Cisco also said that the hacker’s ability to install malicious firmware exists only for older company products. Newer ones are equipped with secure boot capabilities that prevent them from running unauthorized firmware, the company said.

To install their modified bootloader, the US and Japanese advisory said, the threat actors install an older version of the legitimate firmware and then modify it as it runs in memory. The technique overrides signature checks in the Cisco ROM monitor signature validation functions, specifically functions of Cisco’s IOS Image Load test and the Field Upgradeable ROMMON Integrity test. The modified firmware, which consists of a Cisco IOS loader that installs an embedded IOS image, allows the compromised routers to make connections over SSH without being recorded in event logs.