System File Checker (SFC) is a utility in Microsoft Windows that allows users to scan for and restore corrupted Windows system files.

Overview

Microsoft ships this utility with

Windows 98 Windows 98 is a consumer-oriented operating system developed by Microsoft as part of its Windows 9x family of Microsoft Windows operating systems. The second operating system in the 9x line, it is the successor to Windows 95, and was released t ...

Windows 2000 Windows 2000 is a major release of the Windows NT operating system developed by Microsoft and oriented towards businesses. It was the direct successor to Windows NT 4.0, and was released to manufacturing on December 15, 1999, and was officially ...

and all subsequent versions of the Windows NT family of operating systems. In Windows Vista,

Windows 7 Windows 7 is a major release of the Windows NT operating system developed by Microsoft. It was Software release life cycle#Release to manufacturing (RTM), released to manufacturing on July 22, 2009, and became generally available on October 22, ...

and Windows 10, System File Checker is integrated with Windows Resource Protection (WRP), which protects

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keys and folders as well as critical system files. Under Windows Vista, sfc.exe can be used to check specific folder paths, including the Windows folder and the boot folder.

Windows File Protection Windows File Protection (WFP), a sub-system included in Microsoft Windows operating systems of the Windows 2000 and Windows XP era, aims to prevent programs from replacing critical Windows system files. Protecting core system files mitigates proble ...

(WFP) works by registering for notification of file changes in

Winlogon In computing, Winlogon (Windows Logon) is the component of Microsoft Windows operating systems that is responsible for handling the secure attention sequence, loading the user profile on logon, and optionally locking the computer when a screensav ...

. If any changes are detected to a protected system file, the modified file is restored from a cached copy located in a compressed folder at %WinDir%\System32\dllcache. Windows Resource Protection works by setting discretionary access-control lists (DACLs) and

access control list In computer security, an access-control list (ACL) is a list of permissions associated with a system resource (object). An ACL specifies which users or system processes are granted access to objects, as well as what operations are allowed on giv ...

s (ACLs) defined for protected resources. Permission for full access to modify WRP-protected resources is restricted to the processes using the ''Windows Modules Installer service'' (TrustedInstaller.exe). Administrators no longer have full rights to system files.

History

Due to problems with Windows applications being able to overwrite system files in

Windows 95 Windows 95 is a consumer-oriented operating system developed by Microsoft as part of its Windows 9x family of operating systems. The first operating system in the 9x family, it is the successor to Windows 3.1x, and was released to manufacturi ...

, Microsoft has since implemented a number of security measures to protect system files from malicious attacks, corruptions, or problems such as DLL hell. System File Checker was first introduced on

as a GUI utility. It offered scanning and restoration of corrupted system files by matching the version number against a database containing the original version number of the files in a fresh Windows 98 installation. This method of file protection was basic. It determined system files by file extension and

file path A path is a string of characters used to uniquely identify a location in a directory structure. It is composed by following the directory tree hierarchy in which components, separated by a delimiting character, represent each directory. The d ...

. It was able to restore files from the installation media or a source specified by the user. Windows 98 did not offer real-time system file protection beyond file attributes; therefore, no preventive or reactive measure was available. All Windows NT-based operating systems since

introduced real-time file protection, called

(WFP). In addition, the System File Checker utility (sfc.exe) was reimplemented as a more robust command-line utility that integrated with WFP. Unlike the Windows 98 SFC utility, the new utility forces a scan of protected system files using Windows File Protection and allows the immediate silent restoration of system files from the ''DLLCache'' folder or installation media. SFC did not appear on

Windows ME Windows Millennium Edition, or Windows Me (marketed with the pronunciation of the pronoun "me"), is an operating system developed by Microsoft as part of its Windows 9x family of Microsoft Windows operating systems. It is the successor to Win ...

, as it was replaced with System File Protection (SFP). Similar to WFP, SFP offered real-time protection.

Issues

The System File Checker component included with versions of Windows 2000 earlier than Service Pack 4 overrode patches distributed by Microsoft; this was rectified in Windows 2000 Service Pack 4.

Usage

In Windows NT-based operating systems, System File Checker can be invoked via

Windows Command Prompt Command Prompt, also known as cmd.exe or cmd, is the default command-line interpreter for the OS/2, eComStation, ArcaOS, Microsoft Windows (Windows NT family and Windows CE family), and ReactOS operating systems. On Windows CE .NET 4.2, Win ...

(with Admin privilege), with the following command: sfc /scannow (to repair problems) or sfc /verifyonly (no repair) If it finds a problem, it will attempt to replace the problematic files from the ''DLL Cache'' (%WinDir%\System32\Dllcache\). If the file is not in the DLL Cache or the DLL Cache is corrupted, the user will be prompted to insert the Windows installation media or provide the network installation path. System File Checker determines the Windows installation source path from the registry values SourcePath and ServicePackSourcePath. It may keep prompting for the installation media even if the user supplies it if these values are not correctly set. In Windows Vista and onwards, files are protected using

Access control list In computer security, an access-control list (ACL) is a list of permissions associated with a system resource (object). An ACL specifies which users or system processes are granted access to objects, as well as what operations are allowed on giv ...

s (ACLs), however, the above command has not changed. System File Checker in Windows Vista and later Windows operating systems can scan specified files. Also, scans can be performed against an offline Windows installation folder to replace corrupt files, in case the Windows installation is not bootable. For performing offline scans, System File Checker must be run from another working installation of Windows Vista or a later operating system or from the Windows setup DVD or a recovery drive which gives access to the Windows Recovery Environment. In cases where the component store is corrupted, the "System Update Readiness tool" (CheckSUR) can be installed on Windows 7, Windows Vista, Windows Server 2008 R2 or Windows Server 2008, replaced by "

Deployment Image Service and Management Tool The Windows Imaging Format (WIM) is a file-based disk image format. It was developed by Microsoft to help deploy Windows Vista and subsequent versions of the Windows operating system family, as well as Windows Fundamentals for Legacy PCs. Design ...

" (DISM) for Windows 10, Windows 8.1, Windows 8, Windows Server 2012 R2 or Windows Server 2012. This tool checks the store against its own payload and repairs the corruptions that it detects by downloading required files through Windows update.

References

External links