T1042.md

T1042 - Change Default File Association

Description from ATT&CK

When a file is opened, the default program used to open the file (also called the file association or handler) is checked. File association selections are stored in the Windows Registry and can be edited by users, administrators, or programs that have Registry access (Citation: Microsoft Change Default Programs) (Citation: Microsoft File Handlers) or by administrators using the built-in assoc utility. (Citation: Microsoft Assoc Oct 2017) Applications can modify the file association for a given file extension to call an arbitrary program when a file with the given extension is opened.

System file associations are listed under HKEY_CLASSES_ROOT.[extension], for example HKEY_CLASSES_ROOT.txt. The entries point to a handler for that extension located at HKEY_CLASSES_ROOT[handler]. The various commands are then listed as subkeys underneath the shell key at HKEY_CLASSES_ROOT[handler]\shell[action]\command. For example:

• HKEY_CLASSES_ROOT\txtfile\shell\open\command
• HKEY_CLASSES_ROOT\txtfile\shell\print\command
• HKEY_CLASSES_ROOT\txtfile\shell\printto\command

The values of the keys listed are commands that are executed when the handler opens the file extension. Adversaries can modify these values to continually execute arbitrary commands. (Citation: TrendMicro TROJ-FAKEAV OCT 2012)

Atomic Tests

• Atomic Test #1 - Change Default File Association

Atomic Test #1 - Change Default File Association

Change Default File Association From cmd.exe

Supported Platforms: Windows

Inputs

Name	Description	Type	Default Value
extension_to_change	File Extension To Hijack	String	.wav
target_exenstion_handler	Thing To Open	Path	C:\Program Files\Windows Media Player\wmplayer.exe

Run it with command_prompt!

cmd.exe /c assoc #{extension_to_change}="#{target_exenstion_handler}"