Month: July 2020

The standard Windows Registry contains some keys that are not real keys, but instead are symbolic links (or simply, links) to other keys. For example, the key HKEY_LOCAL_MACHINE\System\CurrentControlSet is a symbolic link to HKEY_LOCAL_MACHINE\System\ControlSet001 (in most cases). When working with the standard Registry editor, RegEdit.exe, symbolic links look like normal keys, in the sense that they behave as the link’s target. The following figure shows the above mentioned keys. They look exactly the same (and they are).

There are several other existing links in the Registry. As another example, the hive HKEY_CURRENT_CONFIG is a link to (HKLM is HKEY_LOCAL_MACHINE) HKLM\SYSTEM\CurrentControlSet\Hardware Profiles\Current.

But how to do you create such links yourself? The official Microsoft documentation has partial details on how to do it, and it misses two critical pieces of information to make it work.

Let’s see if we can create a symbolic link. One rule of Registry links, is that the link must point to somewhere within the same hive where the link is created; we can live with that. For demonstration purposes, we’ll create a link in HKEY_CURRENT_USER named DesktopColors that links to HKEY_CURRENT_USER\Control Panel\Desktop\Colors.

The first step is to create the key and specify it to be a link rather than a normal key (error handling omitted):

HKEY hKey;
RegCreateKeyEx(HKEY_CURRENT_USER, L"DesktopColors", 0, nullptr,
	REG_OPTION_CREATE_LINK, KEY_WRITE, nullptr, &hKey, nullptr);

The important part is that REG_OPTION_CREATE_LINK flag that indicates this is supposed to be a link rather than a standard key. The KEY_WRITE access mask is required as well, as we are about to set the link’s target.

Now comes the first tricky part. The documentation states that the link’s target should be written to a value named “SymbolicLinkValue” and it must be an absolute registry path. Sounds easy enough, right? Wrong. The issue here is the “absolute path” – you might think that it should be something like “HKEY_CURRENT_USER\Control Panel\Desktop\Colors” just like we want, but hey – maybe it’s supposed to be “HKCU” instead of “HKEY_CURRENT_USER” – it’s just a string after all.

It turns out both these variants are wrong. The “absolute path” required here is a native Registry path that is not visible in RegEdit.exe, but it is visible in my own Registry editing tool, RegEditX.exe, downloadable from https://github.com/zodiacon/AllTools. Here is a screenshot, showing the “real” Registry vs. the view we get with RegEdit.

This top view is the “real” Registry is seen by the Windows kernel. Notice there is no HKEY_CURRENT_USER, there is a USER key where subkeys exist that represent users on this machine based on their SIDs. These are mostly visible in the standard Registry under the HKEY_USERS hive.

The “absolute path” needed is based on the real view of the Registry. Here is the code that writes the correct path based on my (current user’s) SID:

WCHAR path[] = L"\\REGISTRY\\USER\\S-1-5-21-2575492975-396570422-1775383339-1001\\Control Panel\\Desktop\\Colors";
RegSetValueEx(hKey, L"SymbolicLinkValue", 0, REG_LINK, (const BYTE*)path,
    wcslen(path) * sizeof(WCHAR));

The above code shows the second (undocumented, as far as I can tell) piece of crucial information – the length of the link path (in bytes) must NOT include the NULL terminator. Good luck guessing that 🙂

And that’s it. We can safely close the key and we’re done.

Well, almost. If you try to delete your newly created key using RegEdit.exe – the target is deleted, rather than the link key itself! So, how do you delete the key link? (My RegEditX does not support this yet).

The standard RegDeleteKey and RegDeleteKeyEx APIs are unable to delete a link. Even if they’re given a key handle opened with REG_OPTION_OPEN_LINK – they ignore it and go for the target. The only API that works is the native NtDeleteKey function (from NtDll.Dll).

First, we add the function’s declaration and the NtDll import:

extern "C" int NTAPI NtDeleteKey(HKEY);

#pragma comment(lib, "ntdll")

Now we can delete a link key like so:

HKEY hKey;
RegOpenKeyEx(HKEY_CURRENT_USER, L"DesktopColors", REG_OPTION_OPEN_LINK, 
    DELETE, &hKey);
NtDeleteKey(hKey);

As a final note, RegCreateKeyEx cannot open an existing link key – it can only create one. This in contrast to standard keys that can be created OR opened with RegCreateKeyEx. This means that if you want to change an existing link’s target, you have to call RegOpenKeyEx first (with REG_OPTION_OPEN_LINK) and then make the change (or delete the link key and re-create it).

Isn’t Registry fun?