Medium Severity

BEC/Fraud: Reply-chain manipulation with urgent keywords and self-reply

Labels

BEC/Fraud

Social engineering

Description

Detects suspicious reply messages with urgent language in sender name or email address, minimal body content, and the sender's email address appearing in previous thread content, indicating a self reply.

References

No references.

Sublime Security

Created Mar 11th, 2026 • Last updated Mar 11th, 2026

Feed Source

Sublime Core Feed

Source

GitHub

type.inbound
and subject.is_reply
and length(attachments) == 0
// short current thread
and length(body.current_thread.text) < 25
// only 1 previous thread
and length(body.previous_threads) == 1
and any(recipients.to,
        .email.domain.root_domain != sender.email.domain.root_domain
)
// urgent or authority based keywords in sender display name or local part
and (
  regex.icontains(sender.display_name,
                  '\b(?:(?:crucial|urgent|immediate|important|quick)(?:ly)?|immediatetask|emergency)\b'
  )
  or regex.icontains(sender.email.local_part, '(?:task|ceo|executive)')
)
// sender replied to themselves
and any(body.previous_threads, strings.icontains(.preamble, sender.email.email))

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