Spy agencies have some very smart tools at their disposal for their surveillance and data gathering operations, many leaks have revealed so far, but Russian security agency Kaspersky Lab has uncovered one of the most important such cyber weapon available to the NSA, Reuters reports. According to the company, a country which it decided not to identify, but which the publication confirmed to be the U.S., has been able to sneak malware in a critical part of a computer, which practically made it possible not only to spy on a variety of targets with ease, but also bypass any security measures, and reinfect the machines over and over.

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Apparently, the NSA managed to create a smart virus that can hide in a hard drive’s own firmware that boots up when the PC is turned on. Regular anti-virus programs would not discover the infection, and computers with such drives inside would offer a treasure trove of information to the spy agency controlling them.

According to a former intelligence operative, the NSA did indeed develop the technology, which is valued “as highly as Stuxnet.”

Reuters further reveals that disk drive firmware is viewed by spies and cybersecurity experts as “the second-most valuable real estate on a PC for a hacker, second only to the BIOS code invoked automatically as a computer boots up.”

Drives from multiple companies have been found to contain the virus, including Western Digital, Seagate, Toshiba, IBM, Micro Technology and Samsung. Of those, Western Digital, Seagate and Micron said they have no knowledge of the matter, while Toshiba and Samsung did not comment.

According to Kaspersky, getting the malicious code into a drive’s firmware is not an easy task, and can’t be done with help of public tools, meaning that the NSA had to have access to source codes for hard drives produced by these companies.