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TL;DR
Osaka City confirmed on November 13 that 1,078,713 email addresses linked to its administrative online system were exposed in a breach at Arara Message, an email-delivery service used by system provider TKC. The city and TKC say the leak involved email addresses only; they say message contents, My Number details and application data were not exposed.
Osaka City confirmed November 13 that 1,078,713 email addresses associated with 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 exposed information was limited to addresses, not email contents, My Number details or application data.
The city says the affected addresses belong to recipients of emails sent by the system from July 1 through October 31, 2026. Those messages included account registration and changes, login notifications, application updates and other announcements. Osaka said the count was high partly because the system sent a notice to all registered users on September 16. The figure is a count of email addresses, not confirmed individuals; the city has not published a number of people affected.
Osaka said it identified its users’ addresses using information supplied by TKC, which sorted and matched data received from Arara. TKC told the city on November 4 that the exposed information relevant to its systems was limited to email addresses. The city’s report says it is not able to confirm whether a particular address is included when individuals ask.
Arara has reported a wider incident affecting its service. Its November 6 report put the total at 81,329,467 email addresses, after duplicate entries were removed, across as many as about 380 client companies. That company-wide figure is not a count of Osaka users or people, and should not be confused with the city’s separate total.
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
Why the Exposed Addresses Matter
An exposed address can make a person more susceptible to convincing phishing emails, including messages that impersonate a city office or refer to an account or application. The leak does not establish that attackers accessed Osaka residents’ passwords or application records, but it gives them contact information they could use to target recipients. The city advises people to treat unexpected messages claiming to come from Osaka with caution, avoid entering passwords or payment details after following links, and check information through the city’s website.
The incident also shows how a breach at an outside service can affect users of a public system even when the government agency and its direct software provider report no intrusion into their own environments. Osaka says it has requested a detailed account from TKC and will review how external services handle personal information, their safety measures and coordination with vendors. Those reviews matter because the email service was part of the chain used to communicate with online-system users.
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How the Vendor Breach Reached Osaka
According to the reports summarized by Osaka, TKC and Arara, the access occurred in Arara’s shared service environment, rather than in Osaka City’s own system. Arara recorded activity it described as reconnaissance on October 27. On October 28, an attacker accessed an administrative screen and copied data, and Arara’s service began sending fraudulent credit-card emails. Arara confirmed unauthorized access that day.
Arara initially disclosed about 3.01 million addresses from one shared system on October 29, then reported a broader total as its investigation continued. On October 31, it shut down the environment used by TKC for local-government systems; Osaka’s system could no longer send email from about 5:40 p.m. that day. Arara later reported 81,329,467 addresses after removing duplicates. Osaka issued early notices on November 3 and 4, then released its final count on November 13.
The city says it found no break-in to its own online system, and TKC likewise said the incident did not involve an intrusion into its own systems. Those statements distinguish the reported point of compromise from the systems whose users’ addresses were implicated; they do not remove the exposure caused by the vendor breach.
“Users’ email addresses: 1,078,713 records.”
— Osaka City, in its November 13 report
What the Reports Do Not Establish
The published totals count email addresses rather than people, and the sources do not establish how many unique individuals are represented. Osaka has not said whether it will send individual notices; its report says that option is under consideration. The city also says it cannot confirm an individual address’s status through inquiries.
The city and TKC say the exposed data was limited to email addresses, while the available source material does not describe any confirmed misuse of Osaka users’ addresses. It remains unclear whether any affected users have received targeted fraudulent messages because of this exposure. Osaka has requested a detailed report on the cause, scope and prevention measures, but those findings were not included in the November 13 announcement.
City Review and User Guidance
Osaka says it will seek a detailed report from TKC covering the breach’s cause, scope and steps to prevent a repeat. The city also plans to review its handling of personal information in external services, vendor safety measures and coordination arrangements. No timetable for that review or decision on individual notifications was provided in the report.
For now, users who received messages from the Osaka online system between July and October should be alert to unexpected emails claiming to be from the city. They should not follow suspicious links to provide passwords or card details and can check official information on Osaka City’s website. The city has not published a public address-lookup process, so an individual user’s inclusion remains unconfirmed.
Key Questions
How many Osaka email addresses were exposed?
Osaka City confirmed 1,078,713 email addresses linked to its administrative online system. The count is addresses, not a confirmed number of people.
What information does the city say was leaked?
The city and TKC say the exposed information was email addresses only. They say email contents, My Number details and application data were not leaked.
Who may be affected?
The city identifies recipients of system emails sent from July 1 through October 31, 2026. It says a September 16 message sent to all registered users contributed to the high count, but it cannot confirm whether a specific address was included.
Was Osaka City’s own system hacked?
The city says it found no break-in to its online system. The reported unauthorized access occurred at Arara Message, the email-delivery service used by TKC.
What should users do now?
Treat unexpected messages claiming to be from Osaka City with caution. Do not enter passwords or payment details after clicking links; check city information through its official website. Osaka has not announced an address-lookup tool.
Source: ThorstenMeyerAI.com
