The agency had identified only about $46 million in phony refunds as of late February.
Some of the top-earning users traded over $20,000 on the exchange in a single year.
The IRS’ Individual Master File may be the oldest IT system in government and its failure could disrupt tax processing nationwide.
They're also spoofing tax preparers' websites, security researchers said.
Auditors found a programming error prevented the IRS from sending out notices.
Despite what IRS officials told Congress, the IRS did not have to continue using Equifax services in a bridge contract, a GAO official said.
There were 177 phishing attacks against tax preparers between January and May of this year.
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