Callback phishing via Microsoft comment

calendar Mar 26, 2026  ·
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Detects callback scam messages originating from legitimate Microsoft infrastructure but containing fraudulent content designed to trick recipients into calling scammer phone numbers. The message includes typical callback phishing language around purchases, payments, subscriptions, or support services along with embedded phone numbers, while passing Microsoft's authentication checks.

Sublime rule (View on GitHub)

 1name: "Callback phishing via Microsoft comment"
 2description: "Detects callback scam messages originating from legitimate Microsoft infrastructure but containing fraudulent content designed to trick recipients into calling scammer phone numbers. The message includes typical callback phishing language around purchases, payments, subscriptions, or support services along with embedded phone numbers, while passing Microsoft's authentication checks."
 3type: "rule"
 4severity: "medium"
 5source: |
 6  type.inbound
 7  and length(attachments) == 0
 8  
 9  // Legitimate MicrosoftOnline sending infrastructure
10  // or invites@microsoft.com abuse
11  and (
12    (
13      sender.email.domain.root_domain in ('microsoftonline.com')
14      or sender.email.email == "invites@microsoft.com"
15    )
16  
17    // Callback Phishing
18    and (
19      any(ml.nlu_classifier(body.current_thread.text).intents,
20          .name in ("callback_scam")
21          and .confidence in ("medium", "high")
22          and length(body.current_thread.text) < 1750
23      )
24      or 3 of (
25        strings.ilike(body.current_thread.text, '*purchase*'),
26        strings.ilike(body.current_thread.text, '*payment*'),
27        strings.ilike(body.current_thread.text, '*transaction*'),
28        strings.ilike(body.current_thread.text, '*subscription*'),
29        strings.ilike(body.current_thread.text, '*antivirus*'),
30        strings.ilike(body.current_thread.text, '*order*'),
31        strings.ilike(body.current_thread.text, '*support*'),
32        strings.ilike(body.current_thread.text, '*help line*'),
33        strings.ilike(body.current_thread.text, '*receipt*'),
34        strings.ilike(body.current_thread.text, '*invoice*'),
35        strings.ilike(body.current_thread.text, '*call*'),
36        strings.ilike(body.current_thread.text, '*cancel*'),
37        strings.ilike(body.current_thread.text, '*renew*'),
38        strings.ilike(body.current_thread.text, '*refund*')
39      )
40    )
41    // phone number regex
42    and any([body.current_thread.text, subject.subject],
43            regex.icontains(.,
44                            '\+?([ilo0-9]{1}.)?\(?[ilo0-9]{3}?\)?.[ilo0-9]{3}.?[ilo0-9]{4}',
45                            '\+?([ilo0-9]{1,2})?\s?\(?\d{3}\)?[\s\.\-⋅]{0,5}[ilo0-9]{3}[\s\.\-⋅]{0,5}[ilo0-9]{4}'
46            )
47    )
48  )  
49
50attack_types:
51  - "Callback Phishing"
52tactics_and_techniques:
53  - "Impersonation: Brand"
54  - "Out of band pivot"
55  - "Social engineering"
56detection_methods:
57  - "Content analysis"
58  - "Header analysis"
59  - "Natural Language Understanding"
60  - "Sender analysis"
61id: "8346c7b9-1b46-50e7-b04e-b32969db8737"
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