Data breach exposes up to 14.2 million email logins at six ISPs

Summary

Japanese telecommunications operator KDDI Corporation has disclosed a significant data breach impacting its email systems, which are also used by five other internet service providers. Threat actors exploited a vulnerability in third-party software, potentially exposing the email addresses and passwords of up to 14.2 million customers. KDDI has notified affected ISPs and relevant Japanese authorities, and customers are advised to reset their passwords and enable two-factor authentication.

Full text

Data breach exposes up to 14.2 million email logins at six ISPs By Bill Toulas June 28, 2026 10:13 AM 0 Japanese telecommunications operator KDDI Corporation disclosed a data breach where threat actors gained access to one of its email systems used by five other internet service providers (ISPs) in the country. The company says that it discovered the compromise on June 17 and responded immediately by blocking the attacker and implementing defense measures. The investigation determined that the hackers exploited a vulnerability in an unnamed third-party software that KDDI Corporation used on its system. “Although technical defensive measures have already been implemented for the system, there remains a possibility that customers' email addresses and passwords were obtained by unauthorized third parties as a result of the incident,” KDDI warns. Scale of exposure KDDI is one of Japan’s largest ISPs, with 45,000 employees and an annual revenue of $32.4 billion. It is a public entity that has operated since 2000, following the merger of IDO, DDI, and KDD, Japan's former state-monopoly international telecommunications provider. The company says that the incident impacted the following five ISP operators and their email services: STNet, Inc. JCOM Co., Ltd. Chubu Telecommunications C., Inc. NIFTY Corporation BIGLOBE Inc. Although the investigation into the incident is still underway and the exact number of impacted accounts has yet to be determined, KDDI said it may have exposed the email addresses and passwords of up to 14,22 million customers. This figure includes current and former customers, as well as inactive accounts that may no longer be in use. Another mitigating factor, according to KDDI, is that some passwords were stored in hashed and/or encrypted form, meaning that they cannot be readily abused for account hijacks even if exposed. However, KDDI did not specify what type of encryption was used or what percentage of accounts had passwords stored in plaintext. KDDI says it has been contacting affected ISPs since June 17 and has also notified Japan's Personal Information Protection Commission and the Ministry of Internal Affairs and Communications. The company is currently working with affected ISPs to implement additional security measures to mitigate the risks arising from this exposure. Meanwhile, customers who may have been exposed are advised to reset their email account passwords as soon as possible. If two-factor authentication (2FA) is available, it would be prudent to set it up as well for additional protection. Test every layer before attackers do Security teams log 54% of successful attacks and alert on just 14%. The rest move through your environment unseen.The Picus whitepaper shows how breach and attack simulation tests your SIEM and EDR rules so threats stop slipping by detection. Get the whitepaper Related Articles: Japanese energy firm loses drive with data of 10.9 million clientsLastPass confirms data breach in Klue supply chain attackHealthtech firm Xolis suffers data breach impacting 1.4 million peopleWebinar: Why email security teams are drowning in alertsKlue OAuth breach victim list grows as Icarus hackers claim attack

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