MaxPatrol SIEM

Detects cyberincidents that undermine cyber resilience of a company

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. 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.

Positive Technologies products that cover the technique

MaxPatrol SIEM knowledge base

mitre_attck_persistence: PT-CR-664: Default_File_Association_Modify: The process changed one of the registry branches responsible for determining the default application used to open files with a specific extension in the system

Detection

IDDS0009Data source and componentProcess: Process CreationDescription

Monitor for newly executed processes that may establish persistence by executing malicious content triggered by a file type association.

IDDS0024Data source and componentWindows Registry: Windows Registry Key ModificationDescription

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.

IDDS0017Data source and componentCommand: Command ExecutionDescription

Monitor executed commands and arguments that may establish persistence by executing malicious content triggered by a file type association.