PowerShell V3 Beta–Better NTFS Alternate Data Stream Handling

One of the many new features in Windows PowerShell V3 is better support for alternate data streams (ADS) in NTFS files.  ADS allows an NTFS file to contain additional data that is not part of the “main” stream i.e. the file’s primary content.  Tools like Windows Explorer or even PowerShell’s Get-ChildItem cmdlet don’t show these extra data streams.  In fact the file size reported by both of these tools does not take into account the data stored in the alternate streams.  For more information on ADS check out the NTFS topic on Wikipedia.

A common use of ADS is to indicate that a file downloaded by Internet Explorer came from the Internet Zone.  Files coming from the internet could be potentially dangerous.  Various applications check for this stream and if it is present and contains information indicating the “Internet” zone, they might block access or in the case of PowerShell’s RemoteSigned execution policy, only execute the file if it is signed.

Previous to PowerShell V3, you could use the SysInternals streams.exe tool to list and remove alternate data streams.  A common application of this tool was to delete all streams in a file.  That was a rather crude but effective way to “unblock” a file downloaded from the internet.

This is also one area where CMD.EXE was one up on PowerShell.  From a CMD prompt, you can use “dir /r” to list files and their alternate data streams.  You can also create/overwrite streams with CMD.exe like so “echo.>test.exe:Zone.Identifier” which would “unblock” an internet zone file.  You can also unblock such files by selecting the file’s Properties in Windows Explorer and pressing the “Unblock” button at the bottom right of the general tab.  However this is not convenient if you need to do this to dozens or hundreds of files.  With the PowerShell Community Extensions 2.0, we introduced an Unblock-File cmdlet that would delete only the stream named Zone.Identifier.  That is the stream that Internet Explorer creates when you download a file.  Fortunately with PowerShell V3, we can obsolete that cmdlet because V3 offers several ways to manage alternate data streams.

First up is PowerShell’s own Unblock-File cmdlet which, like the PSCX equivalent, is quite easy to use:

Note that you wouldn’t normally need to prefix Unblock-File with Microsoft.PowerShell.Utility.  In this case, I wanted to make sure I was using the PowerShell Unblock-File and not the one from PSCX.

In addition to using the big gun of Unblock-File you can also manipulate streams with the following cmdlets:

Here is how you can list all the alternate data streams in a file and the contents of any particular data stream:

Note that :$DATA is the main stream i.e. the file’s primary contents.

If you need to clear the contents of a data stream without removing the stream completely, you can use Clear-Content’s –Stream parameter e.g.:

To completely remove the stream, use Remove-Item’s –Stream parameter e.g.:

And if you need to create an alternate stream, you can do so using Add-Content’s –Stream parameter e.g.:

Finally, Set-Content –Stream can be used to modify the content of an existing stream.

The new  Unblock-File cmdlet as well as the upgrades to the *-Content and Get/Remove-Item  cmdlets are a very welcome enhancement to PowerShell’s file handling capabilities.