Disclosure context

About Rhysida

Rhysida is a ransomware group that emerged in June 2023, operating with primarily financial motivations through targeted attacks against critical infrastructure and public sector organizations. The group's origin and potential state affiliations remain unclear, though they operate independently rather than as a traditional Ransomware-as-a-Service model, with limited public documentation regarding connections to other cybercriminal organizations. Rhysida employs double extortion tactics, typically gaining initial access through compromised VPN credentials and exploiting vulnerable public-facing applications before deploying their ransomware payload and exfiltrating sensitive data prior to encryption. The group has demonstrated a particular focus on healthcare and educational institutions, with notable attacks documented by CISA and FBI advisories highlighting their targeting of hospitals and school districts across multiple countries, resulting in significant operational disruptions to critical services. As of late 2024, Rhysida remains an active threat with continued operations targeting organizations primarily in the United States, Canada, and other Western nations, maintaining their focus on high-value sectors where operational disruption can maximize ransom payment likelihood. The group has been linked to 278 public disclosures across our corpus. First observed on a leak site on June 5, 2023; most recent post May 20, 2026. The operation is currently active.

Timeline of this disclosure

December 15, 2025YOKOSUKA GAKUIN listed by Rhysidaon the group's public leak site

Data size: 10 TB
Records: 287 files

Sector and geography

This disclosure adds to ransomware activity in the Education sector, which has 694 disclosures indexed across all operators we track. Geographically, YOKOSUKA GAKUIN is reported in Japan, a country with 108 ransomware disclosures in our corpus.

How we know this. Darkfield monitors public ransomware leak sites continuously, archiving every new disclosure and the data later released against the victim. Each entry on this page is sourced from the operator's own publication and cross-checked against complementary OSINT feeds (RansomLook, ransomware.live, RansomWatch). We do not collect or host stolen data — only the metadata, timestamps and screenshots needed to make the public disclosure searchable and accountable. Records here are corrected when the original post is edited, retracted, or merged with another disclosure.

Ransomware victim disclosure

← All victims

YOKOSUKA GAKUIN

Claimed by Rhysida · listed 5 months ago

10 TB

Data size

287 files records

Age

since listed · data leaked

Status timeline

Listed
Dec 15, 2025
Data leaked

At a glance

Group: Rhysida
Status: Data leaked
Country: Japan
Sector: Education
Listed on leak site: Dec 15, 2025
Data size: 10 TB
Records: 287 files

About the victim

AI dossier — public-source company profile

Yokosuka Gakuin is a private school located in Yokosuka, Kanagawa Prefecture, Japan, established in 1950. The institution provides primary and/or secondary education to students in the Yokosuka area. No further operational details are available from the public site.

Industry: Private K-12 Education
Address: Yokosuka, Kanagawa Prefecture, Japan
Founded: 1950

Attack summary

Severity: high — 683 GB of data across 403,050 files has been fully published publicly. A private school likely holds student PII, guardian information, and staff records, constituting regulated personal data on minors, which warrants a high severity rating.

Rhysida claims to have exfiltrated approximately 683 GB of data comprising 403,050 files from Yokosuka Gakuin and has published all files to public access.

high

Data the group says was taken

AI dossier — extracted from the leak post

School administrative files
Student records (inferred)
Staff records (inferred)
Internal documents (403,050 files, 683 GB)

What the group claims

YOKOSUKA GAKUIN

The leak post

captured from the group's site

STELIA North America is an aerospace and defence company specializing in the design, development, and manufacturing of composites.10 TB of confidential data!Data from major partner companies, such as: Lockheed Martin, Northrop Grumman, Sikorsky, Leonardo, L3Harris, Airbus Atlantic, Boeing, Bombardier, De Havilland, ARDE, MDA.
Data Catalog: 9,3 Tb, 4 324 287 Files 
All files was uploaded to public access, data hunters, enjoy 
[Southold Town Senior ServicesSouthold Police Department](https://www.southoldtownny.gov)
The Town of Southold, New York provides various government services including forms and permits, online payments, and notifications for residents. Southold Police Department is a company that operates in the Local industry.
Data Catalog: 1,7 Tb, 767 693 Files 
All files was uploaded to public access, data hunters, enjoy 
Rohner designs and builds high-quality, custom paint booths and manufactured industrial paint systems. Our quality and service, cutting-edge technology, and engineering experience provide the best value in the finishing equipment industry.
Data Catalog: 3 Tb, 3 924 397 Files 
All files was uploaded to public access, data hunters, enjoy 
The Cheyenne and Ar…

Source

Indexed 5 months ago

This page surfaces a public ransomware disclosure indexed by Darkfield. Original posts come from the operator's own leak site; we cross-check against ransomware.live, RansomLook and RansomWatch where applicable. Share this URL freely.