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What Forensic Analysts should know about
NT ALTERNATE DATA STREAMS (ADS)
Copyright © 1998-2013 Dan Mares

Back in 1997 at an NT security conference, I was reminded about a little known part of the NTFS file system called "Alternate Data Streams". Most of us have never heard of Alternate Data Streams (referred below as ADS). In fact, the typical computer user will probably never have any need to use or deal with ADS.

So what are Alternate Data Streams and why should we bother with them? The explanation that follows is not a technical one, and is therefore technically unsophisticated. It is intended to be useful when you need to provide an explanation to a non-technical person.

Let's start with what most people consider a normal data file. A normal file consists of an entry in the directory which typically contains the following: file name; date; time; size; an indication as to where the files resides on the physical disk (in DOS we call this the starting Cluster location; on NTFS it is information located in the Master File Table, or MFT); and the data of the file, which is contained in a series of bytes of data located in Clusters on the disk. Thatís the simple explanation. Information about the NT Master File Table can be found in many places. One helpful reference is Executive Software International's site at: (http://www.execsoft.com/tech-support/articles/art-0004.htm, and art-0020.htm).

Now, comes NTFS. Disregarding for a moment that files of about 1500 bytes or less can reside entirely within the MFT, the MFT contains a significant amount more information about the file. This information is usually referred to as "attributes." Some authorities list more than 10 attributes of a file. An important point is that attributes can be resident (in the MFT) or nonresident, meaning located somewhere else on the disk.

One of the more significant attributes of a file is the "DATA" attribute. The data attribute points to the data, resident (located within the MFT) or not. This still sounds very simple. We have a file system, a way of tracking files (the directory entry), and attributes about the files. But things become more complex than this.

On NTFS systems, (and only on NTFS), this "DATA" attribute can actually be multiple data attributes pointing to more than one piece of "data". In this case 'data' means the contents of a file, or other information such as security information. Thus the "DATA" attribute can point to more than one file. These additional files are called Alternate Data Streams. I think of them as additional files that are sitting--or more appropriately--hiding behind the visible file.

ADS are sort of like invisible attachments to a file. Their physical information is not included in the results of a DIR or Explorer window. DIR or Explorer will never tell you a file contains ADS. In most cases, if one existed, its size would be so insignificant to the overall size of a physical drive that you wouldnít even notice that unaccounted-for space was being taken up. So how do you look for them? Very carefully.

Actually, before we start looking for Alternate Data Streams, I should first tell you how to create a simple ADS. The easiest way is to use Notepad. Assume we have an existing file called test.txt. It is a text file, sitting out there on the disk. Now let's create a simple ADS, named alternate.txt. Using Notepad, at the prompt, enter the command

C:>notepad test.txt:alternate.txt

Notice the format for creating an ADS. You use the filename of the main data file, then add a colon (:), followed by the name of the ADS you wish to create or access. There should be no spaces within this entire string of characters. (Unless you quote the string, which is another matter.)

Because the ADS doesn't currently exist, Notepad will probably say that alternate.txt does not exist and ask if you want to create it. Answer "yes." Notepad will open an empty file and you can enter any information, just as usual. When you finish, save the file and exit. Look at the size of the test.txt file that DIR shows. The filesize hasnít changed at all and you will not see any indication of alternate.txt anywhere. Interesting. Where did the data go? It went into one of those Alternate Data Streams that is now associated with the original file test.txt.

Now that you have created an ADS, you can go in and create a second, third, fourth, and so on. To access these ADS files later, all you need to do is use Notepad to edit the ADS just as you did when originally creating it.

C:>Notepad test.txt:alternate.txt

Does ADS simply seem like a toy? Well, this toy could be used to hide data "behind" any file within the NTFS file system. Under normal forensic processing you wouldnít even know it existed. If you did a string search on the entire physical drive you might find the text strings if they werenít encrypted or in a binary format. A suspect may very easily create an ADS containing passwords, contacts or other incriminating information and only he would know where it was and by what name it could be seen. An encrypted file could be sent on an NTFS formatted Jazz or Zip disk to someone, and the password might be sent right along with it in the form of an ADS. Binary images could be attached to simple text or document files for later retrieval. There might already be some sophisticated FTP programs that will transfer ADS files. It would certainly be an easy programming task to create one.

Once you've created a text ADS, it is easy to add a binary data file as an ADS. You simply use the command:

C:>type binary.file >> test.txt:binary.mds

This would add the binary.file to the test.txt as an ADS. Although I havenít seen a published program to extract out the binary data from the ADS, creating one is a relatively simple programming task. In fact, Maresware's 32 bit versions of Diskcat, Hash, MD5 and Crckit can detect and perform the appropriate analysis on these ADS. And Maresware's Mdir and Diskcat can identify the existence of ADS files.

Here is a sample output of Mdir when it finds a file with an ADS. Notice the ADS do not carry a different date, so I indicate their presence with the term Alternate Data or ADATA.

Filename size date time TZ attributes

junk_3               103 11/17/1998 12:41w EST A....
junk_3:altdata.txt:   40 ALTERNATE DATA    EST ADATA
junk_3:alternat.txt:  43 ALTERNATE DATA    EST ADATA
junk_3:alternate.txt: 16 ALTERNATE DATA    EST ADATA


For more information on Alternate Data Streams you can research the web using the keyword Alternate Data Streams, or try these sites:

http://merxsoft.com/mersoft-Free/Information/ntfsmds.htm or http://premium.microsoft.com/msdn/library/winresource/dnwinnt/f1d/d1e/s838b.htm and look at the 4th page where ADS is discussed.

One other very important thing to remember: If you are copying a file with an ADS and you are going from NTFS to NTFS, the ADS will tag along. No special command is necessary to maintain ADS integrity. However, ADS are only valid when dealing with NTFS disks. So, if you copy a file having ADS to a non NFTS drive, the ADS are lost. This has its good and bad points.

This has been a primary tutorial dealing with Alternate Data Streams. For more information check out the web.

Copyright © 1999-2003 by Dan Mares

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