sublimemediumRule

Link: Invalid reply-to with recipient details in subject, body, and encoded link

Detects inbound messages with an invalid reply-to address where the recipient's domain SLD appears in the subject, the recipient's local part and domain SLD appear in the body, and the recipient's full email address is base64-encoded within a link fragment.

MITRE ATT&CK

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

initial-accessdefense-evasion

Detection Query

type.inbound
// Invalid reply-to address
and any(headers.reply_to, not .email.domain.valid)
and strings.icontains(subject.base, recipients.to[0].email.domain.sld)
and strings.icontains(body.current_thread.text,
                      recipients.to[0].email.local_part
)
and strings.icontains(body.current_thread.text,
                      recipients.to[0].email.domain.sld
)
and any(body.current_thread.links,
        any(strings.scan_base64(.href_url.fragment),
            . == recipients.to[0].email.email
        )
)

Data Sources

Email MessagesEmail HeadersEmail Attachments

Platforms

Raw Content

name: "Link: Invalid reply-to with recipient details in subject, body, and encoded link"
description: "Detects inbound messages with an invalid reply-to address where the recipient's domain SLD appears in the subject, the recipient's local part and domain SLD appear in the body, and the recipient's full email address is base64-encoded within a link fragment."
type: "rule"
severity: "medium"
source: |
  type.inbound
  // Invalid reply-to address
  and any(headers.reply_to, not .email.domain.valid)
  and strings.icontains(subject.base, recipients.to[0].email.domain.sld)
  and strings.icontains(body.current_thread.text,
                        recipients.to[0].email.local_part
  )
  and strings.icontains(body.current_thread.text,
                        recipients.to[0].email.domain.sld
  )
  and any(body.current_thread.links,
          any(strings.scan_base64(.href_url.fragment),
              . == recipients.to[0].email.email
          )
  )
attack_types:
  - "Credential Phishing"
  - "BEC/Fraud"
tactics_and_techniques:
  - "Social engineering"
  - "Evasion"
  - "Spoofing"
detection_methods:
  - "Header analysis"
  - "Content analysis"
  - "URL analysis"
  - "Sender analysis"
id: "aebbc685-7cbf-56ee-ae32-1a49ddd9dcd6"