Attackers install highly persistent malware implants on Cisco routers

The router implant, dubbed SYNful Knock, provides attackers with highly privileged backdoor access to the affected devices and persists even across reboots. This is different than the typical malware found on consumer routers, which gets wiped from memory when the device is restarted.

SYNful Knock is a modification of the IOS operating system that runs on professional routers and switches made by Cisco Systems. So far it was found by Mandiant researchers on Cisco 1841, 8211 and 3825 "integrated services routers," which are typically used by businesses in their branch offices or by providers of managed network services.

Mandiant, a subsidiary of cybersecurity firm FireEye that specializes in incident response services, has seen the rogue firmware on 14 routers in Mexico, Ukraine, India and the Philippines.

The models confirmed to be affected are no longer being sold by Cisco, but there's no guarantee that newer models are won't be targeted in the future or haven't been already.

Cisco published a security advisory in August warning customers about new attacks that install rogue firmware on routers made by the company.

In the cases investigated by Mandiant the SYNful Knock implant was not deployed through a vulnerability, but most likely through default or stolen administrative credentials. The modifications made to the firmware image were done specifically to keep its size identical to the size of the original one.

The rogue firmware implements a backdoor password for privileged Telnet and console access and also listens for commands contained in specifically crafted TCP SYN packets -- hence the name SYNful Knock.

The custom "knocking" procedure can be used to instruct the rogue firmware to inject additional malicious modules into the router's memory. Unlike the backdoor password though, these modules are not persistent across device reboots.

Router compromises are very dangerous because they give attackers the ability to sniff and modify network traffic, direct users to spoofed websites and launch other attacks against devices, servers and computers located inside isolated networks.

Routers don't typically get the same level of security attention as employee workstations or application servers that companies actually expect to be attacked. They're not protected by firewalls and don't have antimalware products running on them.

"Finding backdoors within your network can be challenging; finding a router implant, even more so," the Mandiant security researchers said Tuesday in a blog post. "This backdoor provides ample capability for the attacker to propagate and compromise other hosts and critical data using this as a very stealthy beachhead."

A Mandiant white paper provides indicators of compromise that can be used to detect SYNful Knock implants, both locally on the routers and at the network level.

"It should be evident now that this attack vector is very much a reality and will most likely grow in popularity and prevalence," the researchers warned.