The Chinese government managed to gain access to the servers of almost 30 U.S. companies, including Apple and Amazon, according to a Bloomberg report.

The infiltration occurred via the placement of tiny microchips, about the size of a rice grain, affixed to motherboards deployed by U.S-based server motherboard specialist Supermicro. Chinese government loyalist groups allegedly infiltrated the company’s supply chain, allowing them to place the malicious chips in the factory during the manufacturing process.

The chips allowed the attackers to create a stealth doorway into any network that included the altered machines, according to Bloomberg.

Bloomberg reported that Amazon Web Services uncovered the malicious chips in 2015 when examining servers manufactured by a company known as Elemental Technologies, which AWS eventually acquired.

Apple and Amazon have denied the report, saying their systems are intact, and there is no evidence of malicious chips or hardware modifications to support the report. Supermicro and China’s Ministry of Foreign Affairs also denied the report.

Bloomberg maintained the veracity of its report by stating thus:

“The companies’ denials are countered by six current and former senior national security officials, who—in conversations that began during the Obama administration and continued under the Trump administration—detailed the discovery of the chips and the government’s investigation.”

The report comes amid increased concerns regarding foreign intelligence agencies infiltrating U.S. and other companies through foreign supply chains.