Link: Free Subdomain host with undisclosed recipients

 1name: "Link: Free Subdomain host with undisclosed recipients"
 2description: |
 3    Detects messages with undisclosed recipients, containing links to free subdomain hosts
 4type: "rule"
 5severity: "medium"
 6source: |
 7  type.inbound
 8  and any(body.links,
 9          .href_url.domain.root_domain in $free_subdomain_hosts
10          and .href_url.domain.subdomain is not null
11          and .href_url.domain.subdomain != "www"
12  )
13  and (length(recipients.to) == 0 or all(recipients.to, .display_name == "Undisclosed recipients"))
14  and length(recipients.cc) == 0
15  and length(recipients.bcc) == 0
16  
17  // negate listmailers & benign threads
18  and not (
19    any(headers.hops, any(.fields, .name == "List-Unsubscribe"))
20    or any(ml.nlu_classifier(body.current_thread.text).intents,
21           .name == "benign" and .confidence == "high"
22    )
23  )
24  and (
25    profile.by_sender().prevalence in ("new", "outlier")
26    or profile.by_sender().any_messages_malicious_or_spam
27  )
28  and not profile.by_sender().any_false_positives
29    
30tags:
31  - "Attack surface reduction"
32tactics_and_techniques:
33  - "Free subdomain host"
34detection_methods:
35  - "Header analysis"
36  - "URL analysis"
37id: "c23d979d-de80-599d-b0ca-c7fef217885e"