Disclosure context

About Atomsilo

Atomsilo is a relatively obscure ransomware group that emerged in December 2021, operating with financial motivations and maintaining a low profile compared to major ransomware operations. The group's origin and potential affiliations remain largely undocumented by major threat intelligence sources, though their limited scope of operations suggests they function as an independent entity rather than a large-scale Ransomware-as-a-Service operation. Based on available victim data, Atomsilo appears to employ targeted attacks against specific sectors, though their exact initial access vectors and technical methodologies have not been extensively documented by major security research organizations. The group has demonstrated a preference for targeting healthcare and financial services sectors, with documented activity primarily affecting organizations in Brazil and Japan, suggesting either regional focus or specific linguistic/cultural targeting capabilities. Atomsilo remains a minor player in the ransomware landscape with only five publicly documented victims, and their current operational status is unclear due to limited public reporting and analysis from major threat intelligence sources. The group has been linked to 5 public disclosures across our corpus. First observed on a leak site on December 21, 2021; most recent post February 24, 2026. The operation is currently active.

Timeline of this disclosure

December 21, 2021Eisai Co., Ltd listed by Atomsiloon the group's public leak site

Sector and geography

This disclosure adds to ransomware activity in the Healthcare sector, which has 1,779 disclosures indexed across all operators we track. Geographically, Eisai Co., Ltd 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

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Eisai Co., Ltd

Claimed by Atomsilo · listed 4 years ago

53m

Age

since listed · data leaked

Status timeline

Listed
Dec 21, 2021
Data leaked

At a glance

Group: Atomsilo
Status: Data leaked
Country: Japan
Sector: Healthcare
Listed on leak site: Dec 21, 2021

About the victim

AI dossier — public-source company profile

Eisai Co., Ltd. is a Japanese multinational pharmaceutical company headquartered in Tokyo, Japan, specializing in neurology and oncology therapeutics. The company develops and markets globally recognized drugs including Aricept (donepezil) for Alzheimer's disease and Halaven (eribulin) for breast cancer. Eisai operates across multiple continents and engages in global health initiatives alongside its core drug discovery and manufacturing activities.

Industry: Pharmaceutical & Biotechnology
Address: 4-6-10 Koishikawa, Bunkyo-ku, Tokyo 112-8088, Japan
Employees: 10000+
Founded: 1941

Attack summary

Severity: high — Eisai is a large multinational pharmaceutical company with disclosed data publication status, indicating confirmed exfiltration of business data. The pharmaceutical sector likely involves sensitive R&D, clinical trial data, and employee/patient-adjacent records, but the lack of specific data inventory detail prevents a critical classification.

The Atomsilo ransomware group claims to have attacked Eisai Co., Ltd. and has published data, indicating confirmed exfiltration; however, the leak post provides no specific details on the volume or nature of the data stolen or whether encryption of systems was also performed.

high

Data the group says was taken

AI dossier — extracted from the leak post

Undisclosed corporate data

Original description

AI-summarised, not from the leak post

Eisai Co., Ltd. is a Japanese multinational pharmaceutical company headquartered in Tokyo. Established in 1941, it's one of the leading firms in the field of neurology and oncology. Eisai produces several drugs used globally such as Aricept for Alzheimer's disease and Halaven for breast cancer. Aside from drug creation, Eisai also engages in global health initiatives.

Sources

Victim siteeisai.com
Leak posthttp://npmh5ahrgakbniuntyc7io4adm6ietbdbuejrfonowqtyqn24or556qd.onion/leaks.html/post.html?id=eisai

Source

Indexed 4 years 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.