# Change Default File Association

Change Default File Association
Technique
ID T1042
Tactic Persistence
Platform Windows
Data Sources Windows Registry, Process command-line parameters, Process monitoring
CAPEC ID CAPEC-556
Contributors Stefan Kanthak, Travis Smith, Tripwire

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.12 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.

## Mitigation

Direct mitigation of this technique is not recommended since it is a legitimate function that can be performed by users for software preferences. Follow Microsoft's best practices for file associations.3

Identify and block potentially malicious software that may be executed by this technique using whitelisting4 tools, like AppLocker,56 or Software Restriction Policies7 where appropriate.8

## 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.