An Introduction to Alternate Data Streams?(ADS)

A Hidden Layer of New Technology File System?(NTFS)

Alternate Data Streams (ADS) is a New Technology File System (NTFS) feature that allows data to be associated with a file or directory without modifying its primary data or attributes.?

Although introduced to provide enhanced functionality, ADS has also sparked debates due to its potential misuse in cybersecurity. This article explores ADS's technical nuances, exploring its design, use cases, and challenges.

What are Alternate Data?Streams?

In NTFS, every file or directory consists of multiple data streams. By default, the file’s primary data is stored in the main data stream, also known as the default data stream.?

ADS allows developers to attach additional data streams to a file, offering a way to embed metadata or supplementary content without altering the original file’s content.

For instance, a file on an NTFS filesystem can have a primary stream (main stream) for the main content and one or more alternate streams for additional metadata.

Syntax Overview

The syntax for working with ADS is pretty straightforward. You can associate an alternate data stream using a colon (:) as a separator

filename:streamname        

For example:

echo "This is an alternate data stream" > document.txt:hiddenstream        

Here, document.txt is the primary file, and hiddenstream is the alternate data stream associated with it.

These alternate streams could be anything, for instance, an executable, a script, a log file, etc.

Practical Use Cases of?ADS

ADS was designed with legitimate use cases in mind. Some of its primary applications are:

1. Storing?Metadata

Alternate Data Streams can store metadata about files without cluttering the primary file content.?

For instance, a text editor might save configuration settings or user preferences in an ADS.

2. Attaching Hidden?Data

Applications can use ADS to store additional data related to a file, such as thumbnails or indexing information, without exposing it in the file’s primary content.

3. Enhanced File Management

Developers can utilize ADS for logging, tagging, or embedding instructions within files.?

For example, a backup application might use ADS to store backup timestamps.

Cybersecurity Challenges with?ADS

While ADS is useful, it has also been exploited for malicious purposes. Because alternate data streams are not visible in traditional file explorers or command-line tools without specific flags, they provide an ideal hiding spot for malware and unauthorized scripts.

1. Data?Hiding

Attackers can embed malicious code or payloads within ADS to evade detection.?

For example, a file might appear benign while carrying a hidden executable within an alternate data stream.

2. Bypassing Security?Tools

Many antivirus and security scanners do not thoroughly inspect alternate data streams, making them an effective tool for malware authors to obfuscate threats.

3. Persistence Mechanism

Threat actors can leverage ADS to maintain persistence on a compromised system.?

For instance, they might store configuration files, encryption keys, or secondary payloads in ADS.

Detecting and Managing Alternate Data?Streams

Understanding how to detect and manage ADS is critical given the potential risks. Here are some tools and techniques:

1. Using Built-in?Commands

The dir command with the /R flag can reveal alternate data streams:

dir /R        

2. PowerShell Scripts

Custom PowerShell scripts can be used to enumerate ADS.?

For example:

# List all alternate data streams in the current directory
Get-ChildItem -Recurse | ForEach-Object { 
    $file = $_
    Get-Item $file.FullName -Stream * | Where-Object Stream -ne ':$Data' | ForEach-Object {
        [PSCustomObject]@{
            FileName = $file.Name
            Stream = $_.Stream
            Length = $_.Length
        }
    }
} | Format-Table -AutoSize        

The explanation of the above script is as follows:

The `Get-ChildItem -Recurse` command retrieves all the files and subdirectories present in the current working directory, which we then pipe the output to a `ForEach-Object` loop that iterates through each item.

The `Get-Item $file.FullName -Stream *` command retrieves all streams associated with a particular item being processed by the loop. The output of this is, in turn, passed to the `Where-Object Stream -ne ':$Data'` which filters out the main stream identified by the tag?`:$Data` (this would contain the main content of the file)

Finally, we pipe the filtered list from above into another loop that iterates through the identified alternate data streams and creates a custom object for each entry found during the process.

We use `Format-Table -AutoSize` command to display the final output in a tabular form.

The output of the above script, in our case, will reveal the alternate data stream hiddenstream that we created in the earlier section:

3. Third-Party Tools

Specialized tools like Sysinternals' Streams can identify and analyze ADS on a system.

Mitigating Risks of?ADS

To balance the utility of ADS with security, organizations and developers can adopt the following practices:

1. Monitor and Audit: Regularly audit systems for unauthorized ADS usage.

2. Restrict Privileges: Limit file system privileges to reduce the risk of ADS exploitation.

3. Educate Users: Train users and administrators on identifying and mitigating ADS risks.

4. Enhance Security Scans: Ensure antivirus and security tools are configured to detect and scan ADS.

This article was originally published on my Medium Blog