INFO: Executing Files by Hyperlink and the File Download Dialog Box

• Microsoft Internet Explorer (Programming) versions 4.0, 4.01, 4.01 SP1, 4.01 SP2, 5.0

SUMMARY

Many Web sites, particularly on corporate intranets, have hyperlinks to files that the user needs to execute on the local system. For obvious security reasons, Internet Explorer prevents any hyperlink action from automatically running any downloaded executable or other possibly dangerous files without displaying some form of dialog box prompt to the user.

In these situations, the File Download dialog box is not preventable in any way. This includes the use of HTML tags, script in the page, hosting the WebBrowser control, and changing security zone options.

Unfortunately, this dialog box can be disruptive to a Web-based interface that relies on these sorts of links to simplify the use of executable files stored on the Web server. This article discusses two alternative safe methods for automatically executing files by hyperlink.

MORE INFORMATION

Internet Explorer displays the File Download dialog box for two primary reasons: safety and choice. To be safe, the browser must never execute a potentially dangerous file on the user's system automatically without potential user intervention. Furthermore, Web sites have traditionally used links to executable files with two different intentions: the site intends for the user either to execute the linked file or to save it. Unfortunately, there is nothing in the HTML specification for the anchor tag (<A>) or any current scripting object model that provides a means for the Web site to indicate which is desired for a particular link. Thus, the burden of choice is on the user. The user must decide whether they want to run or save the file.

Internet Explorer decides whether to display the File Download dialog box primarily by checking the file's extension and looking in the registry for an application that reads that file. For most types of files, the user can clear the "Always ask before opening this type of file" option in the dialog box, which grants permission to Internet Explorer to not display the dialog box in the future for those specific file types. Users can also change this setting in the Edit File Types dialog box accessible from the shell's Folder Options by flipping the "Confirm open after download" check box.

However, Internet Explorer contains a predefined, hard-coded list of file extensions that it inherently distrusts. These extensions correspond to generic executables and other kinds of files that have the capability to harm the user's machine without the proper security safeguards. The File Download dialog box cannot be prevented for any files of these types. The "Always ask before opening" option will be grayed out on the dialog box and you won't be able to select it. Following is the list of the file extensions for these file types:

EXE, COM, BAT, LNK, URL, CMD, INF, REG, ISP, BAS, PCD, MST, PIF, 
SCR, HLP, CHM, HTA, ASP, JS, JSE, VBS, VBE, WS, WSH 

IFRAME Linking

1. Move the files to a special server, share, and directory on the corporate Intranet that the entire intended user audience has permissions to access via Windows UNC.



2. Create a separate directory for each executable file and copy the files into the directories. The directories should be empty except for the single executable file.



3. Wherever a hyperlink to the executable would have been used, include HTML of the following form:

   
   Click on the icon in the following window to run this very special 
   program automatically without annoying dialog boxes:
   
   <IFRAME SRC="\\server\share\directory"></IFRAME> 

   Note that the HTML code points to the directory that the file is located in, not the file itself.

Internet Code Download Linking

1. If the "executable file" is not a signable PE (EXE) such as a .bat file, then the file must be packaged in a .cab file with an INF in the following form:
   

   
   [version]
      signature="$CHICAGO$"
      AdvancedINF=2.0
   [Add.Code]
      file.zzz=file.zzz
   [file.zzz]
      clsid={15589FA1-C456-11CE-BF01-00AA0055595A}
      FileVersion=1,0,0,0
      hook=zzzinstaller
   [zzzinstaller]
      run=%EXTRACT_DIR%\file.zzz 

   Replace the instance of File.zzz above with the executable file to be run.
   
   For more information on packaging the CAB file, refer to the MSDN Web Workshop tutorial Packaging Components for Internet Distribution.



2. Ensure that the EXE (or CAB) is code-signed. If the EXE has not been signed, this can be done using the CryptoAPI Authenticode Code Signing tools. Refer to the CryptoAPI documentation in the MSDN Platform SDK under the "Security" heading for more information.
   
   For security reasons, the process of signing code for an organization is best handled by a central authority that is trusted by the entire organization. Code signing requires either the purchase of costly certificates from external vendors such as VeriSign or the maintenance of a certificate server such as Microsoft Certificate Server on the intranet.



3. Use this example page as a guideline for the link and script necessary to launch the signed code without prompt:
   

   
   <HTML><HEAD><TITLE>Page of executable links</TITLE></HEAD>
   <BODY>
   <BR/>
   
   <!-- hyperlink uses central script function called linkit() -->
   <A HREF="" onclick="return linkit('signed-testfile.exe');">
   SIGNED-CLOCK.EXE</A>
   
   <SCRIPT>
   // linkit puts filename into HTML content and spews it into iframe
   function linkit(filename)
   {
      strpagestart = "<HTML><HEAD></HEAD><BODY><OBJECT CLASSID=" +
         "'CLSID:15589FA1-C456-11CE-BF01-00AA0055595A' CODEBASE='";
      strpageend = "'></OBJECT></BODY></HTML>";
      runnerwin.document.open();
      runnerwin.document.write(strpagestart + filename + strpageend);
      window.status = "Done.";
      return false;  // stop hyperlink and stay on this page
   }
   </SCRIPT>
   
   <!-- hidden iframe used for inserting html content -->
   <IFRAME ID=runnerwin WIDTH=0 HEIGHT=0 SRC="about:blank"></IFRAME><BR/>
   
   </BODY></HTML> 

Additional query words:

Keywords          : kbDHTML kbIE400 kbIE401 kbNavigation kbSecurity kbIE401sp1 kbIE401sp2 kbGrpInet kbIE500 
Version           : WINDOWS:4.0,4.01,4.01 SP1,4.01 SP2,5.0
Platform          : WINDOWS 
Issue type        : kbinfo 

Last Reviewed: June 15, 1999