Open redirect: obunsha.co.jp

name: "Open redirect: obunsha.co.jp"
description: |
  Detects messages containing Obunsha's passnavi redirect service that can be exploited
  to redirect users to malicious sites. This has been used in phishing campaigns.
type: "rule"
severity: "medium"
source: |
  type.inbound
  and any(body.links,
          // Look for Obunsha passnavi URLs
          .href_url.domain.domain == "passnavi.obunsha.co.jp"
          and strings.icontains(.href_url.path, '/ct.html')
          and strings.icontains(.href_url.query_params, 'uri=')
          // Make sure it's not redirecting back to obunsha domains
          and not regex.icontains(.href_url.query_params,
                                  'uri=(?:https?(?:%3a|:))?(?:%2f|\/){2}[^&]*obunsha\.co\.jp(?:\&|\/|$|%2f)'
          )
  )
  // Exclude legitimate Obunsha domains as senders
  and not sender.email.domain.root_domain == "obunsha.co.jp"
  
  // negate highly trusted sender domains unless they fail DMARC authentication
  and (
    (
      sender.email.domain.root_domain in $high_trust_sender_root_domains
      and not headers.auth_summary.dmarc.pass
    )
    or sender.email.domain.root_domain not in $high_trust_sender_root_domains
  )
attack_types:
  - "Credential Phishing"
  - "Malware/Ransomware"
tactics_and_techniques:
  - "Open redirect"
detection_methods:
  - "Sender analysis"
  - "URL analysis"
id: "e972dacf-0316-57d0-b135-38517042b9e8"