If you run notepad elevated (Right click | Run as Administrator), and you try and drag-and-drop a file from Windows Explorer, nothing happens. It looks like it is going to work because the pointer icon changes but the file doesn’t open. Weird, huh?
What’s Going On?
In the traditional NT Security model (prior to Vista), all processes on the same desktop ran with the same security token and had all the same privileges. UAC changed this by allowing processes with different privilege levels on the same desktop.
Lower Privilege Processes Can’t Interfere with Higher Privilege Processes
In order to prevent potential elevation of privilege attacks, certain functionality needs to be blocked. This is implemented through Mandatory Integrity Control (MIC). All processes and all resources (files, registry, etc.) have an integrity level assigned. MIC prevents a standard user process from writing to a protected per machine location like Program Files or the HKLM registry hive. I won’t go too deep into MIC in this post but the following is a great resource if you want more info: Inside Windows Vista User Account Control.
User Interface Privilege Isolation (UIPI)
Okay, back to our drag and drop issue… A “sister” technology that works in conjunction with MIC is UIPI. UIPI blocks Windows messages being sent from process with a lower MIC level to one running at a higher MIC level. Drag-and-drop is implemented via Windows messages. Therefore, if you try and drag-and-drop a file from Windows Explorer (medium MIC) to Notepad running elevated (high MIC), the Windows messages are blocked and drag-and-drop doesn’t work.
You can use ChangeWindowsMessageFilterEx in your application to allow specified Windows messages to not be blocked. Unfortunately, this isn’t recommended as a safe solution for drag and drop due to the messages that drag and drop uses.
Okay. Now What?
The best solution is to only use drag and drop between the same MIC levels. With UAC enabled, Windows Explorer will run at a medium MIC level. Therefore, your application (Notepad in our example) needs to run at medium (or lower) MIC level. The bottom line is that drag and drop from Windows Explorer will not work if your application is elevated. If you find yourself in this situation, you may need to rethink your application design or not support drag-and-drop with UAC enabled.