Incident BriefingsMitsui Sumitomo Insurance policyholder-portal data incident

Mitsui Sumitomo Insurance policyholder-portal data incident

The insurer disconnected its policyholder portal after detecting unauthorized access on November 2.

Last modified

Summary

  • Environment: Policyholder-only customer portal
  • Operational impact: Related services suspended and portal disconnected
  • Financial impact: No incident cost disclosed
  • Record status: Developing record. Reviewed November 9, 2026; updated as evidence emerges.

What happened

Mitsui Sumitomo Insurance detected unauthorized access to its policyholder portal on November 2, 2026 and stopped related services early the following day. Investigation confirmed customer-information leakage. [1]

Impact

  • Approximately 37,000 records for registered portal customers were affected as of November 6. [1]
  • Exposed fields included policy numbers, names, addresses, phone numbers and email addresses. Credit-card and bank-account information were excluded. [1]

Timeline

  1. Unauthorized access detected

    The insurer detected the intrusion at night.

    [1]

  2. Services suspended

    The portal was disconnected from the network.

    [1]

  3. Initial suspension announcement

    Mitsui first announced the service interruption.

    [1]

  4. Data leakage confirmed

    The follow-up notice reports the affected records.

    [1]

  5. Briefing updated

    This briefing was last reviewed and updated on November 9, 2026.

Threat Group & Attack Vector

The initial access method has not been disclosed.

Actors

  • No threat actor group has been identified in the reviewed public evidence.

TTPs

  • No specific MITRE ATT&CK technique is currently mapped for this case.

Response

Mitsui reported no external unauthorized access after suspension. Outside specialists were investigating, and individual notifications were planned. No misuse had been confirmed as of November 6. [1]