Taxpayers who are enrolled online are given a five-digit e-filing code, used to authenticate the user when filing taxes. The IRS said that the attackers tried to match e-filing codes with more than 464,000 Social Security numbers, thought to be stolen elsewhere from other attacks.
By matching the codes to the stolen Social Security numbers, it's theoretically possible to gain access to tax return and filing data.
The agency said an automated bot was thought to have generated e-filing for over 101,000 Social Security numbers. No taxpayer data was compromised or disclosed by IRS systems.
Affected accounts have been marked to help protect against tax-related identity theft.