Attack Sneaks Rootkits Into Linux Kernel

Kernel rootkits are tough enough to detect, but a researcher this week has demonstrated an even sneakier method of hacking Linux.

The attack attack exploits an oft-forgotten function in Linux versions 2.4 and above in order to quietly insert a rootkit into the operating system kernel as a way to hide malware processes, hijack system calls, and open remote backdoors into the machine, for instance. At Black Hat Europe this week in Amsterdam, Anthony Lineberry, senior software engineer for Flexilis, will demonstrate how to hack the Linux kernel by exploiting the driver interface to physically addressable memory in Linux, called /dev/mem.

"One of bonuses of this [approach] is that most kernel module rootkits make a lot noise when they are inserting [the code]. This one is directly manipulating" the memory, so it's less noticeable, he says.

The /dev/mem "device" can be opened like a file, and you can read and write to it like a text file, Lineberry says. It's normally used for debugging the kernel, for instance.

Lineberry has developed a proof-of-concept attack that reads and writes to kernel memory as well as stores code inside the kernel, and he plans to release a framework at Black Hat that lets you use /dev/mem to "implement rootkit-like behaviors," he says.

The idea of abusing /dev/mem to hack the Linux kernel is not really new, he says. "People have known what you can do with these /dev/mem devices, but I have never seen any rootkits with dev/mem before," he says.

[UPDATE: But Linux experts point out that the technique Lineberry is demonstrating at Black Hat indeed been has been deployed before with the so-called SuckIT rootkit, and as far back as the late 1990s with direct kernel-object modification (DKOM) rootkits].

This method of attack is not as simple as writing a kernel module like a lot of rootkit attacks entail, however, Lineberry says. "This is a lot more tedious and requires a lot more in-depth knowledge of kernel internals and systems architecture ... But the framework enables you to leverage this a little more easily."

And the /dev/mem exploitation is just the first step in an attack on Linux. "After you've exploited a box and have root access, at that point you can use it to maintain that access and anything any rootkit can do -- hiding processes, files, and controlling network traffic," Lineberry says.

Linux system administrators typically aren't aware of the potential dangers of leaving /dev/mem exposed, he says. Lineberry says his goal is to educate them on this potential security hole. And there's now a way to defend against such an attack, too: the Linux development community recently issued a patch to locks down /dev/mem, limiting read and write access from the outside, he says.

"The problem with kernel-based rootkits is that the rootkit can mitigate [detection] because it has control," he says. "It's a race in the kernel to see who's going to see who first."

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