A relatively unknown compatibility feature of NTFS, Alternate Data Streams (ADS) provides hackers with a method of hiding root kits or hacker tools on a breached system and allows them to be executed without being detected by the systems administrator.
ADS is the ability to fork file data into existing files without affecting their functionality, size, or display to traditional file browsing utilities like dir or Windows Explorer. Found in all version of NTFS, ADS capabilities where originally conceived to allow for compatibility with the Macintosh Hierarchical File System, HFS; where file information is sometimes forked into separate resources. Alternate Data Streams have come to be used legitimately by a variety of programs, including native Windows operating system to store file information such as attributes and temporary storage.
Amazingly enough, Alternate Data Streams are extremely easy to make and require little or no skill on the part o the hacker. Common DOS commands like “type” are used to create an ADS. These commands are used in conjunction with a redirect [>] and colon [:] to fork one file into another.
For instance: the command
“type c:\anyfile.exe > c:\winnt\system32\calc.exe:anyfile.exe”
will fork the common windows calculator program with an ADS “anyfile.exe.”
Alarmingly files with an ADS are almost impossible to detect using native file browsing techniques like command line or windows explorer. In our example, the file size of calc.exe will show as the original size of 90k regardless of the size of the ADS anyfile.exe. The only indication that the file was changed is the modification time stamp, which can be relatively innocuous.
Once injected, the ADS can be executed by using traditional commands like type, or start or be scripted inside typical scripting languages like VB or Perl. When launched, the ADS executable will appear to run as the original file - looking undetectable to process viewers like Windows Task Manager. Using this method, it is not only possible to hide a file, but to also hide the execution of an illegitimate process.
Unfortunately, it is virtually impossible to natively protect your system against ADS hidden files if you use NTFS
ADS is the ability to fork file data into existing files without affecting their functionality, size, or display to traditional file browsing utilities like dir or Windows Explorer. Found in all version of NTFS, ADS capabilities where originally conceived to allow for compatibility with the Macintosh Hierarchical File System, HFS; where file information is sometimes forked into separate resources. Alternate Data Streams have come to be used legitimately by a variety of programs, including native Windows operating system to store file information such as attributes and temporary storage.
Amazingly enough, Alternate Data Streams are extremely easy to make and require little or no skill on the part o the hacker. Common DOS commands like “type” are used to create an ADS. These commands are used in conjunction with a redirect [>] and colon [:] to fork one file into another.
For instance: the command
“type c:\anyfile.exe > c:\winnt\system32\calc.exe:anyfile.exe”
will fork the common windows calculator program with an ADS “anyfile.exe.”
Alarmingly files with an ADS are almost impossible to detect using native file browsing techniques like command line or windows explorer. In our example, the file size of calc.exe will show as the original size of 90k regardless of the size of the ADS anyfile.exe. The only indication that the file was changed is the modification time stamp, which can be relatively innocuous.
Once injected, the ADS can be executed by using traditional commands like type, or start or be scripted inside typical scripting languages like VB or Perl. When launched, the ADS executable will appear to run as the original file - looking undetectable to process viewers like Windows Task Manager. Using this method, it is not only possible to hide a file, but to also hide the execution of an illegitimate process.
Source: http://www.windowsecurity.com/articles/Alternate_Data_Streams.html
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Sam
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