Hidden Threat: NTFS Alternate Data Streams (ADS)
- August 21st, 2009
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by Rohan Sethi / Aug 20,2009 / Tags: security, ADS, threat
Alternate Data Streams in NTFS (NT file system) is a rather unknown compatibility feature in Windows NT systems. It was introduced in NTFS with the intent of compatibility with HFS, or the old Macintosh Hierarchical File System. The Primary Function of ADS is to hold metadata about files: Writing details in summary of a text document (right clicking the TXT file, selecting properties, and then selecting the summary tab) gets attached as an ADS.
So what’s so special about ADS?
One may think, “A text document’s summary is stored as ADS… so what?” Well, there’s more to it — executable code can also be stored as an alternate data stream without the Timestamp, listed File’s size or running process name being modified. Moreover, files with ADS are almost impossible to be detected by native file browsing techniques like Windows Explorer or the command line; software that can identify them are few and far in between…
Exploiting ADS
The lengths a malicious hacker can go to hide his tracks can be astonishing, and this is what makes ADS the worst nightmare of a System Administrator. Due to the concealed nature of ADS, detecting and preventing execution of malicious code is intricate.
Once a hacker has acquired administrator access on the system, he’ll strip off all information of concern, covering the detection of his presence and will try to install a backdoor (a remote access Trojan) for easy future access. This backdoor needs to be veiled from the system administrator, this is where ADS comes to in – it can be used to hide files on the breached system, evading detection and executing them without the knowledge of the sys admin.
The ability to hide executable code in an invisible form inside ADS can also make viruses difficult to be detected within a file system, because most virus scanners only verify the default data stream of files. Major Anti-virus vendors point out that ADS must be loaded into the memory before execution and thus will be detected with real-time scanning (when a file is scanned after it is loaded in memory (just after commanded to execute), the type of scan is known as a “real-time” scan). The problem with this approach is that many network administrators do not run real-time scanning on their servers or workstations due to performance issues.
Denial of Service (DoS) attacks that exploit the use of ADS also exist. It is the difficulty of detection that increases the threat. For example, it is quite common for an attacker to create a file large enough to fill up the system partition on a Windows NT/2000 system, to crash the server due to lack of space for temporary files. When using the main stream of a file in such an attack, the violating files are easily identified due to their abnormally large size. By using Alternate Streams here, it can be made difficult to detect where the violating files are located on the system. Another attack exploiting ADS can be launched by creating a large number of alternate streams, more than 6,000 on a specific file. If the attacker or the system tries to access the default stream of the file, the system’s response slows considerably and in worst case, the system crashes thus creating a Denial of Service.
Moreover, this vulnerability is not confined to the NTFS file system; any other file system that uses streams for alternate data is vulnerable.
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According to Luis Corrons, Technical Director of PandaLabs, “maybe one of the reasons of this increase is the economic crisis along with the big business that selling this information on the black market, such as credit card numbers, Paypal or Ebay accounts, etc. We have also seen an increase of the distribution and infection of this kind of malware through social networks.”