With the accelerated release schedule of Windows 10 it's common for new features to be regularly introduced. This is especially true of features to mitigate some poorly designed APIs or easily misused behavior. The problems with many of these mitigations is they're regularly undocumented or at least not exposed through the common Win32 APIs. This means that while Microsoft can be happy and prevent their own code from being vulnerable they leave third party developers to get fucked.
One example of these silent mitigations are the additional OBJECT_ATTRIBUTE flags OBJ_IGNORE_IMPERSONATED_DEVICEMAP and OBJ_DONT_REPARSE which were finally documented, in part because I said it'd be nice if they did so. Of course, it only took 5 years to document them since they were introduced to fix bugs I reported. I guess that's pretty speedy in Microsoft's world. And of course they only help you if you're using the system call APIs which, let's not forget, are only partially documented.
While digging around in Windows 10 2004 (ugh... really, it's just confusing), and probably reminded by Alex Ionescu at some point, I noticed Microsoft have introduced another mitigation which is only available using an undocumented system call and not via any exposed Win32 API. So I thought, I should document it.
UPDATE (2020-04-23): According to @FireF0X this was backported to all supported OS's. So it's a security fix important enough to backport but not tell anyone about. Fantastic.
The system call in question is NtLoadKey3. According to j00ru's system call table this was introduced in Windows 10 2004, however it's at least in Windows 10 1909 as well. As the name suggests (if you're me at least) this loads a Registry Key Hive to an attachment point. This functionality has been extended over time, originally there was only NtLoadKey, then NtLoadKey2 was introduced in XP I believe to add some flags. Then NtLoadKeyEx was introduced to add things like explicit Trusted Hive support to mitigate cross hive symbolic link attacks (which is all j00ru's and Gynvael fault). And now finally NtLoadKey3. I've no idea why it went to 2 then to Ex then back to 3 maybe it's some new Microsoft counting system. The NtLoadKeyEx is partially exposed through the Win32 APIs RegLoadKey and RegLoadAppKey APIs, although they're only expose a subset of the system call's functionality.
Okay, so what bug class is NtLoadKey3 trying to mitigate? One of the problematic behaviors of loading a full Registry Hive (rather that a Per-User Application Hive) is you need to have SeRestorePrivilege* on the caller's Effective Token. SeRestorePrivilege is only granted to Administrators, so in order to call the API successfully you can't be impersonating a low-privileged user. However, the API can also create files when loading the hive file. This includes the hive file itself as well as the recovery log files.
* Don't pay attention to the documentation for RegLoadKey which claims you also need SeBackupPrivilege. Maybe it was required at some point, but it isn't any more.
When loading a system hive such as HKLM\SOFTWARE this isn't an issue as these hives are stored in an Administrator only location (c:\windows\system32\config if you're curious) but sometimes the hives are loaded from user-accessible locations such as from the user's profile or for Desktop Bridge support. In a user accessible location you can use symbolic link tricks to force the logs file to be written to arbitrary locations, and to make matters worse the Security Descriptor of the primary hive file is copied to the log file so it'll be accessible afterwards. An example of just this bug, in this case in Desktop Bridge, is issue 1492 (and 1554 as they didn't fix it properly (╯°□°）╯︵ ┻━┻).
RegLoadKey3 fixes this by introducing an additional parameter to specify an Access Token which will be impersonated when creating any files. This way the check for SeRestorePrivilege can use the caller's Access Token, but any "dangerous" operation will use the user's Token. Of course they could have probably implemented this by adding a new flag which will check the caller's Primary Token for the privilege like they do for SeImpersonatePrivilege and SeAssignPrimaryTokenPrivilege but what do I know...
Used appropriately this should completely mitigate the poor design of the system call. For example the User Profile service now uses NtLoadKey3 when loading the hives from the user's profile. How do you call it yourself? I couldn't find any documentation obviously, and even in the usual locations such as OLE32's private symbols there doesn't seem to be any structure data, so I made best guess with the following:
Notice that the TrustKey and Event handles from NtLoadKeyEx have also been folded up into a list of handle values. Perhaps someone wasn't sure if they ever needed to extend the system call whether to go for NtLoadKey4 or NtLoadKeyExEx so they avoided the decision by making the system call more flexible. Also the final parameter, which is also present in NtLoadKeyEx is seemingly unused, or I'm just incapable of tracking down when it gets referenced. Process Hacker's header files claim it's for an IO_STATUS_BLOCK pointer, but I've seen no evidence that's the case.
It'd be really awesome if in this new, sharing and caring Microsoft that they, well shared and cared more often, especially for features important to securing third party applications. TBH I think they're more focused on bringing Wayland to WSL2 or shoving a new API set down developers' throats than documenting things like this.