any(filter(html.xpath(body.html,
                                        '//a[img[@src] or .//img[@src]][.//div[contains(@style, "border:1px solid")] or ancestor::div[contains(@style, "border:1px solid")]]'
                             ).nodes,
                             // the display text is the link we're inspecting
                             ..display_text == .display_text
                      ),
                      // inside this is a reference to the google icon 
                      strings.icontains(.raw, 'gstatic.com/docs/doclist/images/')
          )
  )

attack_types:
  - "Credential Phishing"
tactics_and_techniques:
  - "Free file host"
  - "Free email provider"
  - "Social engineering"
detection_methods:
  - "Content analysis"
  - "Sender analysis"
  - "URL analysis"
id: "b010740b-a462-5dcd-acf9-877783a84534"