BEC/Fraud: Penpal scam

 1name: "BEC/Fraud: Penpal scam"
 2description: "This rule detects messages from individuals looking to establish contact under the guise of seeking friendship or a penpal relationship. Over time, they build trust and then exploit this relationship by asking for money, personal information, or involvement in suspicious activities."
 3type: "rule"
 4severity: "medium"
 5source: |
 6  type.inbound
 7  
 8  // the sender or the reply-to is a freemail provider
 9  and (
10    sender.email.domain.domain in $free_email_providers
11    or any(headers.reply_to,
12           .email.domain.root_domain in $free_email_providers
13           and not sender.email.domain.root_domain in $free_email_providers
14    )
15  )
16  
17  // body contains pen ?pal
18  and regex.contains(body.current_thread.text, 'pen\s?pal')
19  
20  // and NLU Request
21  and any(ml.nlu_classifier(body.current_thread.text).entities,
22          .name == "request"
23  )
24  
25  // not a reply
26  and (length(headers.references) == 0 or headers.in_reply_to is null)
27  
28  // negate highly trusted sender domains unless they fail DMARC authentication
29  and (
30    (
31      sender.email.domain.root_domain in $high_trust_sender_root_domains
32      and not headers.auth_summary.dmarc.pass
33    )
34    or sender.email.domain.root_domain not in $high_trust_sender_root_domains
35  )  
36attack_types:
37  - "BEC/Fraud"
38tactics_and_techniques:
39  - "Free email provider"
40  - "Social engineering"
41detection_methods:
42  - "Content analysis"
43  - "Header analysis"
44  - "Sender analysis"
45id: "a4bdfa17-7527-5ee2-a27b-44d03e190773"