Why you shouldn’t touch Change­Window­Message­Filter with a 10-ft pole…

One of the things introduced with Windows Vista is the concept of something called “user interface privilege isolation”, or an attempt to allow multiple processes to coexist on one desktop even if they are running at different privilege levels, without compromising security. This new change to the security architecture with respect to how the user interface operates is also a significant part of how UAC and Internet Explorer Protected Mode can claim be to be reasonably secure, despite displaying user interfaces from differing security contexts on the same desktop.

(For reference, the Windows GUI model was designed back in the 16-bit cooperative multitasking days, where every task (yes, task – there weren’t processes or threads in Windows in those days) completely trusted every other task. Although things have improved somewhat since then, there are a number of assumptions that are so fundamental to how the Windows GUI model works that it is virtually impossible to eliminate all communication between GUI programs via window messages and still have the system function, which leads to many of the difficulties in securing user interface communications in Windows. As a result, the desktop has traditionally been considered the “security barrier” in terms of the Windows UI, such that processes of differing privilege levels should be isolated on their own desktops. Of course, since only one desktop is displayed locally at a time, this is less than convenient from an end user’s perspective.)

Most of the changes that were made in Vista relate to restrictions on just how processes from different integrity levels or user accounts can communicate eachother using the window messaging system. Window messages are typically split up into several different categories:

  1. System marshalled messages, or messages that come before WM_USER. These are messages that for compatibility with 16-bit Windows (and convenience), are automagically marshalled cross process. For example, this is why you can send the LB_ADDSTRING message to a window located in a different process, even though that message takes a pointer to a string which would obviously not be valid in the remote address space. The windowing system understands the semantics of all system marshalled messages and can thus, as the name implies, marshal them cross-process. This also means that in many circumstances, the system can also perform parameter validation in the cross process case, so that for instance a null pointer passed to LB_ADDSTRING won’t cause the remote process to crash. The system marshalled range also includes messages like WM_DESTROY, WM_QUIT, and the like. Most of the “built in” controls like the Edit and ListBox controls are “grandfathered in” to the system marshalled range, for compatibility reasons, though the newer common controls are not specially handled like this.
  2. The private window class message range, from WM_USER to 0x7FFF. These messages are specific to a particular custom window class (note that common controls, such as Rich Edit, are “custom” window classes even though you might think of them as being built in to the operating system; the windowing system ostensibly has no special knowledge of these controls, unlike the “built-in” controls like ListBox or Edit). Because the format and semantics of these messages are specific to a program-supplied window class, the window manager cannot interpret, marshal, or validate these parameters cross-process.
  3. The private application message range, from WM_APP to 0xBFFF. These window messages are specific to a program (and not a window class), though in practice it is very common for programmers to incorrectly interchange WM_USER and WM_APP for custom window classes that are internal to an application. These window messages are again completely opaque to the window manager and are essentially treated the same as private window class messages. Their intended use is to allow things like application customized subclasses of window classes to communicate with eachother (e.g. in the case where the application hooks a window procedure).
  4. The registered message range, from 0xC000 to 0xFFFF. Window messages here are dynamically assigned similarly to how atoms work in Windows. Specifically, a program passes an arbitrary string to the RegisterWindowMessage routine, which hands the application back a value in the registered message range. Any other program calling RegisterWindowMessage in the same session will receive the same window message value. These messages are, like class- and application- defined messages, opaque to the system. They are used when programs need to communicate cross-process with custom window messages, without the use of some sort of central registry where window messages would have to be permanently registered with Microsoft in order to receive a unique, non-conflicting identifier. By using an arbitrary string value (easy to make unique) and dynamically reserving a numeric message identifier at runtime, no registry is required and programs following the “standard” for that window message can still communicate with eachother. Registered window messages are not marshalled or interpreted by the windowing system.

Now, in Windows Vista, only a subset of the system marshalled messages can be sent cross process when the two processes have differnet integrity levels, and this subset of messages is heavily validated by the system, such that if a program receives a message in the system marshalled range, it can ostensibly “trust” it. Since the windowing system cannot validate custom messages (in any of the other three categories), these are all silently dropped by default in this scenario. This is typically a good thing (consider that many common control messages have pointers in their contracts and lay in the WM_USER range, making it very bad for an untrusted program to be able to send them to a privileged application). However, sometimes, one does need to send custom messages cross process. Vista provides a mechanism for this in the Change­Window­Message­Filter function, which is essentially a way to “poke a hole” in the window message “firewall” that exists between cross-integrity-level processes in Vista.

Now, this might seem like a great approach at first – after all, you’ll only use Change­Window­Message­Filter when you’re sure you can completely validate a received message even if it is from an untrusted source, such that there’s no way something could go wrong, right?

Well, the problem is that even if you do this, you are often opening your program up to attack unintentionally. Consider for a moment how custom window messages are typically used; virtually all the common controls in existance have “dangerous” messages in the custom class message range (e.g. WM_USER and friends). Additionally, many programs and third party libraries confuse WM_USER and WM_APP, such that you may have programs communicating cross process via both WM_USER and WM_APP, via “dangerous” messages that are used to make sensitive decisions or include pointer parameters.

This means that in reality, you can’t really use Change­Window­Message­Filter for a specific window message unless you are absolutely sure that nobody else in your process is listening for that message and can’t be exploited if they receive a malformed (or specially crafted) message. Right away, this pretty much excludes all WM_USER messages, and even WM_APP is highly questionably in my opinion due to how frequently components mix up WM_USER and WM_APP.

Well, that’s still not so bad, is it? After all, you can just ensure that any third party modules you use don’t do stupid things with custom window messages if you’ve got source code to them, right? Well, aside from the fact that that is in reality a pretty implausible scenario, the problem is even worse. There is rampant use of Change­Window­Message­Filter in operating system shipped libraries that your program is already using on Vista, which you have absolutely no control over. For instance, if one looks at Shell32.dll with a disassembler in Vista, one might see this in, say, the SHChangeNotifyRegister function:

mov     edx, 1          ; MSGFLT_ADD
mov     ecx, 401h       ; WM_USER + 1
call    cs:__imp_ChangeWindowMessageFilter

In other words, SHChangeNotifyRegister just promised to the window message firewall that everybody in the entire process fully validates the custom class message WM_USER + 1. Yow! The WM_USER range is the worst of any to be doing that on, especially a low WM_USER value because practically everything that uses a custom window class will use a WM_USER message for something, and the changes of a collision with the set of all custom window classes goes extremely much up that close to the start of the custom window class range. Now, just for kicks, let’s take a look at the SDK headers and see if any of the built in common controls have any interesting messages that match WM_USER + 1, or 0x401:

AclUI.h(142):#define PSPCB_SI_INITDIALOG (WM_USER + 1)
CommCtrl.h(1458):#define TB_ENABLEBUTTON (WM_USER + 1)
CommCtrl.h(2562):#define TTM_ACTIVATE (WM_USER + 1)
CommCtrl.h(6230):#define CBEM_INSERTITEMA (WM_USER + 1)
CommDlg.h(765):#define WM_CHOOSEFONT_GETLOGFONT (WM_USER + 1)

Let’s look at the documentation for some of those window messages in MSDN. Hmm, there’s CBEM_INSERTITEM[A|W], which takes, in lParam, “A pointer to a COMBOBOXEXITEM structure…”. Oh, and there’s also WM_CHOOSEFONT_GETLOGFONT, which uses lParam for a “pointer to a LOGFONT structure…”. Hmm, let me see. Anybody who calls SHChangeNotify is promising that they are not using any ComboBoxEx controls, any choosefont controls, or any of the other many hits for dangerous WM_USER + 1 messages that I didn’t list for space reasons. Oh, and that’s just the built-in controls – what happens if there’s a third party control thrown in there? What if we’re running in Internet Explorer and there’s a custom ActiveX control showing its own user interface there, blissfully unaware that some other code in the process called SHChangeNotify. That means that anybody, anywhere, who calls SHChangeNotify on Vista (or uses a library or function that calls SHChangeNotify internally, which is probably not going to be documented at all being an implementation detail) just reinvented the shatter attack with their program (congratulations!), probably without even realizing it – how would they, if they didn’t take the time to disassemble the API instead of trusting that it just works?

Now, I might be coming off a bit harsh on Microsoft here, but that’s kind of my point. Microsoft puts a lot of effort into security, and they’re the ones who designed and implemented the new the new security improvements on Vista. Sadly, this is hardly an isolated incident in Vista – with a little bit of looking, it’s very easy to find numerous other examples of system libraries that are loaded in used and called all over the place making this sort of error, which represents in my opinion a fundamental lack of understanding of how user interface security works.

Now, if the company that designed and implemented Change­Window­Message­Filter is using it wrong, how many third party developers out there that just want to get their program working under Vista in the quickest way possible with the minimum amount of effort and money spent will do the right thing? MSDN doesn’t even document this entire class of problems with Change­Window­Message­Filter that I can tell, so to be honest I think I can be fairly confident and say “virtually nobody”. It takes someone with a fairly good understanding of how the window messaging system impacts security to recognize and grasp this problem, and that only includes people who are even thinking about security in the first place when they see a function like Change­Window­Message­Filter, which I’m betting are already the vast minority.

This function is one that is pretty much all but impossible to use correctly aside from just maybe messages in the registered message range, which are already expected to be cross process and and not fully trusted (and I’m still skeptical that people will even on average get it right even in that case).

There are almost certainly other loopholes in the UIPI architecture that have yet to be discovered, if for no other reason than that it is a security bolt-on to an architecture that was designed for a single shared address space in a cooperative multitasking system. I wouldn’t say that this is so much the fault of the UIPI folks, but rather a fact that it’s just going to be ridiculously hard to make it completely safe to run programs with multiple privilege levels on the same desktop. And, remember, the “good guys” have to get it right 100% of the time from a security perspective, while the “bad guys” only need to find that one case out of 1000 that got missed in order to break the system.

So, do yourself a favor and stick to the desktop as a security barrier; the 16-bit Windows-derived window messaging system was just not designed to support programs at different privilege levels on the same desktop.

10 Responses to “Why you shouldn’t touch Change­Window­Message­Filter with a 10-ft pole…”

  1. Edgar Barbosa says:

    Hi Skywing,

    First, I would like to say that your blog is simply awesome!!! Keep up the good work.

    I presented at SyScan’07 conference some information about UIPI. Maybe you will find it interesting. The presentation is at

    Edgar Barbosa

  2. Skywing says:

    Thanks – I’ll give it a look! There’s definitely going to be a lot of room for bugs and stuff in the UIPI stuff for awhile, I’d say.

  3. Stewart says:

    I’m guessing here, but wouldn’t it have been more secure if this function had been made to work on a per window basis? That way you could choose to allow messages only for a given window, which in theory you control. Of course that breaks thread messages, but there could be a NULL window handle option for that. The documentation doesn’t really say (or at least I didn’t spot it) whether the filter is per process or per message queue anyway.

  4. Skywing says:

    Yep, definitely seems to me that at a glance at least, it would have been much better to do it on a per-HWND basis.

  5. ak says:

    You could probably do it on a hwnd basis with some special handling in your message loop and/or hooking your own process

  6. Skywing says:

    How do you tell whether a message originiated out of process in that case? As far as I know, win32k still does not expose whether a message originated out of process / from a different thread to user mode.

    Without an authoritative way to determine that a message came from in-process you can’t perform that validation yourself.

  7. Yuhong Bao says:

    For fun, how about running an actual program and set a breakpoint at ChangeWindowMessageFilter?

  8. Skywing says:

    Easily doable; I left that as an exercise to the reader.

    BTW, anything that pops up a shell common file dialog will be vulnerable to the WM_USER + 1 issue I described here.

  9. Michael Morozov says:

    I have an application that relies on cross-process DLL injection. The app communicates with injected DLL via custom messages. I used WM_USER space for these messages, but it looks like UIPI drops them all. What is the best range to use for this functionality to work?

  10. Skywing says:

    I would use RegisterWindowMessage and then allow that via ChangeWindowMessageFilter. However, be aware that you cannot design a cross-process window message interface that passes pointers or other untrusted values or you are creating a security hole.

    For this reason, I would tend to avoid using window at all messages for most cross-process IPC (if possible), as it is typically very difficult to do non-trivial things in a secure fashion using them.