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) 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 execute arbitrary commands.

Detection: Collect and analyze changes to Registry keys that associate file extensions to default applications for execution and correlate with unknown process launch activity or unusual file types for that process.

User file association preferences are stored under [HKEY_CURRENT_USER]\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts and override associations configured under [HKEY_CLASSES_ROOT]. Changes to a user's preference will occur under this entry's subkeys.

Also look for abnormal process call trees for execution of other commands that could relate to Discovery actions or other techniques.

Platforms: Windows

Data Sources: Windows Registry, Process command-line parameters, Process monitoring

Permissions Required: User, Administrator, SYSTEM

Contributors: Stefan Kanthak, Travis Smith, Tripwire

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 assoc #{extension_to_change}="#{target_exenstion_handler}"