Callback phishing via SignFree e-signature request
This rule inspects messages originating from legitimate SignFree infrastructure, with content matching Callback Phishing criteria, in the body, requiring at least one brand name, as well as 3 matching Callback Phishing terms and a phone number.
Sublime rule (View on GitHub)
1name: "Callback phishing via SignFree e-signature request"
2description: "This rule inspects messages originating from legitimate SignFree infrastructure, with content matching Callback Phishing criteria, in the body, requiring at least one brand name, as well as 3 matching Callback Phishing terms and a phone number."
3type: "rule"
4severity: "high"
5source: |
6 type.inbound
7 and length(attachments) == 0
8 and (
9 not beta.profile.by_reply_to().solicited
10 or (
11 beta.profile.by_reply_to().any_messages_malicious_or_spam
12 and not beta.profile.by_reply_to().any_messages_benign
13 )
14 )
15 // Legitimate SignFree sending infratructure
16 and sender.email.domain.root_domain == 'signfree.io'
17 and (headers.auth_summary.spf.pass or headers.auth_summary.dmarc.pass)
18 and (
19 // this section is synced with attachment_callback_phish_with_pdf.yml and attachment_callback_phish_with_img.yml
20 regex.icontains(strings.replace_confusables(body.current_thread.text),
21 '(p.{0,3}a.{0,3}y.{0,3}p.{0,3}a.{0,3}l|ma?c.?fee|n[o0]rt[o0]n|geek.{0,5}squad|ebay|symantec|best buy|lifel[o0]c|secure anywhere|starz|utilities premium|pc security|at&t)'
22 )
23 or any(ml.logo_detect(file.message_screenshot()).brands,
24 .name in (
25 "PayPal",
26 "Norton",
27 "GeekSquad",
28 "Ebay",
29 "McAfee",
30 "AT&T",
31 "Microsoft"
32 )
33 )
34 )
35 and (
36 (
37 // this seciton is synced with attachment_callback_phish_with_img.yml and attachment_callback_phish_with_pdf.yml
38 // however, the 3 of logic and requiring a phone number is specific to this rule in order to reduce FPs
39 // caused by messages which mention cancelling or otherwise managing a subscription
40 // it is also synced and below for message_screenshot OCR output
41 3 of (
42 strings.icontains(body.current_thread.text, 'purchase'),
43 strings.icontains(body.current_thread.text, 'payment'),
44 strings.icontains(body.current_thread.text, 'transaction'),
45 strings.icontains(body.current_thread.text, 'subscription'),
46 strings.icontains(body.current_thread.text, 'antivirus'),
47 strings.icontains(body.current_thread.text, 'order'),
48 strings.icontains(body.current_thread.text, 'support'),
49 strings.icontains(body.current_thread.text, 'help line'),
50 strings.icontains(body.current_thread.text, 'receipt'),
51 strings.icontains(body.current_thread.text, 'invoice'),
52 strings.icontains(body.current_thread.text, 'call'),
53 strings.icontains(body.current_thread.text, 'cancel'),
54 strings.icontains(body.current_thread.text, 'renew'),
55 strings.icontains(body.current_thread.text, 'refund'),
56 regex.icontains(body.current_thread.text, "(?:reach|contact) us at"),
57 strings.icontains(body.current_thread.text, "+1"),
58 strings.icontains(body.current_thread.text, "amount"),
59 strings.icontains(body.current_thread.text, "charged"),
60 strings.icontains(body.current_thread.text, "crypto"),
61 strings.icontains(body.current_thread.text, "wallet address"),
62 regex.icontains(body.current_thread.text, '\$\d{3}\.\d{2}\b'),
63 )
64 // phone number regex
65 and regex.icontains(body.current_thread.text,
66 '\+?([ilo0-9]{1}.)?\(?[ilo0-9]{3}?\)?.[ilo0-9]{3}.?[ilo0-9]{4}',
67 '\+?([ilo0-9]{1,2})?\s?\(?\d{3}\)?[\s\.\-⋅]{0,5}[ilo0-9]{3}[\s\.\-⋅]{0,5}[ilo0-9]{4}'
68 )
69 )
70 or (
71 any(file.explode(file.message_screenshot()),
72 // this seciton is synced with attachment_callback_phish_with_img.yml and attachment_callback_phish_with_pdf.yml
73 // and above for current_thread.text
74 3 of (
75 strings.icontains(.scan.ocr.raw, 'purchase'),
76 strings.icontains(.scan.ocr.raw, 'payment'),
77 strings.icontains(.scan.ocr.raw, 'transaction'),
78 strings.icontains(.scan.ocr.raw, 'subscription'),
79 strings.icontains(.scan.ocr.raw, 'antivirus'),
80 strings.icontains(.scan.ocr.raw, 'order'),
81 strings.icontains(.scan.ocr.raw, 'support'),
82 strings.icontains(.scan.ocr.raw, 'help line'),
83 strings.icontains(.scan.ocr.raw, 'receipt'),
84 strings.icontains(.scan.ocr.raw, 'invoice'),
85 strings.icontains(.scan.ocr.raw, 'call'),
86 strings.icontains(.scan.ocr.raw, 'helpdesk'),
87 strings.icontains(.scan.ocr.raw, 'cancel'),
88 strings.icontains(.scan.ocr.raw, 'renew'),
89 strings.icontains(.scan.ocr.raw, 'refund'),
90 regex.icontains(.scan.ocr.raw, "(?:reach|contact) us at"),
91 strings.icontains(.scan.ocr.raw, '+1'),
92 strings.icontains(.scan.ocr.raw, 'amount'),
93 strings.icontains(.scan.ocr.raw, 'charged'),
94 strings.icontains(.scan.ocr.raw, 'crypto'),
95 strings.icontains(.scan.ocr.raw, 'wallet address'),
96 regex.icontains(.scan.ocr.raw, '\$\d{3}\.\d{2}\b'),
97 )
98 // phone number regex
99 and regex.icontains(.scan.ocr.raw,
100 '\+?([ilo0-9]{1}.)?\(?[ilo0-9]{3}?\)?.[ilo0-9]{3}.?[ilo0-9]{4}',
101 '\+?([ilo0-9]{1,2})?\s?\(?\d{3}\)?[\s\.\-⋅]{0,5}[ilo0-9]{3}[\s\.\-⋅]{0,5}[ilo0-9]{4}'
102 )
103
104 // negate messages with previous threads. While callback phishing with thread hijacking or with current_thread
105 // padded with whitespace and previous threads in the message has been observed, the intetion of using OCR is for image embedded callbacks
106 and not regex.icount(.scan.ocr.raw, '(?:from|to|sent|date|cc|subject):') > 3
107 // this notation of previous threads often only occurs once
108 and not regex.icontains(.scan.ocr.raw, 'wrote:[\r\n]')
109 )
110 )
111 )
112
113attack_types:
114 - "Callback Phishing"
115tactics_and_techniques:
116 - "Exploit"
117 - "Impersonation: Brand"
118 - "Out of band pivot"
119 - "Social engineering"
120detection_methods:
121 - "Computer Vision"
122 - "Content analysis"
123 - "Header analysis"
124 - "Sender analysis"
125 - "URL analysis"
126id: "21381c37-ecc7-5c3c-b7ab-87e936f4b048"