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T1546.001 Change Default File Association

Adversaries may establish persistence by executing malicious content triggered by a file type association. 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 or by administrators using the built-in assoc utility.123 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.4

Item Value
ID T1546.001
Sub-techniques T1546.001, T1546.002, T1546.003, T1546.004, T1546.005, T1546.006, T1546.007, T1546.008, T1546.009, T1546.010, T1546.011, T1546.012, T1546.013, T1546.014, T1546.015, T1546.016
Tactics TA0004, TA0003
Platforms Windows
Permissions required Administrator, SYSTEM, User
Version 1.0
Created 24 January 2020
Last Modified 30 March 2023

Procedure Examples

ID Name Description
G0094 Kimsuky Kimsuky has a HWP document stealer module which changes the default program association in the registry to open HWP documents.6
S0692 SILENTTRINITY SILENTTRINITY can conduct an image hijack of an .msc file extension as part of its UAC bypass process.5


ID Data Source Data Component
DS0017 Command Command Execution
DS0009 Process Process Creation
DS0024 Windows Registry Windows Registry Key Modification