[Investigating][Severity: Critical][Ransomware][Credential abuse]

Ryomo Systems: ransomware attack

On 12 August, an attacker misused a VPN device account to get into the internal network of Ryomo Systems, an IT company in Gunma, and the anomaly was detected early on 14 August. It was a ransomware attack; some servers showed traces of access, and personal data of clients' customers, employees and others may have leaked. Through old work files left on its internal network, about 124,000 customers of Daito Gas, 3,789 student accounts of Isesaki City and about 6,800 customers of Nabari Kintetsu Gas were among those affected.

Disclosed

2026-08-15

Organizations involved

Leaked or at risk

Unknown

Downtime

Unknown

Timeline

Started 2026-08-12
Detected 2026-08-14
Disclosed 2026-08-15
3 days

Attack or cause

Method: [Ransomware][Credential abuse]
Impact: [Data leak]
Entry point: VPN device

Data exposed or at risk

[Name][Postal address][Phone number][Passwords][Education]

Supply chain

Ryomo Systems Co., Ltd.

[Breached]

Ryomo Systems Co., Ltd.

[Breached]

Organizations listed on this site: 5

Organizations involved

[Through a supplier][Critical]Gunma, IsesakiMunicipality3,789 leaked
Isesaki City
Names, school years, account names and passwords of students' learning tablets.
[Breached][Critical]Gunma, KiryuInformation and communications
Ryomo Systems Co., Ltd.
[Through a supplier][Critical]Saitama, FujimiElectricity, gas, heat and water124,000 leaked
Daito Gas Inc.
Names, addresses, phone numbers, gas usage and more of customers from November 2023 to April 2024.
[Through a supplier][Major]Mie, NabariElectricity, gas, heat and water6,800 leaked
Nabari Kintetsu Gas Inc.
Names, addresses and billing numbers of propane gas customers.
[Through a supplier][Major]Nagano, NaganoElectricity, gas, heat and water3,600 leaked
Nagano Toshi Gas Inc.
Results of safety inspections carried out from 1 April to 16 May 2026 (names, addresses, phone numbers, contract numbers).

						Note
Isesaki City	[Through a supplier]	[Critical]	Gunma, Isesaki	Municipality	3,789	Names, school years, account names and passwords of students' learning tablets.
Ryomo Systems Co., Ltd.	[Breached]	[Critical]	Gunma, Kiryu	Information and communications	Unknown
Daito Gas Inc.	[Through a supplier]	[Critical]	Saitama, Fujimi	Electricity, gas, heat and water	124,000	Names, addresses, phone numbers, gas usage and more of customers from November 2023 to April 2024.
Nabari Kintetsu Gas Inc.	[Through a supplier]	[Major]	Mie, Nabari	Electricity, gas, heat and water	6,800	Names, addresses and billing numbers of propane gas customers.
Nagano Toshi Gas Inc.	[Through a supplier]	[Major]	Nagano, Nagano	Electricity, gas, heat and water	3,600	Results of safety inspections carried out from 1 April to 16 May 2026 (names, addresses, phone numbers, contract numbers).

Open in organization search

Sources

[1][Official]当社社内システムへの不正アクセスに関するお知らせ（第3報）Ryomo Systems
[2][News]大阪ガス系2社で顧客情報漏洩の可能性不正アクセスNikkei
[3][Official]業務委託先企業における「不正アクセスによるお客さま情報流出の可能性」についてのお詫びとお知らせ（第二報・事案の概要とお客さまへの個別通知について）Nagano Toshi Gas · 2026-10-05
[4][Official]システムサービス提供事業者へのサイバー攻撃に伴うお客さま情報流出の可能性についてDaito Gas · 2026-09-28
[5][Official]本市委託先への不正アクセスによる情報漏洩の可能性についてIsesaki City · 2026-09-28