关闭windows 文件保护机制 SFC

Hacking Windows File Protection Jeremy Collake Bitsum Technologies .

Useful tools: - Tool to replace WFP protected files. (x32 and x64) [C++ source included] - Tool to disable WFP on specific folders. (x32 only) - Console app to replace in-use files. - Console app to correct PE image checksums.

Quick links: Disable WFP for specific folders until the computer is next rebooted via manual handle manipulation Disable WFP completely until the computer is next rebooted via undocumented SFC API : Disable WFP on a specific file for 1 minute via undocumented SFC API : Disable WFP permanently via patches and undocumented registry value : Disable WFP permanently on specific files via patching the protected file list

Hacking Windows File Protection

Windows File Protection (WFP) is a mechanism that protects system files from being modified or deleted. Introduced in Windows 2000, Windows File Protection was a leap forward in operating system stability since it protected the core modules from being corrupted or updated except by service packs or hotfixes sent from Microsoft.

A big problem prior to Windows 2000 was 'DLL Hell'. Applications would often update system modules with their own versions, regardless if other applications already installed were depending on a different version of that same module. Although Microsoft recommended that application programmers place modules into the program's folder instead of the system folder, few programmers did. WFP solved DLL Hell, along with many other issues.

In order to protect the integrity of the system, Microsoft did not document a way to disable WFP. If they had, programmers would surely begin to circumvent it and have their application installers overwrite system modules with their own versions. Booting to Safe Mode was the only way Microsoft provided for replacing a protected file. In theory, this was a good idea. However, programmers and power users sometimes desire the power to replace or delete protected modules without the cumbersome process of booting to safe mode and back.

Enter the hacks.

In Windows 2000, a hidden registry value to fully disable WFP existed. Unfortunately, this didn't last long after I discovered and posted it to NTBugTraq. Microsoft soon tweaked their code so that the hidden registry value was neutralized (curiously, it wasn't completely removed from the code). I then created patches to re-enable this undocumented value. describes the history of the discovery of the undocumented registry value and creation of the patches to re-enable it.

Since that time, many other techniques to disable or circumvent Windows File Protection have been discovered.

How the System File Checker monitors files for changes

The executable portion of WFP is called the System File Checker (SFC). It exists throughout SFC.DLL, SFC_OS.DLL, SFCFILES.DLL, and SFC.EXE.

The contents of SFC.DLL and SFC_OS.DLL have changed between Windows 2000 and Windows XP. In Windows XP, SFC_OS.DLL contains all the core code and SFC.DLL is merely a proxy to it. The exports of SFC.DLL are forwarded on to SFC_OS.DLL. The SFCFILES.DLL module is simply a data holder that contains a list of all files protected by WFP.

At startup the winlogon service invokes the unnamed export ordinal 1 of SFC_OS.DLL, SfcInitProt. This API launches a new free thread called the 'SFC Watcher Thread'. This thread creates a series of directory change notification events, one for each folder that contains a protected file. The WaitForMultipleObjects API is then called within loop to wait for any of these events to be signalled. Upon event signaling, the modified file(s) are determined and replaced with copies found in the cache folder, if one exists there. If it doesn't, the user is prompted to insert the Windows installation CD so that the file can be replaced from the copy found on it.

Therefore, SFC actually allows for files to be replaced or modified, then overwrites them. A few second delay is incorporated so that time is given for write operations to 'settle down' before the affected file(s) are restored.

Knowing how SFC works means that one can easily disable it by terminating the watcher thread or by closing the directory change notification event handles. This leads us to methods 1 and 2:

The core of Windows File Protection
SFC.DLL	WFP executable content. In XP only a proxy to SFC_OS.DLL.
SFC_OS.DLL	WFP executable content.
SFCFILES.DLL	Contains list of protected files. Exports SfcGetFiles API.
SFC.EXE	System File Checker utility. Utility to scan WFP protected files for changes and replace altered versions.

: Disable WFP for specific folders until the computer is next rebooted via manual handle manipulation

The first technique to disable WFP is to close the directory change notification handles by enumerating the handles that winlogon has opened, determining which ones correspond to the folder(s) we wish to deprotect by querying and comparing the handle names, then closing those handles via ntdll.NtDuplicateHandle (or kernel32.DuplicateHandle). This method is used by WfpAdmin.

: Disable WFP completely until the computer is next rebooted via undocumented SFC API

The second technique is to terminate the SFC Watcher Thread that continually waits for and responds to the directory change notification events to be signalled. Doing this manually isn't very practical since it is diffucult to be sure the right thread has been located. Fortunately, the SFC_OS.DLL exposes a nice unnamed export at ordinal 2: SfcTerminateWatcherThread.. This API accepts no parameters and does exactly as its name implies. However, there is one caveat to using this function: It must be invoked in the process that created the SFC Watcher Thread: winlogon. To accomplish this, virtual memory needs to be allocated in the winlogon process space and a thread procedure that invokes SfcTerminateWatcherThread copied into that memory. The thread procedure should then be invoked using kernel32.CreateRemoteThread and WFP will be disabled until the winlogon process restarts (computer is rebooted).

Ordinal 2:
DWORD WINAPI SfcTerminateWatcherThread();

The return value is 0 if success, or 1 if an error occurred .

: Disable WFP on a specific file for 1 minute via undocumented SFC API

The SFC_OS.DLL module exports another very useful undocumented, unnamed API at oridinal 5: SfcFileException. This handy API will register a temporary SFC exception for specific file, allowing the file to be updated. The period the exception is in place is currently one minute.

Ordinal 5:
DWORD WINAPI SfcFileException(DWORD dwUnknown0, PWCHAR pwszFile, DWORD dwUnknown1);

dwUnknown0	Unknown. Set to 0
pwszFile	Filename
dwUnknown1	Unknown. Set to -1

The return value is 0 if success, or 1 if an error occurred (usually that the file is not protected by WFP).

An example call to this API is:
SetSfcFileException(0, L"c:\\windows\\notepad.exe",-1);

: Disable WFP permanently via patches and undocumented registry value

Prior to Windows 2000 SP1 there was an undocumented registry value that would fully disable WFP. This is the famous 0xffffff9d value I discovered while reverse engineering SFC.DLL in Windows2000. Unfortunately, soon after its discovery Microsoft disabled it. Fortunately, the core code to disable WFP was left in SFC.DLL (later moved to SFC_OS.DLL). Therefore, a simple patch to SFC.DLL or SFC_OS.DLL will re-enable this value. I've created patches for 2K and XP and have generalized the patching procedure so the patch may be applied to all current and (hopefully) future versions of the SFC module without having to worry about a specific patch address.

General patch procedure:

For Windows 2000, the patch is applied to SFC.DLL.
For Windows XP and 2003, the patch is applied to SFC_OS.DLL.

Copy the target file to a temporary one.

Search for the bytes '83 F8 9D 75 07 8B C6'.
Replace the '8B C6' with '90 90'.

You must correct the checksum of the image by using our utility. It can be obtained .

Now set the temporary file to replace the original at boot-time by using our utility. It can be obtained .

Set the SFCDisable value described below and then reboot the computer to complete the process.

Undocumented SFCDisable value:

Key: HKEY_LOCAL_MACHINE\Software\Policies\Microsoft\Windows NT\Windows File Protection
Value name: SFCDisable
Value: 0xFFFFFF9D

: Disable WFP permanently for specific files via patching the protected file list

More simple than patching executable code is simply patching the list of files contained in SFCFILES.DLL. First, copy SFCFILES.DLL to a temporary file. Using a hex editor (i.e. UltraEdit), search for files to disable protection on inside the temporary file. Once found, replacing the first character of the file name with 0 (that is: value 0 NOT ascii '0' character). After completing the modifications, correct the checksum using our utility and set the temporary file to replace the original at boot-time using our utility. Reboot the computer to finish the process.

-Jeremy Collake

Like this article? Please feed me by donating to via paypal ;).

The End

---

 

The following are original publications preserved for historical reasons, misc. technical details, or other useless stuff.
Stop reading here.
 

---

: Original publication of undocumented registry setting to fully disable WFP
(no longer works without binary patches)

---

NTBugTraq Posting:
 

W2k undocumented registry setting fully disables Windows File Protection

From: Jeremy Collake ([e-mail])
Date: Sat Jun 24 2000 - 05:16:46 CDT

6:13am 6/24
 

Summary: Undocumented registry setting allows for
   Windows File Protection (aka System File Checker)
   to be fully disabled.
 

HowTo: Set the SFCDisable value (see Q222473) to
  0xffffff9d.
 

Ok, after spending 6 hours in the guts of sfc.dll, sfcfiles.dll,
and winlogon.exe I have *finally* discovered how to permanently
disable windows file protection. The more I dug into the internals
of SFC, the more I began to think that it would not be as easy as
I first thought it would be - and indeed Microsoft does not want it
to be easy. Windows File Protection, while annoying, does provide
a good degree of system stability and even some level of virus/trojan
protection by preventing system files from being modified without
at least notifying the user. Therefore, I was *very* shocked when
I was looking through a disassembly of sfc.dll and came to the code
that checks the value of the SfcDisable in the WinLogon key.
I see in the code of ordinal 2 (which is the initialization function
that winlogon calls), sticking out like a sore thumb, this:
 

76986A89 push 1
76986A8B cmp eax, ebx
76986A8D pop esi
76986A8E jz loc_76986B97
76986A94 cmp eax, esi
76986A96 jz loc_76986B7A
76986A9C cmp eax, 2
76986A9F jz loc_76986B69
76986AA5 cmp eax, 3
76986AA8 jz short loc_76986AE0
76986AAA cmp eax, 4
76986AAD jz short loc_76986ACF
76986AAF cmp eax, 0FFFFFF9Dh
76986AB2 push ebx
76986AB3 jz loc_76986B86
76986AB9 push offset byte_76981898
76986ABE push edi
76986ABF call sub_7698877D
76986AC4 mov dword_769901D4, ebx
76986ACA jmp loc_76986B97
 

Ok, values 0, 1, 2, 3, and 4 are documented at
, but
what the heck is this 0ffffff9dh value that it accepts?! As you can
see, any value other than 0,1,2,3,4 and 0ffffff9dh are assumed to be
zero, which is the default of SFC enabled with popups enabled. So,
without further delay, I went and plugged 0ffffff9dh into the SfcDisable
value to see what was up. Rebooted. I'll be darned, Microsoft provided
a very,very simple way to fully disable WFP!
 

When booting with this value in the SFCDisable value in the WinLogon
key (HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon), an
event is written to the system log, ID 64032 from Windows File
Protection, with the description:
"Windows File Protection is not active on this system. ".
 

All attempts to replace/delete protected system files succeeded,
just as if I were in safe mode :). I rebooted a few more times and
verified that it is the one value (other than 4=popus disabled) that
is not reset to 0 after the first boot.
 

Needless to say, this is not what Microsoft intended.
 

Well, it's now 6am, hopefully I haven't mucked this up too much in
my delerium.
 

Jeremy Collake
[e-mail]

---

Appendix B: Original publication of binary patches to SFC.DLL or SFC_OS.DLL
 

---

 

Binary Patches:

If you would like to re-enable the undocumented value to disable Windows File Protection, you may apply the appropriate patch and then replace the DLL in the 2k/XP recovery console (boot to CD).  Be sure to set the checksum in the PE header by using Bitsum Technology's SetCSUM utility after patching.

How to find the patch offset yourself for current and hopefully future version of SFC.DLL or SFC_OS.DLL:

Search for the hex byte sequence: '83 F8 9D 75 07 8B C6' and replace the '8B C6' with '90 90'. If more than one occurance of this sequence is found, do not patch unless you know what you are doing. Email me at for more aid. Please do not email me with requests for instructions on how to actually apply the patches listed here. They are intentionally kept technical to prevent layman modification of windows system files which could result in an unbootable system, or worse.

This patch changes the instruction 'mov eax,esi' to two 'nop's. At this location of execution when the undocumented registry value is given, eax holds 0ffffff9d and esi is 0. Therefore, this patch prevents eax from being nullified before the switch on the registry value held in eax that follows.

Windows2000 SP2:

file:            SFC.DLL
virtual offset:  76986C11
physical offset: 6211
original:        8BC6
new:             9090

Windows 2000 SP4:

file:            SFC.DLL
virtual offset:  76986CDB
physical offset: 62DB
original:        8BC6
new:             9090

WindowsXP:

file:            SFC_OS.DLL
virtual offset:  76C6EEB8
physical offset: 0E2B8
original:        8BC6
new:             9090

WindowsXP SP1:

file:            SFC_OS.DLL
virtual offset:  76C6EFBB
physical offset: 0E3BB
original:        8BC6
new:             9090