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Hackers who stole terabytes of data from Ticketmaster and other customers of the cloud storage firm Snowflake claim they obtained access to some of the Snowflake accounts by first breaching a Belarusian-founded contractor that works with those customers.

About 165 customer accounts were potentially affected in the recent hacking campaign targeting Snowflake’s customers, but only a few of these have been identified so far. In addition to Ticketmaster, the banking firm Santander has also acknowledged that their data was stolen but declined to identify the account from which it was stolen. Wired, however, has independently confirmed that it was a Snowflake account; the stolen data included bank account details for 30 million customers, including 6 million account numbers and balances, 28 million credit card numbers, and human resources information about staff, according to a post published by the hackers. Lending Tree and Advance Auto Parts have also said they might be victims as well.

Snowflake has not revealed details about how the hackers accessed the accounts, saying only that the intruders did not directly breach Snowflake’s network. This week, Google-owned security firm Mandiant, one of the companies engaged by Snowflake to investigate the breaches, revealed in a blog post that in some cases the hackers first obtained access through third-party contractors, without identifying the contractors or stating how this access aided the hackers in breaching the Snowflake accounts.