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YCC Information Systems: entrusted partner data may have leaked after file-server ransomware

Ransomware on the file server was confirmed early on April 2, 2026 (first notice April 3). The April 14 fourth notice said entrusted partner data may have leaked; the May 20 fifth notice found no clear outbound traces but did not fully rule out access.

Last updated: 2026-11-09

Overview

No published count (count blank). Network-device admin credentials were likely guessed or obtained. Multiple Yamagata municipalities and firms published victim-side notices.

Timeline

  1. Occurred
  2. Discovered
  3. Disclosed

Organization response

Per the notice, the organisation investigated and contained the incident, notified individuals and offered monitoring or protective resources.

What you can do

Be wary of emails, SMS or calls impersonating the organisation; do not open links or attachments or enter passwords or card details. Change reused passwords on other services.

Sources

  1. 【重要】ランサムウェア攻撃に関するお知らせ（第四報）株式会社YCC情報システム · 2026-11-09
  2. 【重要】当社サーバーへの不正アクセスに関するご報告株式会社YCC情報システム · 2026-11-09
  3. 【重要】ランサムウェア攻撃に関するお知らせ（第三報）株式会社YCC情報システム · 2026-11-09
  4. 【重要】ランサムウェア攻撃に関するお知らせ（第五報）株式会社YCC情報システム · 2026-11-09
  5. 【重要】ランサムウェア攻撃に関するお知らせ（第二報）株式会社YCC情報システム · 2026-11-09

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