First published on TechNet on Oct 31, 2005
Last week when I was testing the latest version of
(RKR) I ran a scan on one of my systems and was shocked to see evidence of a rootkit. Rootkits are cloaking technologies that hide files, Registry keys, and other system objects from diagnostic and security software, and they are usually employed by malware attempting to keep their implementation hidden (see my
article from thre June issue of Windows IT Pro Magazine for more information on rootkits). The RKR results window reported a hidden directory, several hidden device drivers, and a hidden application:
Given the fact that I’m careful in my surfing habits and only install software from reputable sources I had no idea how I’d picked up a real rootkit, and if it were not for the suspicious names of the listed files I would have suspected RKR to have a bug. I immediately ran
to look for evidence of code that would activate the rootkit each boot, but I came up empty with both tools. I next turned to
, a tool I wrote for
Inside Windows 2000
and that lets you explore the internals of a live system using the Microsoft kernel debugger, to determine what component was responsible for the cloaking.
Rootkits that hide files, directories and Registry keys can either execute in user mode by patching Windows APIs in each process that applications use to access those objects, or in kernel mode by intercepting the associated kernel-mode APIs. A common way to intercept kernel-mode application APIs is to patch the kernel’s system service table, a technique that I pioneered with Bryce for Windows back in 1996 when we wrote the first version of
. Every kernel service that’s exported for use by Windows applications has a pointer in a table that’s indexed with the internal service number Windows assigns to the API. If a driver replaces an entry in the table with a pointer to its own function then the kernel invokes the driver function any time an application executes the API and the driver can control the behavior of the API.
It’s relatively easy to spot system call hooking simply by dumping the contents of the service table: all entries should point at addresses that lie within the Windows kernel; any that don’t are patched functions. Dumping the table in Livekd revealed several patched functions:
I listed one of the intercepting functions and saw that it was part of the Aries.sys device driver, which was one of the images I had seen cloaked in the $sys$filesystem directory:
Armed with the knowledge of what driver implemented the cloaking I set off to see if I could disable the cloak and expose the hidden processes, files, directories, and Registry data. Although RKR indicated that the \Windows\System32\$sys$filesystem directory was hidden from the Windows API, it’s common for rootkits to hide directories from a directory listing, but not to prevent a hidden directory from being opened directly. I therefore checked to see if I could examine the files within the hidden directory by opening a command prompt and changing into the hidden directory. Sure enough, I was able to enter and access most of the hidden files:
Perhaps renaming the driver and rebooting would remove the cloak, but I also wanted to see if Aries.sys was doing more than cloaking so I copied it to an uncloaked directory and loaded it into
, a powerful disassembler I use in my exploration of Windows internals. Here’s a screenshot of IDA Pro’s disassembly of the code that calculates the entries in the system service table that correspond to the functions it wants to manipulate:
I studied the driver’s initialization function, confirmed that it patches several functions via the system call table and saw that its cloaking code hides any file, directory, Registry key or process whose name begins with “$sys$”. To verify that I made a copy of Notepad.exe named $sys$notepad.exe and it disappeared from view. Besides being indiscriminate about the objects it cloaks, other parts of the Aries code show a lack of sophistication on the part of the programmer. It’s never safe to unload a driver that patches the system call table since some thread might be just about to execute the first instruction of a hooked function when the driver unloads; if that happens the thread will jump into invalid memory. There’s no way for a driver to protect against this occurrence, but the Aries driver supports unloading and tries to keep track of whether any threads are executing its code. The programmer failed to consider the race condition I’ve described. They’ll have to come up with a new approach to their rootkit sooner or later anyway, since system call hooking
does not work at all on x64 64-bit versions of Windows
After I finished studying the driver's code I rebooted the system. The cloak was gone as I expected and I could see all the previously hidden files in Explorer and Registry keys in Regedit. I doubted that the files had any version information, but ran my
utility on them anyway. To my surprise, the majority did have identifying product, file and company strings. I had already recognized Dbghelp.dll and Unicows.dll as Microsoft Windows DLLs by their names. The other files claimed to be part of the “Essential System Tools” product from a company called “First 4 Internet”:
I entered the company name into my Internet browser’s address bar and went to
. I searched for both the product name and Aries.sys, but came up empty. However, the fact that the company sells a technology called XCP made me think that maybe the files I’d found were part of some content protection scheme. I Googled the company name and came across
, confirming the fact that they have deals with several record companies, including Sony, to implement Digital Rights Management (DRM) software for CDs.
The DRM reference made me recall having purchased a CD recently that can only be played using the media player that ships on the CD itself and that limits you to at most 3 copies. I scrounged through my CD’s and found it, Sony BMG’s
Get Right with the Man
(the name is ironic under the circumstances) CD by the Van Zant brothers. I hadn’t noticed when I purchased the CD from Amazon.com that it’s protected with DRM software, but if I had looked more closely at the text on the Amazon.com web page I would have known:
The next phase of my investigation would be to verify that the rootkit and its hidden files were related to that CD’s copy protection, so I inserted the CD into the drive and double-clicked on the icon to launch the player software, which has icons for making up to three copy-protected backup CDs:
Process Explorer showed the player as being from Macromedia, but I noticed an increase in CPU usage by $sys$DRMServer.exe, one of the previously cloaked images, when I pressed the play button. A look at the Services tab of its process properties dialog showed it contains a service named “Plug and Play Device Manager”, which is obviously an attempt to mislead the casual user that stumbles across it in the Services MMC snapin (services.msc) into thinking that it’s a core part of Windows:
I closed the player and expected $sys$DRMServer’s CPU usage to drop to zero, but was dismayed to see that it was still consuming between one and two percent. It appears I was paying an unknown CPU penalty for just having the process active on my system. I launched
and Regmon to see what it might be doing and the Filemon trace showed that it scans the executables corresponding to the running processes on the system every two seconds, querying basic information about the files, including their size,
times each scan. I was quickly losing respect for the developers of the software:
I still had to confirm the connection between the process and the CD’s player so I took a closer look at each process. Based on the named pipe handles I saw they each had opened when I looked in Process Explorer’s handle view I suspected that the player and $sys$DRMServer communicated via named pipes and so I launched Filemon, checked Named Pipes in the Volumes menu, and confirmed my theory:
At that point I knew conclusively that the rootkit and its associated files were related to the First 4 Internet DRM software Sony ships on its CDs. Not happy having underhanded and sloppily written software on my system I looked for a way to uninstall it. However, I didn’t find any reference to it in the Control Panel’s Add or Remove Programs list, nor did I find any uninstall utility or directions on the CD or on First 4 Internet’s site. I checked the
and saw no mention of the fact that I was agreeing to have software put on my system that I couldn't uninstall. Now I was mad.
I deleted the driver files and their Registry keys, stopped the $sys$DRMServer service and deleted its image, and rebooted. As I was deleting the driver Registry keys under HKLM\System\CurrentControlSet\Services I noted that they were either configured as boot-start drivers or members of groups listed by name in the HKLM\System\CurrentControlSet\Control\SafeBoot subkeys, which means that they load even in Safe Mode, making system recovery extremely difficult if any of them have a bug that prevents the system from booting.