Business Email Compromise (BEC) attempt from untrusted sender

 1name: "Business Email Compromise (BEC) attempt from untrusted sender"
 2description: |
 3    Detects potential Business Email Compromise (BEC) attacks by analyzing text within the email body from first-time senders.
 4type: "rule"
 5severity: "medium"
 6source: |
 7  type.inbound
 8  and any(ml.nlu_classifier(body.current_thread.text).intents,
 9          .name in ("bec") and .confidence == "high"
10  )
11  // negating legit replies
12  and not (
13    (
14      (
15        subject.is_reply
16        or subject.is_forward
17        // out of office auto-reply
18        // the NLU model will handle these better natively soon
19        or strings.istarts_with(subject.subject, "Automatic reply:")
20      )
21      and (
22        length(headers.references) > 0
23        or headers.in_reply_to is not null
24      )
25    )
26    or (
27      // calendar invite responses
28      regex.icontains(subject.base, '(?:Accepted|Declined|New Time Proposed|Tentative):')
29      and any(attachments, .content_type == "text/calendar")
30      and profile.by_sender_email().solicited
31    )
32  )
33  and (
34    not profile.by_sender().solicited
35    or (
36      profile.by_sender().any_messages_malicious_or_spam
37      and not profile.by_sender().any_messages_benign
38    )
39  )
40  
41  // negate highly trusted sender domains unless they fail DMARC authentication
42  and (
43    (
44      sender.email.domain.root_domain in $high_trust_sender_root_domains
45      and not headers.auth_summary.dmarc.pass
46    )
47    or sender.email.domain.root_domain not in $high_trust_sender_root_domains
48  )  
49
50attack_types:
51  - "BEC/Fraud"
52tactics_and_techniques:
53  - "Social engineering"
54detection_methods:
55  - "Content analysis"
56  - "Header analysis"
57  - "Natural Language Understanding"
58  - "Sender analysis"
59id: "96d4c35a-ca53-559e-9db3-349dbbdffc20"