• Sublime Core Feed
Medium Severity

Free Email Provider Sender with Mismatched Provider Reply-To

Labels

BEC/Fraud
Credential Phishing
Free email provider
Social engineering
Header analysis
Sender analysis

Description

Detects when a sender using a free email provider includes a reply-to address from a different free email provider, which is a common social engineering tactic.

References

No references.

Sublime Security
Created May 23rd, 2025 • Last updated May 23rd, 2025
Feed Source
Sublime Core Feed
Source
GitHub
type.inbound
and sender.email.domain.root_domain in $free_email_providers
and length(headers.reply_to) > 0
and any(headers.reply_to,
        .email.domain.root_domain in $free_email_providers
        and .email.domain.root_domain != sender.email.domain.root_domain
        and .email.domain.root_domain not in ("googlegroups.com")
)
// secureserver.net seems to rewrite the sender local part to be the reply-to domain for bounces
// observed in many newsletter sent via secureserver.net
and not (
  strings.istarts_with(sender.email.domain.domain, 'bounces.')
  and sender.email.domain.root_domain == 'secureserver.net'
  and all(headers.reply_to,
          strings.istarts_with(sender.email.local_part, .email.local_part)
          and strings.iends_with(sender.email.local_part, .email.domain.domain)
  )
)
// lists.riseup.net send from the list address and use the reply-to of the sender
// the sender is within the X-Original-From header and contains the full "From" header
and not (
  sender.email.domain.domain == "lists.riseup.net"
  and any(headers.hops,
          any(.fields,
              .name =~ "X-Original-From"
              and any(headers.reply_to,
                      strings.icontains(..value, .email.email)
              )
          )
  )
)
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