---
title: "Mitsui Sumitomo Insurance policyholder-portal data incident"
description: "Mitsui Sumitomo Insurance confirmed leakage of approximately 37,000 policyholder-portal records."
incident_type: "Cybersecurity incident"
status: "active"
last_modified: "2026-11-09"
canonical_url: "https://www.ally.security/incidents/mitsui-sumitomo-policyholder-portal-data-incident-2026"
markdown_url: "https://www.ally.security/incidents/mitsui-sumitomo-policyholder-portal-data-incident-2026.md"
stix_url: "https://www.ally.security/incidents/mitsui-sumitomo-policyholder-portal-data-incident-2026/stix.json"
---

# Mitsui Sumitomo Insurance policyholder-portal data incident

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

Last modified Nov 9, 2026 · 1 source

## Summary

- **Environment:** Policyholder-only customer portal
- **Operational impact:** Related services suspended and portal disconnected
- **Financial impact:** No incident cost disclosed

## What happened

[Mitsui Sumitomo Insurance](https://www.ms-ins.com/) 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](#source-1)

## Impact

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

## Timeline

### November 2, 2026 — Unauthorized access detected

The insurer detected the intrusion at night. [1](#source-1)

### November 3, 2026 — Services suspended

The portal was disconnected from the network. [1](#source-1)

### November 4, 2026 — Initial suspension announcement

Mitsui first announced the service interruption. [1](#source-1)

### November 6, 2026 — Data leakage confirmed

The [follow-up notice](https://www.ms-ins.com/news/fy2026/pdf/1106_1.pdf) reports the affected records. [1](#source-1)

### November 9, 2026 — 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](#source-1)

## Assets

[Download the case-scoped STIX 2.1 bundle](<https://www.ally.security/incidents/mitsui-sumitomo-policyholder-portal-data-incident-2026/stix.json>)

## Sources

Primary source records used to research this incident.

<a id="source-1"></a>

### Mitsui Sumitomo Insurance policyholder-portal information-leakage notice

official · Mitsui Sumitomo Insurance · Nov 6, 2026

<https://www.ms-ins.com/news/fy2026/pdf/1106_1.pdf>

<details>
<summary>Evidence ledger</summary>

Review the supporting structured claims.

1. **Resulted In · 100% confidence · current**  
   Mitsui Sumitomo Insurance policyholder-portal data breach: Approximately 37,000 records belonging to registered portal customers were affected as of November 6. Fields included policy numbers, names, addresses, phone numbers and email addresses; card and bank-account information were excluded.
2. **Resulted In · 100% confidence · current**  
   Mitsui Sumitomo Insurance policyholder-portal data breach: Related services stopped
3. **Occurred At · 100% confidence · current**  
   Policyholder-portal intrusion detected: 2026-11-02
4. **Occurred At · 100% confidence · current**  
   Data leakage and record count confirmed: 2026-11-06
5. **Resulted In · 100% confidence · current**  
   Mitsui Sumitomo Insurance policyholder-portal data breach: Data leakage and record count confirmed
6. **Affected Organization · 100% confidence · current**  
   Mitsui Sumitomo Insurance policyholder-portal data breach: Mitsui Sumitomo Insurance
7. **Resulted In · 100% confidence · current**  
   Mitsui Sumitomo Insurance policyholder-portal data breach: Initial service-suspension announcement
8. **Resulted In · 100% confidence · current**  
   Mitsui Sumitomo Insurance policyholder-portal data breach: Mitsui Sumitomo Insurance detected unauthorized access to its policyholder-only portal on the night of November 2, 2026. It stopped related services early on November 3 and later confirmed customer-information leakage.
9. **Resulted In · 100% confidence · current**  
   Mitsui Sumitomo Insurance policyholder-portal data breach: The portal was disconnected from the network. Mitsui said no external unauthorized access occurred after suspension and that outside specialists were carrying out a forensic investigation.
10. **Resulted In · 100% confidence · current**  
   Mitsui Sumitomo Insurance policyholder-portal data breach: Policyholder-portal intrusion detected
11. **Occurred At · 100% confidence · current**  
   Initial service-suspension announcement: 2026-11-04
12. **Resulted In · 100% confidence · current**  
   Mitsui Sumitomo Insurance policyholder-portal data breach: Mitsui planned individual customer notices and had not confirmed misuse of the exposed information as of November 6.
13. **Occurred At · 100% confidence · current**  
   Related services stopped: 2026-11-03
14. **Discovered At · 100% confidence · current**  
   Mitsui Sumitomo Insurance policyholder-portal data breach: 2026-11-02
15. **Disclosed At · 100% confidence · current**  
   Mitsui Sumitomo Insurance policyholder-portal data breach: 2026-11-04

</details>