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 (length(headers.references) > 0 or headers.in_reply_to is not null)
22    )
23    or (
24      // calendar invite responses
25      regex.icontains(subject.base,
26                      '(?:Accepted|Declined|New Time Proposed|Tentative):'
27      )
28      and any(attachments, .content_type == "text/calendar")
29      and profile.by_sender_email().solicited
30    )
31  )
32  and (
33    not profile.by_sender().solicited
34    or (
35      profile.by_sender().any_messages_malicious_or_spam
36      and not profile.by_sender().any_messages_benign
37    )
38  )
39  
40  // negate highly trusted sender domains unless they fail DMARC authentication
41  and not (
42    sender.email.domain.root_domain in $high_trust_sender_root_domains
43    and coalesce(headers.auth_summary.dmarc.pass, false)
44  )  
45attack_types:
46  - "BEC/Fraud"
47tactics_and_techniques:
48  - "Social engineering"
49detection_methods:
50  - "Content analysis"
51  - "Header analysis"
52  - "Natural Language Understanding"
53  - "Sender analysis"
54id: "96d4c35a-ca53-559e-9db3-349dbbdffc20"