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.(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 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.(Citation: TrendMicro TROJ-FAKEAV OCT 2012)

Atomic Tests

Atomic Test #1 - Change Default File AssociationChange Default File Association From cmd.exe of hta to notepad.

Upon successful execution, cmd.exe will change the file association of .hta to notepad.exe. Supported Platforms: windows Elevation Required (e.g. root or admin)#### Attack Commands: Run with command_prompt

assoc .hta=txtfile

Invoke-AtomicTest T1546.001 -TestNumbers 1

Cleanup:

assoc  .hta=htafile

Invoke-AtomicTest T1546.001 -TestNumbers 1 -Cleanup

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.