Last time, we wondered whether the ERROR_KEY_DELETED error code could be the result of the user logging off while a program was running. I noted that this is primarily an issue for services, though you can force it to happen for regular user-session applications.

It involves taking advantage of the Reg­Override­Predef­Key which lets you redirect the registry operations performed by the current program to another registry key. The intention of this function was to allow you to override a registry root key like HKEY_CLASSES_ROOT so you can capture the registry keys written by an installer. But we’re going to use it to force an ERROR_KEY_DELETED error from code that tries to open HKEY_CURRENT_USER\Software.

Note that this is just a parlor trick! There is no practical use for this demonstration.

#include <windows.h>
#include <wil/resource.h>
#include <wil/result_macros.h>

int main(int, char**)
{
    wil::unique_hkey originalSoftware;
    FAIL_FAST_IF_WIN32_ERROR(RegOpenKeyExW(HKEY_CURRENT_USER,
        L"Software", 0, KEY_READ, &originalSoftware));

    wil::unique_hkey originalContoso;
    DWORD disposition;
    FAIL_FAST_IF_WIN32_ERROR(RegCreateKeyExW(originalSoftware.get(),
        L"Contoso", 0, nullptr, REG_OPTION_VOLATILE,
        KEY_READ | KEY_WRITE, nullptr,
        &originalContoso, &disposition));
    if (disposition != REG_CREATED_NEW_KEY) {
        printf("Unexpected HKCU\\Software\\Contoso key\n");
        return 0;
    }

    FAIL_FAST_IF_WIN32_ERROR(RegOverridePredefKey(
        HKEY_CURRENT_USER, originalContoso.get()));

    FAIL_FAST_IF_WIN32_ERROR(
        RegDeleteKeyW(originalSoftware.get(), L"Contoso"));

    wil::unique_hkey key;
    LSTATUS error = RegOpenKeyExW(HKEY_CURRENT_USER, L"Software", 0, KEY_READ, &key);
    if (error == ERROR_KEY_DELETED)
    {
        printf("It happened!\n");
    } else {
        printf("It didn't happen.\n");
    }

    return 0;
}

First, we create a key to the original HKEY_CURRENT_USER\Software, and then create a Contoso subkey under it. If there was already a Contoso subkey, then we bail out because our trick won’t work. (We’ll have to pick another key to use as our sacrificial lamb.)

After creating the Contoso subkey, we pass it to Reg­Override­Predef­Key to make it the new HKEY_CURRENT_USER. Any future references to HKEY_CURRENT_USER will use our originalContoso key instead of the original HKEY_CURRENT_USER. (Note that our original keys still refer to their original unredirected versions.)

Our final step for preparing the trick is deleting the Contoso key from the original HKEY_CURRENT_USER\Software. Since we had redirected HKEY_CURRENT_USER to refer to the original HKEY_CURRENT_USER\Software\Contoso key, deleting that key means that HKEY_CURRENT_USER is now a reference to a deleted key.

The next line of code is our victim. It tries to open HKEY_CURRENT_USER\Software, which is an attempt to reference a Software subkey under the original now-deleted HKEY_CURRENT_USER\Software\Contoso key. Since the key has been deleted, the error is ERROR_KEY_DELETED.

I’m not saying this is what was causing the original code to get an ERROR_KEY_DELETED error when trying to open HKEY_CURRENT_USER\Software. I’m just saying that this is one way it can happen, though it is extremely contrived.