In a troubling new development, threat actors looking for different ways to break into and remain undetected on enterprise networks appear to have begun targeting routers connecting businesses to the Internet.

Once considered a largely theoretical risk, backdoored business routers could soon pose a big problem for enterprises, security vendor FireEye warned in a report released Tuesday.

FireEye said it has discovered at least 14 instances worldwide where attackers have managed to successfully replace the firmware on Cisco business routers with a malicious implant dubbed SYNful Knock by the security vendor.

The implant is basically a clandestine modification of the router’s lOS image and allows attackers to maintain persistence on a compromised system even through reboots, FireEye said. The vendor described the implant as fully modular and customizable in design and capable of being remotely updated after installation.

The implant gives attackers a way to load different modules and executable files on the compromised router and provides attackers with unrestricted access to the system via a backdoor password.

The backdoor is extremely hard to detect on the compromised Cisco routers. It gives attackers a way to not only maintain persistence but also a way to compromise other systems and data on the network to which the router is connected, FireEye said.

According to FireEye, there is no evidence that the attackers exploited any zero-day vulnerability or other flaw to install the malicious implant on the 14 Cisco routers. Instead, the routers likely had default credentials that gave the attackers easy access to the system. Or, the attackers somehow managed to gain access to the credentials in order to install the backdoor.

The routers on which the malicious implant was discovered include the now discontinued Cisco 1841, Cisco 2811 and Cisco 3825 integrated services routers. Other models are likely vulnerable to the poisoned implant as well based on the similarity in their code base and functionality, the FireEye report noted.

However, there is no reason to conclude that only Cisco routers are being targeted with malicious firmware implants. “We believe that the detection of SYNful Knock is just the tip of the iceberg when it comes to attacks utilizing modified router images (regardless of vendor),” FireEye said.

“As attackers focus their efforts on gaining persistent access, it is likely that other undetected variants of this implant are being deployed throughout the globe.”

The threat from modified router implants of the type discovered by FireEye has been considered largely theoretical especially in actual use, the report said. But there’s evidence suggesting that a growing number of such implants are available in the wild.

As one example, the report pointed to a recent alert from Cisco warning customers about an evolution in attacks against the Cisco IOS platform.

The alert noted that Cisco had observed some instances where attackers gained physical or administrative access to a Cisco router and replaced the IOS bootstrap image with a malicious implant.

“In all cases seen by Cisco, attackers accessed the devices using valid administrative credentials,” the network giant noted. They then used a legitimate field upgrade process to install the malicious implant on their systems, Cisco said. No vulnerability was exploited in any of the cases that Cisco has seen so far, the company added.

The FireEye report outlines some measures that enterprise can take if they believe their router is compromised. The most effective measure is to reimage the router with a clean IOS image from Cisco, the company said.

“After fixing the routers, focus on the rest of the network,” FireEye said. “If the router did not have default credentials, then the infection must have occurred some other way. A compromise assessment should follow.”