Execution

Creating a benign text file:

attacker@victim
echo "this is benign" > benign.txt
Get-ChildItem

Hiding an evil.txt file inside the benign.txt

attacker@victim
cmd '/c echo "this is evil" > benign.txt:evil.txt'

Note how the evil.txt file is not visible through the explorer - that is because it is in the alternate data stream now. Opening the benign.txt shows no signs of evil.txt. However, the data from evil.txt can still be accessed as shown below in the commandline - type benign.txt:evil.txt:

Additionally, we can view the data in the notepad as well by issuing:

attacker@victim
notepad .\benign.txt:evil.txt

Observations

Note that powershell can also help finding alternate data streams:

Get-Item c:\experiment\evil.txt -Stream *
Get-Content .\benign.txt -Stream evil.txt

References