Cisco Offers Free Tool To Detect SYNful Knock Router Malware

Cisco Systems enterprise users now have a way to check if their network routers might have been infected by the recently disclosed SYNful Knock malware.

The company yesterday released a new Python script that organizations can use to scan their networks for potentially infected routers. The free tool works by looking for routers on the network that answer to the malware’s specific “knock,” William McVey, technical lead at Cisco’s Talos Threat Intelligence group said in a blog post.

The tool can be used to detect hosts compromised with currently known versions of SYNful Knock, McVey said. “But it cannot establish that a network does not have malware that might have evolved to use a different set of signatures,” he added in a somewhat confusingly worded caveat.

"The tool injects custom crafted packets at the Ethernet layer (layer 2) and monitors and parses the responses,” McVey said. “This functionality requires that the tool be run with root privileges.”

SYNful Knock is basically malware code that allows attackers to gain nearly undetectable and persistent remote control over certain Cisco business routers.

Security vendor FireEye, which issued an alert on the issue last week, described it as a malicious implant designed to replace and masquerade as the legitimate firmware on a handful of now discontinued Cisco router models (Cisco 1841, Cisco 2811 and Cisco 3825). Other models are likely impacted as well, FireEye said, based on its observation of the malware and the impacted systems.

SYNful Knock gives attackers complete administrative control over a compromised router via a backdoor password and provides them with a platform from which to launch attacks against other systems and routers on the same network. FireEye said it has discovered at least 14 Cisco network routers, used by businesses to connect to the Internet, infected with the poisoned implant.

Contrary to what some might expect, the attack is not the result of a security flaw in any of the affected Cisco products. Instead, in each case the attackers appear to have managed to either gain actual physical access to the devices, or used administrative credentials to break into the systems and plant the malware.

Attacks involving the swapping out and replacing of firmware in a commercial-grade router with a rogue version have up to now been considered largely theoretical. The appearance of SYNful Knock suggests otherwise and shows that threat actors have begun exploring ways to backdoor the critical network routers that organizations use to connect to the Internet.

As FireEye noted in its report last week, SYNful Knock could well be the first of a new kind of attack tactic involving the use of modified router images to gain remote control of the devices. The same kind of malicious firmware that was implanted on the Cisco routers can be loaded on routers from other vendors as well.

“Routers are one of the Holy Grail targets for attackers because they lie outside of many normal security protections,” says Lamar Bailey, leader of Tripwire's Vulnerability and Exposures Research Team. “Modifying firmware for your own needs or to add new features is a common practice and has been used to great success on home routers and access points,” Bailey says. “This is just the same practice used on a grander scale.”