THE DARK SIDE OF NTFS

What is NTFS?
It is the abbreviation of New Technology File System - Windows NT's preferred file system.
What is an alternate data stream (ADS)?
In NTFS, a file consits of different data streams. One stream holds the security information (access
rights and such things), another one holds the "real data" you expect to be in a file. There may be another
stream with link information instead of the real data stream, if the file actually is a link. And there may
be alternate data streams, holding data the same way the standard data stream does.
What is wrong with alternate data streams?
I could say: Nothing, they work as expected and as documented (yes, Microsoft did document this
feature).
But stop - there is something wrong: They are totally hidden. You can have a file with 1 byte in the
official main data stream and some hundred MB in one or more alternate data streams. What do you
expect the dir command, file manager or explorer to show as the size of this file? It is 1 byte!
That means a user can hide quite a lot of data in alternate data
streams and nobody will know?
So it is.
But a user does need certain special priviledges to use
alternate data streams?
No. Even guest can create such streams in every file where he has write access for.

How does somebody create an ADS?
You can do it on the command prompt, like notepad visible.txt:hidden.txt. This will
create an hidden stream hidden.txt in the file visible.txt. It doesn't matter if the file exists
or not.
How does somebody copy data into an ADS?
type atextfile > visible.txt:hidden2.txt.
This will create another hidden stream hidden2.txt in the file visible.txt.
How does somebody copy text data from an ADS into a
"normal" file?
more < visible.txt:hidden2.txt > newfile.txt.
This will create a file newfile.txt from the hidden stream hidden2.txt in the file visible.
txt.
How does somebody copy binary data from an ADS into a
"normal" file?
cat visible.txt:hidden.exe > hack.exe.
This will create a file hack.exe from the hidden stream hidden.exe in the file visible.txt.
(Cat is a tool from the Ressource Kit.)
How does somebody delete an ADS?
Let us assume you know there is a file important.exe with an ADS attached to it. The file is very
important and the ADS very dangerous. You need to hold the main stream and delete the ADS. Let us
assume there is no FAT drive on your network, otherwise you could move the file to this drive and than
move it back again. All you need to do is:
ren important.exe temp.exe
cat temp.exe > important.exe
del temp.exe
The method above does not work when the ADS is attached to a directory. If you need to remove,
for instance c:\Windows:harmful.exe without reinstalling Windows, you could use this trick. (If you use
NT 5.x, you need a copy of Notepad.exe from NT 4!)

1. Open the ADS with Notepad:
C:\NT4Tools\Notepad.exe c:\Windows:harmful.exe
2. Delete the entire content of the ADS
3. Close notepad. It will ask whether you want to save your changes
4. Answer YES
5. Notpad will tell you that the file is empty and that it will delete it
Now you are done, the ADS is gone.
Can somebody add an ADS to a directory entry instead of a
file?
Yes, it works the same way.
What possibilities does Microsoft provide to check if there are
alternate data streams on my NTFS disks?
Do you expect they care?
But if the sum of available and used storage on a hard disk is
much less than it's size, I want to know a way to check if there
are alternate data streams on my NTFS disks!
You can move all files onto a FAT drive and back to the NTFS drive. This way all alternate data streams
will be deleted, because FAT does not know how to save this kind of data.