SAN FRANCISCO (MarketWatch) -- United States officials issued a confidential technical alert to merchants Thursday outlining how hackers stole personal credit data last fall from millions of Target Corp. TGT, +0.82% shoppers. According to media reports, the alert aims to arm retailers and other businesses with timely information they can use to prevent further security breaches. The Wall Street Journal reported that malware used against Target escaped detection by antivirus software, and that parts of it were written in Russian. It was not clear, however, who was behind the attack or where they were operating at the time. Target has warned the personal credit data of perhaps as many as 70 million customers may have been compromised as a result of the attack.