> JapanHacked_
[Contained][Severity: Major][Misdelivery]

Mizuho Bank: emails of 5,572 customers misdelivered to a partner, which then sent mismatched notices to customers

It mistakenly sent the email addresses of 5,572 customers to a partner, which then emailed customers notices that did not match them.

Disclosed

2025-06-14

Organizations involved

1

Leaked or at risk

Unknown

Downtime

Unknown

What happened

  1. It mistakenly sent the data to a partner. [1] [2]

  2. The partner emailed customers with mismatched content. [1] [2]

  3. It made the announcement. [1] [3]

Impact

The information that reached the partner was the email addresses and related details of 5,572 customers. [1] [2]

Timeline

  1. Started 2025-06-12
  2. Detected 2025-06-13
  3. Disclosed 2025-06-14
  4. 2 days

Attack or cause

Method
[Misdelivery]
Impact
[Data leak]
Entry point
E-mail

Data exposed or at risk

[E-mail address]

Organizations involved

Open in organization search

Sources

  1. [1][Official]お客さま情報の漏えいに関するお詫びとご報告についてMizuho Bank, Ltd. · 2025-06-14
  2. [2][News]みずほ銀、顧客5572人のメアドなどを提携先に誤送信ITmedia · 2025-06-16
  3. [3][News]お客さま情報の厳格な管理を求められる金融機関がメール誤送信 ～ 株式会社みずほ銀行ScanNetSecurity · 2025-06-30 08:05