Japan Warns Firms After String of Data Breaches: Patch, Vet Vendors, Hold Less Data

Japan's National Cybersecurity Office, part of the Cabinet Secretariat, issued an advisory on 9 October 2026 after confirming several cases in which attackers broke into the systems of companies that hold large amounts of personal data and stole it.

The office named three entry routes: exploited web-system vulnerabilities, compromise through the supply chain, and poorly secured data.

It urged companies that handle large volumes of personal or sensitive information to tighten defences without delay.

1. Web Systems: Assume Attack Is Coming

Internet-facing systems should be run on the assumption that they "could be attacked at any time."

• Patch quickly: apply OS, middleware, CMS and application updates promptly, including on internal servers, because attackers exploit vulnerabilities soon after they are disclosed.
• Retire the old: drop end-of-life products, shut down unused systems, and disable unnecessary ports and services.
• Harden access: require multi-factor authentication, ban guessable passwords, keep admin rights to a minimum, and remove leavers' accounts quickly.
• Plan for breach: keep logs, monitor for abnormal traffic and login attempts, and keep an incident-response plan ready.

2. Supply Chain: Manage Contractors as Your Own

Attackers often get in through a weaker contractor and then move on to the real target.

• Check contractors' and sub-contractors' security policies, certifications and audit results on an ongoing basis, not just when the contract is signed.
• Write data handling, access control, logging and breach reporting into contracts.
• Share only the minimum data, revoke access when work ends, and make sure data is deleted afterwards.
• Map key suppliers and dependencies, and reassess the risk regularly.

3. Data: Hold Less, Protect More

Attackers increasingly go after the data itself, both to extort victims by threatening to publish it and to sell it.

• Keep an inventory of what data is held, where it sits and who owns it, and delete it once it is no longer needed.
• Grant least-privilege access, separate read rights from write rights, and review permissions regularly.
• Encrypt data at rest and in transit, never store credentials in plaintext, protect backups, and check cloud storage and database exposure settings.
• Log and monitor access to sensitive data.
• Given the number of recent intrusions, the office recommends reviewing access logs now for signs of suspicious traffic or bulk downloads.

A Board-Level Issue

The office said cyber risk is no longer an IT-only problem, because a breach can halt operations and set off customer fallout.

With attack methods, including the misuse of AI, growing more sophisticated, it asked executives to treat cybersecurity as a management priority and fund it accordingly.

It also asked victim organisations to share technical attack details quickly with the office, the relevant ministries and specialist agencies.