What a Data Breach Is

A data breach is an incident in which personal or confidential information is accessed by an unauthorized third party. The causes vary widely: intrusion into a company database, information leaked through employee error, or inadequate security on the service provider's side.

Breaches tend to be large because attackers do not go after users one at a time. They go after the databases that businesses hold. When an entire customer table is taken in a single intrusion, email addresses, passwords, credit card details, and home addresses all leak together. Have I Been Pwned, which lets you search leaked addresses, listed more than 1,000 breach incidents and roughly 17.8 billion addresses as of August 2026.

The reason a notice reaches you at all is that the operator has legal obligations to meet. In Japan, when personal data that could cause financial harm if misused leaks - credit card numbers, or login credentials for services with payment features - the Act on the Protection of Personal Information (amended in 2020, in force from April 2022) requires the operator to report the incident to the Personal Information Protection Commission. Where the leak may have been carried out for improper purposes, the reporting deadline is within 60 days of discovery. In other words, by the time a notice or a news report tells you about a breach, the attacker has usually held the data for a while, and what you do from that point decides how far the damage spreads.

What to Do First When a Breach Comes to Light

1. Establish the Scope of the Impact

Start by pinning down exactly which service leaked what. Read the breach notification email and the news coverage carefully, and confirm the types of information involved (email addresses, passwords, credit card numbers, and so on).

2. Change Your Password Immediately

Change the password for the affected service right away. If you have reused the same password on other services, every one of those passwords needs changing as well. Use a password manager to set a unique, strong password for each service.

3. Turn On Two-Factor Authentication

If any of your services still lack it, take this opportunity to enable two-factor authentication. Even when a password leaks, two-factor authentication can stop an unauthorized login.

4. When Credit Card Details Are Involved

If your credit card number may have leaked, contact your card issuer and ask them to suspend the card and issue a replacement. Go through your statements and check for transactions you do not recognize.

How to Check Whether Your Information Has Leaked

Several services let you check whether your email address or password appears in a past data breach.

  • Have I Been Pwned (haveibeenpwned.com): enter an email address and see whether it appears in known breach databases
  • Built-in browser features: Chrome and Firefox can check whether your saved passwords have been exposed
  • Monitoring in a password manager: many password managers offer breach monitoring

Make a habit of using these services periodically to see whether your information has been exposed. Browser autofill also accumulates personal details such as your name and address, so it helps to understand how autofill knows your name and to manage what it keeps.

Secondary Damage to Watch for After a Breach

Beyond the direct damage of a data breach, the leaked information itself invites secondary attacks.

  • Phishing attacks: leaked details make social engineering far more convincing
  • Credential stuffing: leaked email and password pairs are replayed against other services to attempt logins
  • Impersonation: your personal details are used to pose as you and open new accounts
  • Targeted attacks: leaked information becomes the basis for an attack aimed at one specific person

Preparing for Future Breaches

Preventing a data breach outright is difficult, but preparing so that the damage stays small is well within reach.

  • Use a different password for every service
  • Enable two-factor authentication wherever it is offered
  • Delete accounts you no longer need
  • Give services only the personal information they actually need
  • Use email alias features so that each service sees a different address
  • Choose privacy-focused services

Checking your browser security score on IP Check-san as part of your routine, so that you know how safe your connection environment is, also helps you catch trouble early and head it off.

Related Glossary Terms

Act on the Protection of Personal Information (APPI) Japan's primary law governing the proper handling of personal information by bus… Ransomware Malware that encrypts files and entire systems on infected devices, then demands… Data Breach An incident where personal information or confidential data held by an organizat… Dark Web Monitoring A service that continuously scans dark web marketplaces, forums, paste sites, an… Incident Response A systematic, structured process for detecting, containing, eradicating, and rec…

Frequently Asked Questions

I received a data breach notification. What should I do first?

First change your password and enable two-factor authentication. Then check what kinds of data were exposed; if credit card details are included, contact your card issuer. Also change the password on any other service where you reused the same one.

How can I check whether my data has been leaked?

Enter your email address at Have I Been Pwned (haveibeenpwned.com) to see whether it appears in past data breaches. Checking periodically is recommended.