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[Investigating][Severity: Moderate][Intrusion]

Mitsui Sumitomo and Aioi Nissay Dowa: unauthorized access detected, policyholder sites taken offline

Mitsui Sumitomo Insurance and Aioi Nissay Dowa detected suspicious access to their policyholder sites and took them offline from 3 November. No data leak has been confirmed.

Disclosed

2026-11-03

Organizations involved

2

Leaked or at risk

Unknown

Downtime

Unknown

What happened

  1. Unauthorized access hit the policyholder sites of the two MS&AD group insurers. [3]

  2. Both companies took their policyholder pages and other services offline, first announcing it as emergency maintenance. [1] [2] [3]

  3. Both updated their notices, explaining that the services were stopped after suspicious third-party access was detected. [1] [2]

Impact

At Mitsui Sumitomo Insurance Company, Limited, nine services were down on the morning of 4 November: the policyholder page, one-day insurance, online travel, golf and cycling insurance, My Agent, a disaster alert service, the corporate web service and e-G1. At Aioi Nissay Dowa Insurance Co., Ltd., the policyholder page, One Day Supporter, e-travel overseas insurance, My Agent, the corporate web service and e-G1 are down. [1] [2]

No data leak has been confirmed, and both companies are investigating. [3]

Timeline

  1. Detected 2026-11-02
  2. Disclosed 2026-11-03
  3. 1 days

Attack or cause

Method
[Intrusion]
Impact
[Service outage]
Entry point
Unknown

Data exposed or at risk

–

Services hit

Open in service search

Organizations involved

Open in organization search

Sources

  1. [1][Official]ご契約者さま専用ページ等のサービス一時停止について【更新】Mitsui Sumitomo Insurance · 2026-11-04
  2. [2][Official]ご契約者さま専用ページのサービス一時停止についてAioi Nissay Dowa Insurance · 2026-11-04
  3. [3][News]三井住友海上とあいおい、不正アクセス検知 顧客向けサイト一時停止Nikkei · 2026-11-04 12:44