sublimemediumRule

Attachment: Small text file with link containing recipient email address

Attach text file is less than 1000 bytes and contains a recipients email address. Seen in the wild carrying credential phishing links.

MITRE ATT&CK

T1566 T1566.001 T1566.002 T1598 T1036 T1027

defense-evasioninitial-access

Detection Query

type.inbound
and any(attachments,
        .content_type in~ ("text/plain", "text")
        // negate calendar invites
        and not (
          .file_extension == "ics"
          or .content_type in ("application/ics", "text/calendar")
        )
        and any(file.explode(.),
                .size < 1000
                and any(.scan.url.urls,
                        any(recipients.to,
                            strings.icontains(..url, .email.email)
                            and .email.domain.valid
                        )
                )
        )
)
and (
  not profile.by_sender().solicited
  or (
    profile.by_sender().any_messages_malicious_or_spam
    and not profile.by_sender().any_messages_benign
  )
)

Data Sources

Email MessagesEmail HeadersEmail Attachments

Platforms

Tags

Attack surface reduction

Raw Content

name: "Attachment: Small text file with link containing recipient email address"
description: "Attach text file is less than 1000 bytes and contains a recipients email address. Seen in the wild carrying credential phishing links."
type: "rule"
severity: "medium"
source: |
  type.inbound
  and any(attachments,
          .content_type in~ ("text/plain", "text")
          // negate calendar invites
          and not (
            .file_extension == "ics"
            or .content_type in ("application/ics", "text/calendar")
          )
          and any(file.explode(.),
                  .size < 1000
                  and any(.scan.url.urls,
                          any(recipients.to,
                              strings.icontains(..url, .email.email)
                              and .email.domain.valid
                          )
                  )
          )
  )
  and (
    not profile.by_sender().solicited
    or (
      profile.by_sender().any_messages_malicious_or_spam
      and not profile.by_sender().any_messages_benign
    )
  )
tags:
 - "Attack surface reduction"
attack_types:
  - "Credential Phishing"
tactics_and_techniques:
  - "Evasion"
  - "Social engineering"
detection_methods:
  - "File analysis"
  - "URL analysis"
id: "c0472c9d-1e06-5177-90b1-f57620944ff5"