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TL;DR
Osaka City confirmed on November 13 that 1,078,713 email addresses connected to its administrative online system were exposed in a breach at Arara Message, an email service used by system provider TKC. The city and TKC say the leak involved email addresses only; account details and application data were not exposed. People who received system emails from July through October should treat unexpected messages claiming to be from the city with caution.
Osaka City confirmed November 13 that 1,078,713 email addresses associated with its administrative online system were exposed in a breach at Arara Message, an email delivery service used by system provider TKC Corporation. The city and TKC say the exposed information was limited to email addresses, but the incident may leave affected users more vulnerable to convincing messages pretending to come from the city.
The city’s final report says the affected addresses belonged to recipients of emails sent by the system from July 1 through October 31, 2026. Those messages covered registrations, email or password changes, logins, applications, application-status updates and announcements. Osaka said the total was high partly because the system sent a message to all registered users on September 16.
The breach occurred at Arara Message, which TKC used to distribute email, rather than in Osaka’s own online system, according to the city’s account. TKC said it received data from Arara on November 4 and began matching the addresses to customer systems. Arara reported that 81,329,467 email addresses were taken across its service after duplicates were removed; it said the addresses came from up to about 380 client companies and should not be treated as a count of people.
Osaka and TKC say email contents, My Number identifiers and application data were not leaked. The city said it has not found a break-in to its system, and TKC likewise said its own systems were not breached. Those statements concern the systems under their control; the confirmed exposure was at the external email provider.
Osaka email leak · Oct 22 – Nov 13, 2026
How Osaka’s user emails leaked through a vendor’s vendor
No break-in at the city’s system or at its provider has been found, they say. The attack hit the email service one step further down.
Where the addresses were (schematic)
What happened
- Oct 22Arara Message briefly stopped for a PHP vulnerability responseper TKC
- Oct 28Arara confirms unauthorized access; its service starts sending fake card emails
- Oct 31Arara shuts its whole service down; from about 17:40 Osaka’s system cannot send email
- Nov 2TKC tells the city that user addresses may have leaked
- Nov 4Only email addresses affected, says TKC; Arara hands it the leaked addresses
- Nov 13The city confirms the count and warns of fake emails in its name
info-osaka-online@city.osaka.lg.jp between July 1 and Oct 31, 2026. Addresses, not people.Sources: City of Osaka, notice and press releases of Nov 3, 4 and 13, 2026; TKC Corporation, reports one to six; Arara and PayCloud Holdings disclosures. The chain is a schematic.
Graphic: Powered by Thorsten Meyer AI
How the Leak Could Affect Users
The exposed addresses can be used to target people with messages that appear to relate to Osaka services, including account notices or application updates. The city has not said that attackers used the addresses to send messages specifically to affected Osaka users, but the breach makes unexpected city-branded email a reasonable point of caution.
Users should avoid entering passwords, payment-card details or other sensitive information through links in unsolicited messages. Instead, they can open the city’s official website directly and check for notices there. An email address alone does not give an attacker access to an account, but it can help make a fraudulent message seem relevant or familiar.
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The Breach Through Arara Message
Arara’s reports describe suspicious activity beginning with apparent reconnaissance on October 27. On October 28, an attacker accessed an administrative screen in a shared environment and copied data, according to the company’s account. Arara also said its service began sending fake credit-card messages that day. The company confirmed unauthorized access on October 28 and reported the incident to police and Japan’s Personal Information Protection Commission.
Arara later found that data had also been taken through its service as the attacker accessed additional servers between October 28 and 31. It shut down the environment used by TKC for local-government systems on October 31, after which Osaka’s system could no longer send email. Arara’s reported total rose as its investigation developed: it first disclosed about 3.01 million addresses from one shared system, then about 77 million, and on November 6 reported 81,329,467 addresses after deduplication.
Osaka issued early notices on November 3 and 4 before publishing its final confirmation on November 13. Initial information from TKC had warned that email addresses, names and other details might be involved; TKC later told the city that only email addresses were affected. The final count for Osaka was based on information Arara supplied to TKC, which matched the data to the city’s system.
“Users’ email addresses, 1,078,713 records.”
— Osaka City, in its November 13 report
Exposure Checks and Follow-Up Findings
Osaka has not published a list that lets individuals check whether their address was included. The city says it cannot answer inquiries about specific addresses; it said individual notifications were under consideration. It is also not known how many distinct people are represented by the 1,078,713 Osaka records, because an email address count is not necessarily a people count.
The city says email addresses were the only information exposed, while the investigation and vendor reporting have developed over time. Osaka has asked TKC for a detailed report on the cause, scope and steps to prevent a recurrence. The material available does not establish whether the stolen addresses have been used in targeted scams, or whether additional findings will change the account of what was exposed.
City Reviews Vendor Security Measures
Osaka says it will review how personal information is handled by external services, its safety-management measures and its coordination with vendors. It has asked TKC to provide a fuller account of the incident and its prevention measures. The city has not given a date for that report or confirmed whether it will send individual notices.
For now, users can monitor the city’s official website for updates and treat unexpected messages claiming to concern Osaka online services cautiously. Do not use a link in an unsolicited email to sign in or provide payment information; access the service through a known official address instead.
Key Questions
What information did Osaka confirm was exposed?
Osaka confirmed that 1,078,713 email addresses associated with its administrative online system were exposed. The city and TKC say email contents, My Number details and application data were not leaked.
Who may be affected?
The addresses were recipients of system emails sent from July 1 to October 31, 2026. Osaka says a message sent to all registered users on September 16 contributed to the total. The city has not published an individual address lookup.
Was Osaka City’s online system hacked?
The city says it has found no break-in to its online system. According to Osaka and TKC, the unauthorized access occurred at Arara Message, the email delivery service used by TKC.
What should users do about emails claiming to be from Osaka?
Treat unexpected messages cautiously. Do not enter passwords or card details through email links; visit the city’s official website directly to check information or access a service.
Can users check whether their email address was exposed?
Not through an individual check announced by the city. Osaka says it cannot answer inquiries about whether a specific address was included and said individual notifications were still under consideration.
Source: ThorstenMeyerAI.com
