A security researcher has discovered a method of hacking Cisco routers with only basic knowledge about the targeted device.

Although researchers have found various vulnerabilities in Cisco routers, exploits mostly have been focused on hacks of specific IOS router configurations, which require targeted and skilled attacks. But Felix "FX" Lindner, a researcher with Recurity Labs, demonstrated last week (PDF) at the 25th Chaos Communication Congress in Berlin a technique that lets an attacker execute code remotely on Cisco routers, regardless of their configuration.

"The bottom line is that before, all IOS exploits had to know the exact IOS image running on the target to get code execution. This is approximately a 1 to 100,000 chance, [and] a professional attacker doesn't risk his 0-day exploit when the odds are stacked against him like that," Linder says. "I [demonstrated] that at least on one group of Cisco routers, there is a way to execute code without knowing the IOS image version [they are] running."

Lindner says his exploit method is independent of a router vulnerability, and applies only to stack-buffer overflow bugs. He was able to execute memory writes and to disable CPU caches on Cisco routers running on the PowerPC CPU. Lindner hasn't yet tested his technique on larger, more expensive Cisco routers, but plans to do so eventually.

Security researcher Dan Kaminsky says FX's hack disproves conventional wisdom in enterprises that routers are at low risk of attack, and that patching them is riskier than an attack due to the potential network outages that patching can incur.

"Patching is hard. Patching something that, if there's a failure, causes widespread network outages. Having a vaguely credible reason not to patch, like 'the attacker would have to build an attack specifically targeted to my specific version of IOS,' has been something of a mantra for those who haven't wanted to risk updating their systems," says Kaminsky, who is director of penetration testing for IOActive. "The mantra is [now] visibly, irrevocably disproved. FX has shown that however much changes from one version to another, there's a little piece of every Cisco router that's the same, and he can use that piece to attack most of the hardware out there."

The research also broke the barrier of one exploit, one router: "Three-and-a-half years ago it was international news that one guy got one exploit working on one Cisco router," he says. "Today, it's now clear that it's possible, given a flaw in IOS, to reliably exploit many versions of many routers.

"Something we thought was difficult, for no good reason, is now shown to be so very simple after all," Kaminsky says.

Lindner says his research shows that detection of Cisco IOS attacks should be focused on the payload of an exploit, such as a backdoor program, rather than on the exploit itself.

To protect against a widespread router attack, Cisco should encourage users to run more standard versions of IOS, as well as make IOS more modular to simplify patching for its customers with customized versions, Kaminsky says.

The bottom line is that enterprises need to change their tune when it comes to their IOS patching policy. When a remote-code execution bug is discovered in one of their routers, they should treat it like any other system vulnerability, Kaminsky says. "They should treat it as equivalent to a vulnerability against sensitive servers or domain controllers, and act accordingly," he says. And that may require working closely with Cisco on just how to do that, he adds.

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