DKOM Opens Door to Malware Rootkits

Much malware comes with a kernel rootkit component. Subverting the Windows kernel is indeed the best way to conceal malicious activities on infected systems. To achieve this, many types of malware load malicious device drivers that enjoy full access to all kernel objects. However, this technique is somewhat noisy, and loading a new driver is not really stealthy.

At McAfee Labs we recently ran across a W32/IRCBot.gen.ac sample that uses Direct Kernel Object Manipulation (DKOM) to hide itself without loading a new driver. This technique seems impossible at first sight because modifying kernel memory pages from userland is not allowed. However, W32/IRCBot.gen.ac takes advantage of an undocumented function exported by ntdll.dll that provides debugging functionalities at the kernel level.

NtSystemDebugControl(), despite being undocumented, has been known for many years. It provides simple functions such as reading from and writing to any location within the kernel memory. And this is exactly what a piece of malware needs to manipulate kernel objects.

W32/IRCBot.gen.ac starts by checking what version of Windows it’s running on. This technique won’t work under Windows Vista or Windows 7. If the infected machine is not running Windows XP, W32/IRCBot.gen.ac gives up and doesn’t try to hide itself.

If it does find Windows XP, W32/IRCBot.gen.ac opens the current process’ token to ensure it has the SeDebugPrivilege, which is required to call NtSystemDebugControl().

To find the process list in the kernel memory, W32/IRCBot.gen.ac retrieves the address of the global variable PsInitialSystemProcess, which points to the EPROCESS structure of the system process.

W32/IRCBot.gen.ac can now find the process list in memory and go through it to find its own process. It then removes itself from the process list by calling NtSystemDebugControl() to write to kernel memory.

The malicious process is no longer visible in the Windows Task Manager or other tools such as Process Explorer. However, monitoring TCP connections will quickly reveal the presence of an offending process whose name can’t be found.

Rootkit Detective also detects processes hidden via DKOM.

Accessing kernel memory from userland is really bad, but it appears this hole has been plugged in later versions of Windows. Using this method of calling NtSystemDebugControl() to access kernel memory is not trivial, and we don’t expect this technique to be used widely. And this is a good thing because according to Artemis, Windows XP is still the most widely deployed operating system in corporate environments. My colleagues Igor Muttik and Dmitry Gryaznov, and Joel Yonts of Advanced Auto Parts demonstrated this during McAfee’s Focus 09 conference.

Nevertheless, I offer another reminder that the bad guys never hesitate to exploit any feature, whether documented or not, as long as they can gain control over innocent machines.

Log into privileged user accounts only when required, and keep your anti-virus software up to date!