Configuring Blocked File Extensions

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Microsoft provides the ability to restrict certain kinds of files from being uploaded or retrieved, based on the file extension. For example, a file with the .exe file extension could potentially contain code that runs on client computers when it is downloaded. Because it has the .exe file extension, the file can be run on demand when it is downloaded. If files with the .exe file extension are blocked, users can neither upload nor download a file with the .exe extension, and potentially dangerous content in the .exe file cannot be downloaded. This feature does not prevent all exploits based on file types, nor is it designed to do so.

By default, several standard file extensions are blocked, including any file extensions that are treated as executable files by Windows Explorer. Files with curly braces { or } are also blocked automatically. The file extensions blocked by default are:

File extension	File type
.ade	Microsoft Access project extension
.adp	Microsoft Access project
.app	Application file
.bas	Microsoft Visual Basic class module
.bat	Batch file
.chm	Compiled HTML Help file
.class	Java class file
.cmd	Microsoft Windows NT Command Script
.com	Microsoft MS-DOS program
.cpl	Control Panel extension
.crt	Security certificate
.dll	Windows dynamic link library
.exe	Excutable program
.fxp	Microsoft Visual FoxPro compiled program
.hlp	Help file
.hta	HTML application
.ins	Internet Naming Service
.isp	Internet Communication settings
.jse	JScript Encoded Script file
.lnk	Shortcut
.mda	Microsoft Access add-in program
.mdb	Microsoft Access program
.mde	Microsoft Access MDE database
.mdt	Microsoft Access data file
.mdw	Microsoft Access workgroup
.mdz	Microsoft Access wizard program
.msc	Microsoft Common Console Document
.msi	Microsoft Windows Installer package
.msp	Windows Installer update
.mst	Visual Test source files
.ops	Microsoft Office profile settings file
.pcd	Photo CD image or Microsoft Visual Test compiled script
.pif	Shortcut to MS-DOS program
.prf	System file
.prg	Program source file
.reg	Registration entries
.scf	Windows Explorer command file
.scr	Screen saver
.sct	Windows Script Component
.shb	Windows shortcut
.shs	Shell Scrap Object
.url	Uniform Resource Locator (Internet shortcut)
.vb	Visual Basic Scripting Edition (VBScript)file
.vbe	VBScript Encoded Script file
.vbs	VBScript file
.wsc	Windows Script Component
.wsf	Windows Script file
.wsh	Windows Script Host Settings file

The list of file extensions is controlled for the entire server or server farm and is recorded in the configuration database. Because the list of blocked file types is maintained by file extension, all files that use a file extension on the list cannot be uploaded or downloaded, irrespective of the file's intended use. If .asp is on the list of extensions to block, the feature blocks all .asp files on the server, even if they're used to support Web site features on another server in the server farm. If a file ends in a period (.), the preceding characters are checked against the list of blocked file extensions as well. For example, if .exe is on the list of blocked file extensions, a file called "filename.exe." is also blocked. The following list shows different ways of representing the same file, all of which are blocked if the .hta extension is on the list of blocked file extensions:

• filename.hta

• filename.hta.

• filename.hta.{3050F4D8-98B5-11CF-BB82-00AA00BDCE0B}

• filename.hta::$DATA

You can determine which files are blocked for Web sites on your servers by modifying the list of blocked file extensions. You can block additional file extensions (up to 1024 file types) by adding them to the list in the SharePoint Central Administration pages, or remove a block by deleting the file extension from the list. When you change the list of file extensions, the change affects both new files being added to a Web site and files already posted to a Web site. For example, if a document library contains a .doc file, and you add the .doc file extension to the list of blocked file extensions, users will no longer be able to open the .doc file in the document library. Users will be able to rename or delete a file with a blocked file extension, but will not be able to perform any other actions.

Add or remove a file type from the list of blocked file extensions

1. Click Start , point to All Programs , point to Administrative Tools , and then click SharePoint Central Administration .

2. On the SharePoint Central Administration page, under Security Configuration , click Manage blocked file types .

3. On the Manage List of Blocked File Types page, perform one of the following actions:

   • To add a file type, click in the list and type the extension.

   • To delete a file type, delete the file extension from the list.

4. Click OK .