The Internal Revenue Service was the target of an attack that used stolen Social Security numbers and other taxpayer data to obtain PINs that can be used to file tax returns electronically.

The attack occurred in January and targeted an IRS Web application that taxpayers use to obtain their so-called Electronic Filing (E-file) PINs. The app requires taxpayer information such as name, Social Security number, date of birth and full address.

Attackers attempted to obtain E-file PINs corresponding to 464,000 unique SSNs using an automated bot, and did so successfully for 101,000 SSNs before the IRS blocked it.

The personal taxpayer data used during the attack was not obtained from the IRS, but was stolen elsewhere, the agency said in a statement. The IRS is notifying affected taxpayers via mail and will monitor their accounts to protect them from tax-related identity theft.

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