Cisco Systems officials are warning customers of a series of attacks that completely hijack critical networking gear by swapping out the valid ROMMON firmware image with one that’s been maliciously altered.

The attackers use valid administrator credentials, an indication the attacks are being carried out either by insiders or people who have otherwise managed to get hold of the highly sensitive passwords required to update and make changes to the Cisco hardware. Short for ROM Monitor, ROMMON is the means for booting Cisco’s IOS operating system. Administrators use it to perform a variety of configuration tasks, including recovering lost passwords, downloading software, or in some cases running the router itself. In an advisory published Wednesday company officials wrote:

In all cases seen by Cisco, attackers accessed the devices using valid administrative credentials and then used the ROMMON field upgrade process to install a malicious ROMMON. Once the malicious ROMMON was installed and the IOS device was rebooted, the attacker was able to manipulate device behavior. Utilizing a malicious ROMMON provides attackers an additional advantage because infection will persist through a reboot.

No product vulnerability is leveraged in this attack, and the attacker requires valid administrative credentials or physical access to the system to be successful. The ability to install an upgraded ROMMON image on IOS devices is a standard, documented feature that administrators use to manage their networks. No CVE ID will be assigned.

The significance of the advisory isn’t that the initial firmware can be replaced. As indicated, that’s a standard feature not only with Cisco gear but just about any computing device. What’s important is that attackers are somehow managing to obtain the administrative credentials required to make unauthorized changes that take control of the networking gear. The advisory doesn’t say how the attackers are obtaining the credentials.