Service abuse: DocSend share from newly registered domain

calendar Jan 12, 2026 · Attack surface reduction  ·
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This Attack Surface Reduction (ASR) rule matches on DocSend notifications with recently registered reply-to domains.

Sublime rule (View on GitHub)

 1name: "Service abuse: DocSend share from newly registered domain"
 2description: "This Attack Surface Reduction (ASR) rule matches on DocSend notifications with recently registered reply-to domains."
 3type: "rule"
 4severity: "high"
 5source: |
 6  type.inbound
 7  
 8  // Legitimate DocSend sending infratructure
 9  and sender.email.email == "no-reply@docsend.com"
10  and headers.auth_summary.spf.pass
11  and headers.auth_summary.dmarc.pass
12  
13  // the message needs to have a reply-to address
14  and length(headers.reply_to) > 0
15  
16  // reply-to email address has never received an email from your org
17  and not any(headers.reply_to, .email.email in $recipient_emails)
18  
19  // new reply-to
20  and any(headers.reply_to, network.whois(.email.domain).days_old < 30)  
21tags:
22 - "Attack surface reduction"
23attack_types:
24  - "BEC/Fraud"
25  - "Credential Phishing"
26tactics_and_techniques:
27  - "Evasion"
28  - "Free file host"
29  - "Impersonation: Brand"
30  - "Social engineering"
31detection_methods:
32  - "Content analysis"
33  - "Header analysis"
34  - "Sender analysis"
35id: "3bc152f2-6722-57be-b924-055c35fa1e60"

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