KDDI Corporation Data Breach Exposes up to 14.2 Million Email Logins
Japanese telecommunications operator KDDI Corporation disclosed a data breach where threat actors gained unauthorized access to one of its email systems. This system was also used by five other internet service providers (ISPs) in Japan. The company discovered the compromise on June 17, 2026, and responded by blocking the attacker and implementing defensive measures. The investigation determined that hackers exploited a vulnerability in unnamed third-party software. Up to 14.2 million email addresses and passwords, including those of current, former, and inactive customers, may have been exposed. Some passwords were stored in hashed and/or encrypted form, but the company did not specify the encryption type or the percentage of plaintext passwords. KDDI has reported the incident to Japanese privacy and telecommunications regulators. The breach impacts KDDI Corporation and its partner ISPs.
Signal context
First seen: Jun 28, 2026
Last updated: Jun 29, 2026
Status: Public signal
Key points
- Japanese telecommunications operator KDDI Corporation disclosed a data breach.
- Threat actors gained unauthorized access to an email system shared with five other ISPs.
- Compromise discovered on June 17, 2026, and publicly disclosed on June 28, 2026.
Signal analysis
BetaThis analysis groups the signal by industry, likely incident action and impacted security area. It helps compare this signal with other published signals without treating the labels as final determinations.
Sector: Information
Likely country: 🇯🇵 Japan
inferred from signal text
The feed marks multiple actor roles. Treat this as a review signal rather than a final attribution.
- Source type: outside the affected organization
- Source type: supplier or third-party involvement
Impact area: Confidentiality
Likely asset: User or customer data
- 32 signals in the same sector
- 97 signals with the same likely impact area
- 1 signal linked to this organization/domain
External sources
Data breach exposes up to 14.2 million email logins at six ISPs - Bleeping Computerhttps://www.bleepingcomputer.com/news/security/data-breach-exposes-up-to-142-million-email-logins-at-six-isps/Public source from bleepingcomputer.com.
KDDI Data Breach Impacts up to 14.2 Million Email Accounts at Six ISPs - Security Affairshttps://securityaffairs.com/194387/data-breach/kddi-data-breach-impacts-up-to-14-2-million-email-accounts-at-six-isps.htmlPublic source from securityaffairs.com.
KDDI Data Breach Impacts up to 14.2 Million Email Accounts at Six ISPs - Security Affairshttps://securityaffairs.co/wordpress/160628/data-breach/kddi-data-breach.htmlPublic source from securityaffairs.co.
KDDI Email System Breach Exposes Up to 14.2 Million Credentials Across Six Japanese ISPs - Rescanahttps://www.rescana.com/post/kddi-email-system-breach-exposes-up-to-14-2-million-credentials-across-six-japanese-ispsPublic source from rescana.com.
Related signals
Grouped by why the signal is relevant.
STNet, Inc. Affected by KDDI Corporation Data Breach
STNet, Inc., a Japanese internet service provider, was impacted by a data breach originating from an email system provided by KDDI Corporation. Threat actors gained unauthorized access to this shared system by exploiting a vulnerability in third-party software. This led to the potential exposure of up to 14.2 million email addresses and passwords across all affected ISPs. STNet customers' email addresses and passwords may have been compromised.
KDDI Web Communications Customer Data Affected by KDDI Email System Breach
KDDI Web Communications, a subsidiary of KDDI, was impacted by the data breach in KDDI Corporation's email system, disclosed on June 28, 2026. The incident, caused by a third-party software vulnerability, led to the potential exposure of up to 14.2 million email addresses and passwords belonging to customers across six Japanese ISPs, including KDDI Web Communications.
JCOM Co., Ltd. Affected by KDDI Corporation Data Breach
JCOM Co., Ltd., a Japanese internet service provider, was impacted by a data breach originating from an email system provided by KDDI Corporation. Threat actors gained unauthorized access to this shared system by exploiting a vulnerability in third-party software. This led to the potential exposure of up to 14.2 million email addresses and passwords across all affected ISPs. JCOM customers' email addresses and passwords may have been compromised.
BIGLOBE Inc. Affected by KDDI Corporation Data Breach
BIGLOBE Inc., a Japanese internet service provider, was impacted by a data breach originating from an email system provided by KDDI Corporation. Threat actors gained unauthorized access to this shared system by exploiting a vulnerability in third-party software. This led to the potential exposure of up to 14.2 million email addresses and passwords across all affected ISPs. BIGLOBE Inc. customers' email addresses and passwords may have been compromised.
NIFTY Corporation Affected by KDDI Corporation Data Breach
NIFTY Corporation, a Japanese internet service provider, was impacted by a data breach originating from an email system provided by KDDI Corporation. Threat actors gained unauthorized access to this shared system by exploiting a vulnerability in third-party software. This led to the potential exposure of up to 14.2 million email addresses and passwords across all affected ISPs. NIFTY Corporation customers' email addresses and passwords may have been compromised.
Polymarket Suffers $3 Million Crypto Drain via Third-Party Vendor Compromise
Prediction market platform Polymarket disclosed a breach where hackers compromised a third-party vendor. The attackers injected malicious code into the Polymarket website for some users, leading to the draining of approximately $3 million in pUSD (USDC-backed) from at least 11 user wallets. The stolen funds were subsequently swapped and moved to Ethereum. Polymarket has contained the incident, committed to full refunds for affected users, and is in the process of notifying victims. This incident highlights supply-chain vulnerabilities in web frontends and crypto platforms.