In a series of remarkably persistent social engineering attacks against European companies, hackers linked to North Korea copied internal data and used the anonymity tool Tor to hide their tracks, German security firm HvS-Consulting revealed this week.

The campaign against firms in the manufacturing and electrical industries started in February with fake job offers sent via Linkedin and resulted in the theft of valuable data, the security researchers wrote. According to the report, the attacks were ongoing as late as last month.