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.(Citation: Microsoft Change Default Programs)(Citation: Microsoft File Handlers)(Citation: Microsoft Assoc Oct 2017) 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 <code>HKEY_CLASSES_ROOT\.[extension]</code>, for example <code>HKEY_CLASSES_ROOT\.txt</code>. The entries point to a handler for that extension located at <code>HKEY_CLASSES_ROOT\\[handler]</code>. The various commands are then listed as subkeys underneath the shell key at <code>HKEY_CLASSES_ROOT\\[handler]\shell\\[action]\command</code>. For example: 

* <code>HKEY_CLASSES_ROOT\txtfile\shell\open\command</code>
* <code>HKEY_CLASSES_ROOT\txtfile\shell\print\command</code>
* <code>HKEY_CLASSES_ROOT\txtfile\shell\printto\command</code>

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.(Citation: TrendMicro TROJ-FAKEAV OCT 2012)
Change Default File Association
source_name: mitre-attack  url: https://attack.mitre.org/techniques/T1546/001  external_id: T1546.001     source_name: Microsoft Change Default Programs  description: Microsoft. (n.d.). Change which programs Windows 7 uses by default. Retrieved July 26, 2016.  url: https://support.microsoft.com/en-us/help/18539/windows-7-change-default-programs     source_name: Microsoft File Handlers  description: Microsoft. (n.d.). Specifying File Handlers for File Name Extensions. Retrieved November 13, 2014.  url: http://msdn.microsoft.com/en-us/library/bb166549.aspx     source_name: Microsoft Assoc Oct 2017  description: Plett, C. et al.. (2017, October 15). assoc. Retrieved August 7, 2018.  url: https://docs.microsoft.com/windows-server/administration/windows-commands/assoc     source_name: TrendMicro TROJ-FAKEAV OCT 2012  description: Sioting, S. (2012, October 8). TROJ_FAKEAV.GZD. Retrieved August 8, 2018.  url: https://www.trendmicro.com/vinfo/us/threat-encyclopedia/malware/troj_fakeav.gzd     