Credential phishing: Generic document sharing

  1name: "Credential phishing: Generic document sharing"
  2description: |
  3  Detects credential phishing attempts using generic document sharing language
  4  where the sender claims to have sent a document for review, but the link
  5  doesn't point to legitimate file sharing services.  
  6type: "rule"
  7severity: "medium"
  8source: |
  9  type.inbound
 10  // exclude if it's a reply to an existing conversation
 11  and not length(body.previous_threads) > 0
 12  and (
 13    // subject contains document sharing language
 14    regex.icontains(subject.subject,
 15                    '\b(has\s+sent\s+you|sent\s+you|shared\s+with\s+you|document\s+to\s+review|file\s+to\s+review|proposal\s+document|new\s+document|document\s+.{0,20}assigned)\b'
 16    )
 17    or strings.icontains(subject.subject, 'document to review')
 18    or strings.icontains(subject.subject, 'file to review')
 19    or strings.icontains(subject.subject, 'sent you')
 20  )
 21  and (
 22    // body contains document sharing language
 23    regex.icontains(body.current_thread.text,
 24                    '\b(document\s+I\s+sent|proposal\s+document|see\s+the\s+below|document.*review|file.*review|let\s+me\s+know\s+what\s+you\s+think|shared.{0,50}document)\b'
 25    )
 26    or strings.icontains(body.current_thread.text, 'document I sent')
 27    or strings.icontains(body.current_thread.text, 'proposal document')
 28    or strings.icontains(body.current_thread.text, 'let me know what you think')
 29    // account for image-as-content
 30    or (
 31      length(body.current_thread.text) < 10
 32      and (
 33        regex.icontains(beta.ocr(file.message_screenshot()).text,
 34                        '\b(document\s+I\s+sent|proposal\s+document|see\s+the\s+below|document.*review|file.*review|let\s+me\s+know\s+what\s+you\s+think|shared.{0,50}document)\b'
 35        )
 36        or strings.icontains(beta.ocr(file.message_screenshot()).text,
 37                             'document I sent'
 38        )
 39        or strings.icontains(beta.ocr(file.message_screenshot()).text,
 40                             'proposal document'
 41        )
 42        or strings.icontains(beta.ocr(file.message_screenshot()).text,
 43                             'let me know what you think'
 44        )
 45      )
 46    )
 47  )
 48  // has links that look like file attachments but aren't
 49  and any(body.links,
 50          // display text looks like a file
 51          (
 52            regex.icontains(.display_text,
 53                            '\.(pdf|doc|docx|goto|xls|xlsx|ppt|pptx)'
 54            )
 55            or regex.icontains(.display_text, '\d+kb|\d+mb')
 56            or strings.icontains(.display_text, 'document')
 57            or strings.icontains(.display_text, 'proposal')
 58            or strings.icontains(.display_text, 'review')
 59            // account for image-as-content
 60            or (length(body.current_thread.text) < 10 and length(body.links) == 1)
 61          )
 62          // but the URL doesn't point to legitimate file sharing
 63          and .href_url.domain.root_domain not in (
 64            "sharepoint.com",
 65            "google.com",
 66            "drive.google.com",
 67            "dropbox.com",
 68            "box.com",
 69            "onedrive.com",
 70            "1drv.ms",
 71            "aka.ms",
 72            "microsoft.com",
 73            "office.com",
 74            "docusign.com",
 75            "adobesign.com",
 76            "hellosign.com",
 77            "signable.app"
 78          )
 79          // and points to suspicious domains
 80          and (
 81            .href_url.domain.tld in $suspicious_tlds
 82            or .href_url.domain.root_domain in $url_shorteners
 83            or .href_url.domain.domain in $url_shorteners
 84            or .href_url.domain.root_domain in $free_file_hosts
 85            or .href_url.domain.domain in $free_file_hosts
 86            // or it's a forms/survey platform being abused in self_service_creation_platform_domains
 87            or .href_url.domain.root_domain in $self_service_creation_platform_domains
 88            or .href_url.domain.domain in $self_service_creation_platform_domains
 89          )
 90  )
 91  // negate highly trusted sender domains unless they fail DMARC authentication
 92  and (
 93    (
 94      sender.email.domain.root_domain in $high_trust_sender_root_domains
 95      and not headers.auth_summary.dmarc.pass
 96    )
 97    or sender.email.domain.root_domain not in $high_trust_sender_root_domains
 98  )
 99  and (
100    profile.by_sender().solicited == false
101    or profile.by_sender_email().prevalence == "new"
102    or profile.by_sender_email().days_since.last_contact > 30
103    or (
104      profile.by_sender().any_messages_malicious_or_spam
105      and not profile.by_sender().any_messages_benign
106    )
107    // or it's a spoof of the org_domain
108    or (
109      sender.email.domain.domain in $org_domains
110      and not (
111        headers.auth_summary.spf.pass
112        or coalesce(headers.auth_summary.dmarc.pass, false)
113      )
114    )
115  )
116  and not profile.by_sender().any_messages_benign
117    
118attack_types:
119  - "Credential Phishing"
120  - "BEC/Fraud"
121tactics_and_techniques:
122  - "Social engineering"
123  - "Evasion"
124  - "Impersonation: Employee"
125detection_methods:
126  - "Content analysis"
127  - "Natural Language Understanding"
128  - "URL analysis"
129  - "Sender analysis"
130id: "9f0e1d2c-3b4a-5c6d-7e8f-9a0b1c2d3e4f"