Attachment: Encrypted PDF with credential theft body

 1name: "Attachment: Encrypted PDF with credential theft body"
 2description: "Attached PDF is encrypted, and email body contains credential theft language. Seen in-the-wild impersonating e-fax services."
 3type: "rule"
 4severity: "medium"
 5source: |
 6  type.inbound
 7  and any(filter(attachments, .file_type == "pdf"),
 8          any(file.explode(.),
 9              any(.scan.exiftool.fields, .key == "Encryption")
10              or (
11                .scan.entropy.entropy > 7
12                and any(.scan.strings.strings, strings.icontains(., "/Encrypt"))
13              )
14          )
15          // Encrypted PDFs do not have child nodes with any data
16          and all(filter(file.explode(.), .depth > 0), .size == 0)
17  )
18  and (
19    any(ml.nlu_classifier(body.current_thread.text).intents,
20        .name == "cred_theft" and .confidence in ("medium", "high")
21    )
22    or any(ml.nlu_classifier(beta.ocr(file.message_screenshot()).text).intents,
23           .name == "cred_theft" and .confidence in ("medium", "high")
24    )
25    or (
26      (
27        regex.icontains(body.current_thread.text,
28                        'PDF\s*(?:Access|Preview|Unlock|Decrypt)\s*(?:Pass)?code',
29                        '(Access|Preview|Unlock|Decrypt|Pass)\s*(?:word|code)\s*to?\s*PDF\s*is?\s*:'
30        )
31        or (
32          (
33            length(body.current_thread.text) <= 10
34            or (body.current_thread.text is null)
35          )
36          and any(body.previous_threads,
37                  regex.icontains(.text,
38                                  'PDF\s*(?:Access|Preview|Unlock|Decrypt)\s*(?:Pass)?code',
39                                  '(Access|Preview|Unlock|Decrypt|Pass)\s*(?:word|code)\s*to?\s*PDF\s*is?\s*:'
40                  )
41          )
42        )
43      )
44    )
45  )
46  // not forwards/replies
47  and not (
48    (length(headers.references) > 0 or headers.in_reply_to is not null)
49    and (subject.is_forward or subject.is_reply)
50    and length(body.previous_threads) >= 1
51  )
52  and (
53    (
54      profile.by_sender_email().prevalence in ("new", "outlier")
55      and not profile.by_sender_email().solicited
56    )
57    or (
58      profile.by_sender_email().any_messages_malicious_or_spam
59      and not profile.by_sender_email().any_messages_benign
60    )
61    or (
62      length(recipients.to) == 0
63      or all(recipients.to,
64             strings.ilike(.display_name, "undisclosed?recipients")
65      )
66    )
67    or (
68      length(recipients.to) == 1
69      and any(recipients.to, .email.email == sender.email.email)
70    )
71  )
72  // negate highly trusted sender domains unless they fail DMARC authentication
73  and (
74    (
75      sender.email.domain.root_domain in $high_trust_sender_root_domains
76      and not headers.auth_summary.dmarc.pass
77    )
78    or sender.email.domain.root_domain not in $high_trust_sender_root_domains
79  )  
80attack_types:
81  - "Credential Phishing"
82tactics_and_techniques:
83  - "Encryption"
84  - "Evasion"
85  - "PDF"
86  - "Social engineering"
87detection_methods:
88  - "Content analysis"
89  - "Exif analysis"
90  - "File analysis"
91  - "Natural Language Understanding"
92  - "Sender analysis"
93id: "c9596c9a-0465-5364-8523-542e6d25a8f7"