Yahoo built custom software to comply with a government request to monitor all of its users' emails, according to former employees.
That meant hundreds of millions of Yahoo Mail accounts were scanned, either for the NSA or FBI, the former employees told Reuters.
This is the first known time a U.S. company agreed to a spy agency's request to search all emails, instead of storing them or scanning a select number of accounts.
Yahoo is a law abiding company, and complies with the laws of the United States.
Yahoo gave Reuters a brief statement and declined further comment.
It's not clear what data was handed over to authorities, or if any data was offered at all.
Why this one stands out
U.S. phone and Internet companies have turned over bulk metadata to authorities before (as Edward Snowden revealed). But Yahoo's alleged surveillance stands out for its scope, and that the company built software to do the job itself.
Yahoo CEO Melissa Mayer reportedly complied with the directive because the company thought it would lose the inevitable legal battle, despite legal experts telling Reuters that Yahoo could have challenged it.
WATCH | Here's what that Yahoo hack could mean to its users, which probably included you at some point.
It's not clear if this is in any way related to Yahoo's massive email hack last month.
So shortly after "Russian" hackers hacked 500 million Yahoo accounts, Yahoo itself hacked all user accounts— zerohedge (@zerohedge) October 4, 2016
But the irony of the hack was not lost on Twitter.
Other users were outraged.
Yahoo's failure to tell its own security team stuck out as well.
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