Change Default File Association

What the technique is

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.

What MITRE says you would watch

• AN0170

  Detects modification of registry keys used for default file handlers, followed by anomalous process execution from user-initiated file opens. This includes tracking changes under HKCU and HKCR for file extension mappings, and correlating them with new or suspicious handler paths launching unusual child processes (e.g., PowerShell, cmd, wscript).