NTT DOCOMO data breach leaks up to 300,000 customers' details

Japanese mobile carrier NTT DOCOMO says an unauthorised outside party broke into its Docomo Online Shop and My docomo account portal and that information on up to about 300,000 customers has leaked. In a second notice timed at 18:30 Japan time (09:30 UTC) on Thursday, the company lists names, addresses, dates of birth, mobile numbers and contact phone numbers among the data taken.

DOCOMO says it detected the unauthorised access on Wednesday, November 4, and stopped all procedures on both services from about 01:05 Japan time on Thursday (16:05 UTC on Wednesday). It described that at first only as emergency maintenance. The second notice says all access is now blocked and no further leak has been found since the shutdown, but that the leak happened through access before it.

What DOCOMO has and has not said

The cause and the route the intruder took are still under investigation, according to the notice, as are the full range and number of affected records. DOCOMO says it will contact affected customers once the details are known. It gives no date for restoring the two services.

The company warns that the leaked information may be misused and that customers may receive suspicious emails, SMS messages or calls from people posing as DOCOMO or related parties, and asks them not to open links or attachments in such messages and not to give out personal information, passwords or card details.

A separate network outage

The breach notice came on the same day as a nationwide disruption to DOCOMO's 5G Standalone service. Calls, data and SMS for 5G SA subscribers have been hard to use since about 11:18 Japan time on Thursday, the company said in a separate notice and on X. Other 5G, 4G, MVNO and emergency calls are not affected. DOCOMO says the cause of the outage is still being investigated, but that so far it has found no trace of unauthorised access or anything indicating a cyberattack, before or after the outage began. The notice advises 5G SA users to switch their phone to 4G and restart it or toggle airplane mode.

DOCOMO posted the breach notice on its X account as well. Customers can reach its information centre on 151 from a DOCOMO phone or 0120-800-000 from other lines.