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The US Internal Revenue Service (IRS) announced yesterday it was the target of a malware attack last month, during which identity thieves attempted to generate E-file PINS for more than 464,000 taxpayers.

According to a statement, the agency reported the attackers used an automated bot against an application on the website that provides personal identification numbers (PINs) for filing tax returns online.

Identity thieves used personal data and Social Security Numbers (SSNs) stolen from other sources to carry out the attack.

“Based on our review, we identified unauthorized attempts involving approximately 464,000 unique SSNs, of which 101,000 SSNs were used to successfully access an E-file PIN,” said the agency.

However, no personal taxpayer data was compromised or disclosed by IRS systems.

While IRS cybersecurity experts continue to assess the situation, the IRS is working closely with other agencies and industry partners, as well as the Treasury Inspector General for Tax Administration.

Affected taxpayers will be notified via mail if their personal information was used in an attempt to access the IRS application.

“The IRS is also protecting their accounts by marking them to protect against tax-related identify theft,” read the statement.

In May 2015, the agency announced a data breach of 100,000 taxpayers’ information. The incident involved criminals using stolen SSNs, dates of birth, and other “out of wallet” information, such as high school mascots and spouses’ names, to submit IRS’ ‘Get Transcript’ applications.

The ‘Get Transcript’ feature allows taxpayers to view their tax statements, as well as previous tax return information.

In this case, criminals likely leveraged the information accessed to file fraudulent tax returns and credit applications.