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TL;DR
Osaka City confirmed on November 13 that 1,078,713 email addresses associated with its administrative online system were exposed after a breach at email service Arara Message, used by system provider TKC. The city and TKC say the leak was limited to email addresses; they say message contents, My Number data and application information were not exposed.
Osaka City confirmed November 13 that 1,078,713 email addresses associated with users of its administrative online system were exposed after attackers accessed Arara Message, an email delivery service used by system provider TKC Corporation. The city and TKC say the breach exposed email addresses only, not message contents or application data, but the incident could make affected users targets for convincing scam emails.
The city’s final report says the exposed addresses belonged to recipients of messages sent by the system between July 1 and October 31, 2026. Those messages included account registration and login notices, password or email changes, application updates and city announcements. Osaka said the total is high in part because it sent a message to all registered users on September 16.
Arara Message reported 81,329,467 email addresses taken across its service after removing duplicates, drawn from as many as about 380 client companies, according to the supplied source material. That figure counts addresses, not people. Osaka’s confirmed number refers to addresses matched to its system; no count of affected individuals has been published.
TKC told Osaka that it received the stolen address data from Arara on November 4 and began sorting it by customer and system. The city says the addresses were identified using information supplied by the service provider and matched by TKC. Osaka has asked TKC for a detailed report on the cause, scope and steps to prevent a recurrence. It says individual notifications are under consideration, but it cannot tell people whether a particular address is affected.
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
Scam Risks for Osaka Users
The disclosed data is limited, according to the city and TKC, but an email address can still help scammers tailor messages that appear to come from a government office. People who received Osaka online-service emails during the stated period may see more plausible messages about applications, accounts or supposed security problems. The breach does not establish that such messages have been sent to Osaka users; it does mean unexpected email claiming to be from the city warrants caution.
Users should avoid entering passwords or payment-card details after following links in unsolicited or unexpected messages. Instead, they can visit the city’s website directly to check announcements and account issues. An email address leak alone does not mean a password or financial account has been compromised, and the city says those kinds of information were not included in this incident.
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How the Vendor Breach Reached Osaka
The incident occurred in a third-party service chain. TKC provided Osaka’s online system and used Arara Message to distribute email. Osaka says no intrusion into its own online system was found; TKC likewise said no breach was found in its own usage environment. The exposure was traced to Arara’s service, according to the city’s account and the companies’ reports.
The supplied chronology says Arara observed apparent reconnaissance on October 27 and unauthorized access to an administrative screen in a shared environment on October 28. The company detected data theft and shut down the environment used for local-government systems on October 31. Osaka’s system then could not send email. The city issued earlier updates before publishing its final address count on November 13. The November 2 date in some accounts refers to a later Arara report, not the start of the activity described in the source material.
““Because a notice email was sent from the system to everyone registered as a user on Wednesday, September 16, 2026, the number is very large.””
— Osaka City, explaining the size of its affected-address count
Individual Exposure Still Unconfirmed
The city has published the total number of addresses matched to its system, but says it cannot confirm whether a particular address is included. It has not said whether it will send direct notices to affected users; individual notifications are still under consideration. The number of people affected is also unknown, because the published totals count email addresses rather than individuals.
Osaka and TKC say the exposed information was limited to email addresses, and that message bodies, My Number identifiers and application data were not leaked. The supplied material does not include an independent technical review of those findings. The city has requested further details from TKC on the incident’s cause, scope and prevention measures, so its review is ongoing.
City Review and User Guidance
Osaka says it will review how personal information is handled by external services, its safety-management measures and coordination with vendors. It has asked TKC for a detailed account of the breach and its response. The city has not provided a date for that report or a decision on whether it will notify users individually.
For now, users should treat unexpected messages claiming to be from Osaka City with care, avoid opening links to enter sensitive information, and check the city’s website directly for service updates. The city’s confirmed count applies to email addresses in messages sent from July 1 through October 31, 2026; whether a specific address was exposed cannot currently be checked through the city.
Key Questions
What information did the Osaka breach expose?
Osaka and TKC say email addresses only were exposed. They say message contents, My Number data and application information were not leaked.
Who may be affected?
The city’s count covers addresses that received system emails between July 1 and October 31, 2026. Osaka says it cannot confirm whether an individual address is affected.
Does the 1,078,713 figure mean that many people were affected?
No. The figure is a count of email addresses, not people. The number of individuals represented has not been published.
What should users do if they receive an email claiming to be from Osaka?
Be cautious with unexpected messages. Do not enter passwords or card details through email links; visit Osaka City’s website directly to check whether a notice is genuine.
Source: ThorstenMeyerAI.com
