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TL;DR
Osaka City confirmed on November 13 that 1,078,713 email addresses associated with its administrative online system leaked after an intrusion at Arara Message, an email service used by system provider TKC. The city and TKC say the exposed data was limited to email addresses; users should treat unexpected messages claiming to be from the city with caution and verify them through official channels.
Osaka City confirmed November 13 that 1,078,713 email addresses associated with its administrative online system were exposed after an attacker accessed an email delivery service used by the system’s provider, TKC Corporation. The city and TKC say the exposed information was limited to email addresses, not message contents, Japan’s My Number identifiers or application data.
The city’s final report says the affected addresses belong to recipients of system emails sent from info-osaka-online@city.osaka.lg.jp between July 1 and October 31, 2026. Those messages covered registration, account changes, logins, applications and application-status updates, as well as announcements. Osaka said the count is high in part because the system sent a notice to all registered users on September 16. People who had an account at that time should assume their address may be included, although the city says it cannot confirm whether any particular address was affected.
The breach occurred at Arara Message, the email delivery service TKC used, rather than at Osaka’s online system, according to the city’s report. TKC says it received the exposed addresses from Arara on November 4 and matched them against customer systems. Arara reported that 81,329,467 email addresses were taken across its service after duplicates were removed, involving data associated with as many as about 380 client companies. That is a count of email addresses, not a confirmed number of individuals.
Arara’s reports describe unauthorized access to an administrative environment on October 28, followed by activity in which data was copied and the service was used to reach additional servers. Arara shut down the environment used for local-government systems on October 31, and Osaka’s system could no longer send emails from around 5:40 p.m. Japan time. TKC and Osaka say they found no intrusion into their own respective systems. The city has asked TKC for a detailed account of the cause, affected information and steps to prevent a recurrence.
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
What Osaka Users Should Watch For
An exposed address can make it easier for a criminal to send a message that appears relevant to a person’s dealings with the city. The source material does not establish that the stolen addresses have been used in a phishing campaign targeting Osaka users, but the breach creates a reason to scrutinize unexpected messages about accounts, applications or payments. A message may use a real city service or a user’s likely activity to seem credible without proving that it came from Osaka.
People who receive an unexpected email claiming to be from the city should avoid clicking its links or entering passwords, payment-card details or other personal information through a linked page. They can instead type the city’s official website address into a browser or use contact details obtained independently from an official source. The confirmed exposure is limited to email addresses, according to the city and TKC; there is no basis in the published findings to say that account passwords or application records were taken.
The incident also highlights a supply-chain risk: a government service can depend on outside companies to deliver routine messages. Osaka says it will review how personal information is handled by external services, vendor safety measures and cooperation arrangements. The review matters beyond this one system because email addresses held by a delivery provider may relate to customers of multiple organizations.
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How the Email Service Was Breached
Osaka’s online administrative system was provided by TKC Corporation, which used Arara Message to send notifications. Arara’s parent, PayCloud Holdings, and Arara disclosed the incident in stages. Arara’s initial report on October 29 put the number of addresses taken from one shared system at about 3.01 million. A later report said data had also been copied through the service and gave a broader estimate; its November 6 report put the total at 81,329,467 addresses after removing duplicates.
TKC initially told Osaka on October 30 that it had received a report indicating no leak from TKC’s usage environment. After further findings from Arara, TKC told the city on November 2 that email addresses and other details might have been affected across its systems. On November 4, TKC reported that the Osaka-related exposure was limited to email addresses. Osaka issued earlier notices on November 3 and 4 before publishing its final confirmation on November 13.
The dates matter: November 2 was the date of a report, not the date of the intrusion described in the source material. Arara’s account places the relevant activity from apparent reconnaissance on October 27 through the shutdown on October 31. The city says it identified its affected addresses using information Arara supplied to TKC, which TKC then sorted and matched to its customer systems.
““Users’ email addresses, 1,078,713 records.””
— Osaka City, in its November 13 report
What the Published Findings Do Not Show
The city has not published a count of distinct people affected. Its figure is 1,078,713 email addresses, and Arara’s larger service-wide figure also counts addresses rather than people. The public report does not establish whether every address corresponds to one person or whether some people had more than one address in the system.
Osaka has not said whether it will send individual notices; it says such notices are under consideration. It also says it cannot tell users whether a specific address appears in the exposed data. The city has requested a more detailed report from TKC, and the material available so far does not provide the complete technical findings or a full account of how the attacker gained access.
Osaka and TKC say no breach of their own systems was found, and both say the exposed Osaka-related information was limited to addresses. Those statements describe the findings they have reported; the city says its review and vendor inquiries are continuing. The source material does not confirm that the stolen addresses have been used to target affected users.
Osaka’s Vendor Review and User Guidance
Osaka says it has asked TKC for a detailed report covering the cause, scope and measures to prevent another incident. The city also plans to re-examine its handling of personal information in external services, vendor security practices and coordination with suppliers. It has not given a date for completing that work or said when it will decide whether to contact users individually.
For now, users can treat unexpected email claiming to come from Osaka City as unverified, avoid entering credentials or card details through links in such messages, and check the city’s website directly for service information. The city’s final report is the source for its confirmed count and stated data scope; further details may change if TKC or Arara provides additional findings.
Key Questions
Was my Osaka City email address exposed?
If you received an email from the city’s administrative online system between July 1 and October 31, 2026, your address may be among those affected. Osaka says it cannot confirm exposure for individual addresses.
What information does Osaka say leaked?
Osaka and TKC say the exposed Osaka-related data was limited to email addresses. They say email contents, My Number identifiers and application data were not leaked.
Was Osaka City’s own system hacked?
The city says it found no break-in to its online system. The reported intrusion occurred at Arara Message, the email delivery service used by TKC, the system provider.
What should I do if I receive a suspicious message?
Do not click unexpected links or enter passwords or payment information from a message claiming to be from the city. Check the matter through Osaka’s official website or contact details obtained independently.
Does the figure mean more than one million people were affected?
The city confirmed 1,078,713 email address records, not a count of distinct people. No published figure establishes how many individuals those addresses represent.
Source: ThorstenMeyerAI.com
