sublimemediumRule

Reconnaissance: Empty subject with mismatched reply-to from new sender

Message with no subject line from a new sender where the reply-to address differs from the sender address, potentially indicating header manipulation or impersonation tactics.

MITRE ATT&CK

T1566.002 T1534 T1656 T1566 T1566.001 T1598 T1036 T1027

defense-evasioninitial-access

Detection Query

type.inbound
and profile.by_sender_email().prevalence == "new"
and subject.base == ""
and length(headers.reply_to) > 0
and sender.email.email not in map(headers.reply_to, .email.email)

Data Sources

Email MessagesEmail HeadersEmail Attachments

Platforms

Tags

Attack surface reduction

Raw Content

name: "Reconnaissance: Empty subject with mismatched reply-to from new sender"
description: "Message with no subject line from a new sender where the reply-to address differs from the sender address, potentially indicating header manipulation or impersonation tactics."
type: "rule"
severity: "medium"
source: |
  type.inbound
  and profile.by_sender_email().prevalence == "new"
  and subject.base == ""
  and length(headers.reply_to) > 0
  and sender.email.email not in map(headers.reply_to, .email.email)
tags:
  - "Attack surface reduction"
attack_types:
  - "BEC/Fraud"
  - "Credential Phishing"
tactics_and_techniques:
  - "Evasion"
  - "Social engineering"
  - "Spoofing"
detection_methods:
  - "Header analysis"
  - "Sender analysis"
id: "12f4bd45-a236-55c9-b16a-1bff6070e250"