It said a bug in its software meant information that people believed was private had been accessible by third parties. Google said up to 500,000 users had been affected.
According to a report in the Wall Street Journal, the company knew about the issue in March but did not disclose it.
The WSJ quoted an internal Google memo that said doing so would draw “immediate regulatory interest”.
In a statement, the firm said the issue was not serious enough to inform the public.
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