The U.S. government sent a confidential, 16-page technical bulletin to retailers and other merchants on Thursday that describes the malicious software and techniques used to attack Target late last year.
The report provides steps to identify the malicious software used by criminal hackers that went undetected by anti-virus software when it infected Target's network, according to Tiffany Jones, a senior vice president with the security intelligence firm iSIGHT Partners, which helped draft the document.
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"This report was generated so that we could get it into the hands of commercial entities so that they had information they needed to protect themselves," she told Reuters in an interview.
ISight's John Watters told CNBC that the malicious code was written in Russian and was put up for sale last summer, meaning the actual hackers could have come from anywhere.
They have named the code "KAPTOXA" (pronounced Car-toe-sha), which means a Russian code string.
—CNBC.com contributed to this report.