Suspicious attachment with unscannable Cloudflare link

name: "Suspicious attachment with unscannable Cloudflare link"
description: "A PDF or Office document contains suspicious URLs that lead to Cloudflare-protected pages with turnstile CAPTCHA gates. The sender uses deceptive display names and subjects indicating urgency or authority."
type: "rule"
severity: "medium"
source: |
  type.inbound
  and any(attachments,
          (
            .file_extension in $file_extensions_macros
            or .file_extension == "pdf"
            or .file_type in ("pdf", "doc", "docx", "xls", "xlsx")
            or .content_type in ("application/pdf")
          )
          and any(file.explode(.),
                  // few links
                  0 < length(.scan.url.urls) < 20
                  // fewer unique root domain links
                  and length(distinct(.scan.url.urls, .domain.root_domain)) < 10
                  // sender domain matches no body domains
                  and all(.scan.url.urls,
                          .domain.root_domain != sender.email.domain.root_domain
                  )
          )
  )
  
  // negate bouncebacks and undeliverables
  and not any(attachments,
              .content_type in (
                "message/global-delivery-status",
                "message/delivery-status"
              )
  )
  
  // suspicious subject or display name
  and (
    regex.icontains(subject.subject,
                    "termination.*notice",
                    "38417",
                    ":completed",
                    "[il1]{2}mit.*ma[il1]{2} ?bo?x",
                    "[il][il][il]egai[ -]",
                    "[li][li][li]ega[li] attempt",
                    "[ng]-?[io]n .*block",
                    "[ng]-?[io]n .*cancel",
                    "[ng]-?[io]n .*deactiv",
                    "[ng]-?[io]n .*disabl",
                    "action.*required",
                    "abandon.*package",
                    "about.your.account",
                    "acc(ou)?n?t (is )?on ho[li]d",
                    "acc(ou)?n?t.*terminat",
                    "acc(oun)?t.*[il1]{2}mitation",
                    "access.*limitation",
                    "account (will be )?block",
                    "account.*de-?activat",
                    "account.*locked",
                    "account.*re-verification",
                    "account.*security",
                    "account.*suspension",
                    "account.has.been",
                    "account.has.expired",
                    "account.will.be.blocked",
                    "account v[il]o[li]at",
                    "activity.*acc(oun)?t",
                    "almost.full",
                    "app[li]e.[il]d",
                    "authenticate.*account",
                    "been.*suspend",
                    "clos.*of.*account.*processed",
                    "confirm.your.account",
                    "courier.*able",
                    "crediential.*notif",
                    "deactivation.*in.*progress",
                    "delivery.*attempt.*failed",
                    "document.received",
                    "documented.*shared.*with.*you",
                    "dropbox.*document",
                    "e-?ma[il1]+ .{010}suspen",
                    "e-?ma[il1]{1} user",
                    "e-?ma[il1]{2} acc",
                    "e-?ma[il1]{2}.*up.?grade",
                    "e.?ma[il1]{2}.*server",
                    "e.?ma[il1]{2}.*suspend",
                    "email.update",
                    "faxed you",
                    "fraud(ulent)?.*charge",
                    "from.helpdesk",
                    "fu[il1]{2}.*ma[il1]+[ -]?box",
                    "has.been.*suspended",
                    "has.been.limited",
                    "have.locked",
                    "he[li]p ?desk upgrade",
                    "heipdesk",
                    "i[il]iega[il]",
                    "ii[il]ega[il]",
                    "incoming e?mail",
                    "incoming.*fax",
                    "lock.*security",
                    "ma[il1]{1}[ -]?box.*quo",
                    "ma[il1]{2}[ -]?box.*fu[il1]",
                    "ma[il1]{2}box.*[il1]{2}mit",
                    "ma[il1]{2}box stor",
                    "mail on.?hold",
                    "mail.*box.*migration",
                    "mail.*de-?activat",
                    "mail.update.required",
                    "mails.*pending",
                    "messages.*pending",
                    "missed.*shipping.*notification",
                    "missed.shipment.notification",
                    "must.update.your.account",
                    "new [sl][io]g?[nig][ -]?in from",
                    "new voice ?-?mail",
                    "notifications.*pending",
                    "office.*3.*6.*5.*suspend",
                    "office365",
                    "on google docs with you",
                    "online doc",
                    "password.*compromised",
                    "periodic maintenance",
                    "potential(ly)? unauthorized",
                    "refund not approved",
                    "report",
                    "revised.*policy",
                    "scam",
                    "scanned.?invoice",
                    "secured?.update",
                    "security breach",
                    "securlty",
                    "signed.*delivery",
                    "statement is ready",
                    "status of your .{314}? ?delivery",
                    "susp[il1]+c[il1]+ous.*act[il1]+v[il1]+ty",
                    "suspicious.*sign.*[io]n",
                    "suspicious.activit",
                    "temporar(il)?y deactivate",
                    "temporar[il1]{2}y disab[li]ed",
                    "temporarily.*lock",
                    "un-?usua[li].activity",
                    "unable.*deliver",
                    "unauthorized.*activit",
                    "unauthorized.device",
                    "undelivered message",
                    "unread.*doc",
                    "unusual.activity",
                    "upgrade.*account",
                    "upgrade.notice",
                    "urgent message",
                    "urgent.verification",
                    "v[il1]o[li1]at[il1]on security",
                    "va[il1]{1}date.*ma[il1]{2}[ -]?box",
                    "verification ?-?require",
                    "verification( )?-?need",
                    "verify.your?.account",
                    "web ?-?ma[il1]{2}",
                    "web[ -]?ma[il1]{2}",
                    "will.be.suspended",
                    "your (customer )?account .as",
                    "your.office.365",
                    "your.online.access"
    )
    or any($suspicious_subjects, strings.icontains(subject.subject, .))
    or regex.icontains(sender.display_name,
                       "Admin",
                       "Administrator",
                       "Alert",
                       "Assistant",
                       "Billing",
                       "Benefits",
                       "Bonus",
                       "CEO",
                       "CFO",
                       "CIO",
                       "CTO",
                       "Chairman",
                       "Claim",
                       "Confirm",
                       "Critical",
                       "Customer Service",
                       "Deal",
                       "Discount",
                       "Director",
                       "Exclusive",
                       "Executive",
                       "Fax",
                       "Free",
                       "Gift",
                       "/bHR/b",
                       "Helpdesk",
                       "Human Resources",
                       "Immediate",
                       "Important",
                       "Info",
                       "Information",
                       "Invoice",
                       '\bIT\b',
                       "Legal",
                       "Lottery",
                       "Management",
                       "Manager",
                       "Member Services",
                       "Notification",
                       "Offer",
                       "Operations",
                       "Order",
                       "Partner",
                       "Payment",
                       "Payroll",
                       "President",
                       "Premium",
                       "Prize",
                       "Receipt",
                       "Refund",
                       "Registrar",
                       "Required",
                       "Reward",
                       "Sales",
                       "Secretary",
                       "Security",
                       "Service",
                       "Signature",
                       'SSA?\.gov',
                       "Storage",
                       "Support",
                       "Sweepstakes",
                       "System",
                       "Tax",
                       "Tech Support",
                       "Update",
                       "Upgrade",
                       "Urgent",
                       "Validate",
                       "Verify",
                       "VIP",
                       "Webmaster",
                       "Winner",
    )
    or any(attachments,
           (
             .file_extension in $file_extensions_macros
             or .file_extension == "pdf"
             or .file_type in ("pdf", "doc", "docx", "xls", "xlsx")
             or .content_type in ("application/pdf")
           )
           and any(file.explode(.),
                   any(.scan.url.urls, strings.ends_with(.url, ".exe"))
                   or any(ml.nlu_classifier(.scan.ocr.raw).intents,
                          .name == "cred_theft"
                   )
           )
    )
  )
  and any(attachments,
          (
            .file_extension in $file_extensions_macros
            or .file_extension == "pdf"
            or .file_type in ("pdf", "doc", "docx", "xls", "xlsx")
            or .content_type in ("application/pdf")
          )
          and any(file.explode(.),
                  any(.scan.url.urls,
                      (
                        strings.icontains(ml.link_analysis(., mode="aggressive").final_dom.display_text,
                                          "cloudflare"
                        )
                        // includes the turnstile CAPTCHA
                        or (
                          strings.icontains(ml.link_analysis(., mode="aggressive").final_dom.raw,
                                            'https://challenges.cloudflare.com/turnstile/'
                          )
                          // has a short body length indicating the page is gated behind the turnstile instead
                          // of just including the turnstile
                          and length((
                                       ml.link_analysis(., mode="aggressive").final_dom.display_text
                                     )
                          ) < 200
                        )
                      )
                      and not (
                        ( // a Cloudflare error page
                          strings.ilike(ml.link_analysis(., mode="aggressive").final_dom.display_text,
                                        "*error code*"
                          )
                          and any(ml.link_analysis(., mode="aggressive").final_dom.links,
                                  strings.icontains(.href_url.query_params,
                                                    "utm_source=errorcode"
                                  )
                          )
                        ) // a cookie warning mentioning Cloudflare
                        or regex.icontains(ml.link_analysis(., mode="aggressive").final_dom.display_text,
                                           "cookie.{0,50}Cloudflare"
                        )
                        or ml.link_analysis(., mode="aggressive").effective_url.domain.root_domain in (
                          "marketbeat.com"
                        )
                      )
                  )
          )
  )
  and (
    not profile.by_sender().solicited
    or (
      profile.by_sender().any_messages_malicious_or_spam
      and not profile.by_sender().any_messages_benign
    )
  )
  
  // 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:
  - "Credential Phishing"
tactics_and_techniques:
  - "Evasion"
  - "PDF"
  - "Social engineering"
  - "Impersonation: Employee"
  - "Impersonation: VIP"
detection_methods:
  - "File analysis"
  - "URL analysis"
  - "Sender analysis"
  - "Content analysis"
  - "Header analysis"
  - "Natural Language Understanding"
id: "00f92b6f-7449-5c93-ba29-b406c57bf121"