The agency on Tuesday said hackers last month used malware to automatically generate E-file personal identification numbers that could help them steal tax refunds. The attack involved 464,000 Social Security numbers that had already been stolen, of which 101,000 were used to successfully generate an E-file PIN. Those PINs are sometimes used by filers submitting their returns electronically.

The IRS said no personal taxpayer information was compromised and is notifying by mail those taxpayers whose data was targeted. The agency is taking the added step of marking any targeted accounts to protect them against tax-related ID theft.