Digitally signing scripts

Find out more about digitally signing scripts in this excerpt from Chapter 28 of Don Jones' "Managing Windows with VBScript and WMI."

Managing Windows with VBScript and WMI Get a glimpse inside Don Jones' book "Managing Windows with VBScript and WMI" with this series of book excerpts. Below is an excerpt from Chapter 28, "Scripting Security." Click for the complete book excerpt series or purchase the book.



Digitally Signing Scripts

A signed script includes a digital signature as a block comment within the file. You need to be using the WSH 5.6 or later XML format, because it contains a specific element for storing the certificate. Take Listing 28.1 as an example.

  • Script Signer
  • This script signs another script for you. Just run it with the appropriate command-line parameters shown, or run it with no parameters to receive help on the correct usage.

    Listing 28.1 Signer.vbs. This script signs another one.



  • Script Signer -- Explained
  • This script is stored in an XML format, which describes its command-line parameters. That's what the first block of XML does.

    <job> <runtime> <named name="file" helpstring="The script file to sign" required="true" type="string" /> <named name="cert" helpstring="The certificate name" Required="true" type="string" /> <named name="store" helpstring="The certificate store" Required="false" type="string" /> </runtime>

    Then, the actual script begins. It checks first to see that both the "cert" and "file" command-line arguments were provided; if they weren't, the script displays the help information and exits.

    <script language="vbscript"> Dim Signer, File, Cert, Store If Not WScript.Arguments.Named.Exists("cert") Or _ Not WScript.Arguments.Named.Exists("file") Then WScript.Arguments.ShowUsage() WScript.Quit End If

    Assuming everything was provided, the script creates a new Scripting.Signer object and passes it the file and certificate command-line arguments.

    Set Signer = CreateObject("Scripting.Signer") File = WScript.Arguments.Named("file") Cert = WScript.Arguments.Named("cert")

    If a specific certificate store is specified, that's passed to the Signer objects, too.

    If WScript.Arguments.Named.Exists("store") Then Store = WScript.Arguments.Named("store") Else Store " " End If

    Finally, the Signer's SignFile method is called to actually sign the target script file. The file is opened, and its signature is written to a comment block.

    Signer.SignFile(File, Cert, Store) </script> </job>

    Note that anyone can get into the file and modify its signature. However, the signature no longer matches the script, and it cannot pass the trust test conducted by WSH. Similarly, any changes to the script's code, after it is signed, fail the trust test.

    Click for the next excerpt in this series: Running Only Signed Scripts.


    Click for book details or purchase the book.
    This was first published in April 2005

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