Attachment: RFC822 containing suspicious file sharing language with links from untrusted sender

calendar Sep 29, 2026  ·
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This rule identifies messages with an RFC822 attachment contains language indicative of suspicious file-sharing activity. It checks both the original sender and the nested sender against highly trusted domains. The original message is unsolicited, and has not been previously flagged as a false positive.

Sublime rule (View on GitHub)

 1name: "Attachment: RFC822 containing suspicious file sharing language with links from untrusted sender"
 2description: "This rule identifies messages with an RFC822 attachment contains language indicative of suspicious file-sharing activity. It checks both the original sender and the nested sender against highly trusted domains. The original message is unsolicited, and has not been previously flagged as a false positive."
 3type: "rule"
 4severity: "medium"
 5source: |
 6  type.inbound
 7  and any(attachments,
 8          .file_type == "unknown"
 9          and (.content_type == "message/rfc822" or .file_extension in ('eml'))
10          and regex.contains(file.parse_eml(.).subject.subject,
11                             '(shared.{0,30}with you|View Document)'
12          )
13          and 0 < length(file.parse_eml(.).body.links) < 10
14          and file.parse_eml(.).sender.email.email not in $recipient_emails
15  
16          // exclude bounce backs & read receipts
17          and not strings.like(file.parse_eml(.).sender.email.local_part,
18                               "*postmaster*",
19                               "*mailer-daemon*",
20                               "*administrator*"
21          )
22          and not regex.imatch(file.parse_eml(.).subject.subject,
23                               "(undeliverable|read:).*"
24          )
25          and not any(file.parse_eml(.).attachments,
26                      .content_type == "message/delivery-status"
27          )
28  
29          // negate highly trusted sender domains in the nested eml unless they fail DMARC
30          and not (
31            file.parse_eml(.).sender.email.domain.root_domain in $high_trust_sender_root_domains
32            and coalesce(file.parse_eml(.).headers.auth_summary.dmarc.pass, false)
33          )
34          // negate org domains that passed dmarc
35          and not file.parse_eml(.).sender.email.domain.root_domain in $org_domains
36  )
37  
38  // negate highly trusted sender domains unless they fail DMARC authentication
39  and not (
40    sender.email.domain.root_domain in $high_trust_sender_root_domains
41    and coalesce(headers.auth_summary.dmarc.pass, false)
42  )
43  and (
44    not profile.by_sender().solicited
45    or (
46      profile.by_sender().any_messages_malicious_or_spam
47      and not profile.by_sender().any_messages_benign
48    )
49  )
50  and not profile.by_sender().any_messages_benign  
51
52attack_types:
53  - "Credential Phishing"
54tactics_and_techniques:
55  - "Evasion"
56  - "Social engineering"
57detection_methods:
58  - "File analysis"
59  - "Header analysis"
60  - "Natural Language Understanding"
61  - "Sender analysis"
62id: "d96854d7-d0a2-5342-a363-cee1ad51e7c9"
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