MUI files under the hood

Have you ever copied after Vista a system file like notepad.exe onto the desktop and tried to execute it? Have you ever tried after Vista to modify the resources of a system file like regedit.exe? It’s most likely that neither of the two was a successful operation.

This will be very brief because the topic is very limited and because of my lack of time: bear with me. 🙂

If you try to copy, for instance, notepad.exe onto the desktop and run it in a debugger you will notice that it fails in its initialization routine when trying to load its accelerators. You take a look at the HINSTANCE passed to LoadAccelerators and notice that it’s NULL. You open notepad.exe in a resource viewer and notice that it doesn’t contain accelerator resources. Thus, you realize that the global instance is associated to some external resource as well. Go back to the system folder where you took the system executable and you’ll notice language directories such as “en-US”. Just copy the one which identifies the language of your system to the same directory of notepad.exe. You’ll notice that now notepad.exe runs correctly.

Vista introduced the separation between binary and language dependent resources to allow a single Windows image to contain more than just one language. You can obtain more information about the development aspects on MSDN.

The language directory contains files with names such as “notepad.exe.mui”, one for every file they provide resources for (including dlls). These are very basic PE files which contain only a resource directory and are loaded into the address space of the process as they are.

These files are associated to the main file in two ways:

1) By name: just rename the notepad to test.exe and the MUI file accordingly and it still works.
2) Via resource, as we’ll see.

If you open both notepad.exe and its MUI file with a resource viewer, you’ll see they both contain a “MUI” resource. What this data contains can be roughly understood from the MSDN or SDK:

You’ll find this structure in WinNls.h. However, this structure is for GetFileMUIInfo, it doesn’t match the physical data.

The first DWORD is clearly a signature. If you change it, the MUI is invalidated and notepad won’t run. It is followed by another DWORD describing the size of the structure (including the signature).

These are the two checksums:

These two checksums are probably in the same order of the structure. They both match the ones contained in the MUI file and if you change the second one, the application won’t run.

There are no other association criteria: I changed both the main file and the MUI file (by using a real DLL and just replacing the resource directory with the one of the MUI file) and it still worked.

About the second matter mentioned in the beginning: modification of resources. If you try to add/replace an icon to/in notepad.exe you will most likely not succeed. This is because as mentioned in the MSDN:

There are some restrictions on resource updates in files that contain Resource Configuration(RC Config) data: LN files and the associated .mui files. Details on which types of resources are allowed to be updated in these files are in the Remarks section for the UpdateResource function.

Basically, UpdateResource doesn’t work if the PE file contains a MUI resource. Now, prepare for an incredibly complicated and technically challenging hack to overcome this limitation… Ready? Rename the “MUI” resource to “CUI” or whatever, now try again and it works. Restore the MUI resource name and all is fine.

The new build of the CFF Explorer handles this automatically for your comfort.

This limitation probably broke most of the resource editors for Win32. Smart.

PDF Insider Demo

A small, on the fly video presentation of my new utility.



The emphasis of this application stands on parsing correctly the PDF format, if it can’t because the PDF is malformed (very common among malicious PDFs), then it provides the tools to read the objects nonetheless. I tested it on many PDF (also malicious ones) and it handles all of them very well.

As I have written this application in five days, there are still some small features I’d like to add, but most of the code is already there. I started the development of it quite some time ago on a weekend while I was sick at home and have found only now the time to finish it.

I have no plans about how and when to release it yet, but some friends of mine will start using it in real world scenarios.

P.S. Thanks to Alessandro Gario for the throughout testing.

Filter Monitor 1.0.1

This week, after months of development of bigger projects, I found some time to windbg “ntoskrnl.exe” and write a utility. It is called Filter Monitor and shows some key filters installed by kernel mode components.

“As you probably all know the Service Descriptor Table has been a playground on x86 for all sorts of things: rootkits, anti-viruses, system monitors etc. On x64 modifying the Service Descriptor Table is no longer possible, at least not without subverting the Patch Guard technology.

Thus, programs have now to rely on the filtering/notification technologies provided by Microsoft. And that’s why I wrote this little utility which monitors some key filters.

Since I haven’t signed the driver of my utility, you have to press F8 at boot time and then select the “Disable Driver Signature Enforcement” option. If you have a multiple boot screen like myself, then you can take your time. Otherwise you have to press F8 frenetically to not miss right moment.

A disclaimer: the boot process can be a bit annoying, but the utility should be used on virtualized systems anyway, as I haven’t fully tested it yet. I doubt that it will crash your system, I guess the worst scenario is that it won’t list some filters. It should work on any Windows system starting from Vista RTM and I have provided an x86 version and an x64 version. But the truth is that I have tested only the x64 version on Windows 7 RTM. Last but not least, I can’t guarantee that this utility will work on future versions of Windows, it relies heavily on system internals.

Now, let’s run it. The supported filters/notifications at the time are these: Registry, Create Process, Create Thread and Load Image. “Registry” stands for CmRegisterCallback filters. “Create Process” for PsSetCreateProcessNotifyRoutine callbacks. “Create Thread” for PsSetCreateThreadNotifyRoutine callbacks. And “Load Image” for PsSetLoadImageNotifyRoutine callbacks.

The “Additional Info” in the list view provides internal information like the address of the callback function.

There are some default filters registered by system components, but, as you can notice, there are also Kaspersky components. That’s because some filters (like the registry filter) are not used by system components and I needed a tool which would make use of these filters for my little demonstration.

The version of Kaspersky I have installed is the latest one available on the internet which is: 9.0.0.463.

I created for this demonstration a little executable called “k-test” (what you see on the desktop are three copies of the same executable) which copies itself in a directory called “borda” in the “Roaming” directory of the operating system. It then creates a value in the Run key of the registry to execute itself at each start-up. Finally, it launches itself from the “Roaming” directory and ends.

This is a typical malware behavior. Beware that the signature of the application itself is not contained in the databases of Kaspersky as I have written it on the fly, but it detects the suspicious behavior, stops execution and deletes the file. And it does this every time I launch the test application.

Now let’s get to the part where I show an additional functionality of the Filter Monitor which is the ability to remove registered filters and see what happens if I remove the filters installed by klif.sys, which is the “Kaspersky Lab Interceptor and Filter” driver. As the name suggests, this driver intercepts and filters: it installs all four of typologies of filters listed by the Filter Monitor. On x86 instead of calling CmRegisterCallback it additionally hooks about 60 functions of the Service Descriptor Table (which is a lot), but that’s no longer possible on x64.

So, let’s remove the filters and re-launch k-test. It works now.

Final disclaimer: It is not my intent to comment on security features of anti-viruses, I just wanted to present my new tool and show its functionalities. I was already familiar with the internals of Kaspersky before writing this utility.

I hope you enjoyed the presentation.”

P.S. A huge thanks goes to Alessandro Gario for providing me with all the different versions of ntoskrnl.exe.

Native Blocks Pre-Alpha

Here’s a presentation of my new tool. I called it Native Blocks. It’s a re-assembler basically. Since a written presentation would take me too much time I prepared a video presentation.

Again, this is a pre-alpha. This tool will soon support other technologies such as java, actionscript and maybe even x86. Right now it only supports .NET (and even .NET support will be hugely improved, like for instance supporting the direct modification of assemblies without having to use Rebel.NET).

The development of this tool depends mainly on the interest of people.

As I can be considered a student from now on, I would like to earn some extra money by writing tools such as this one. I have still my job as consultant, but it’s a very limited partime, because I just became a student.

This tool is in my opinion pretty good, it is not only good for deobfuscation purposes but also patching and assembling on the fly.

If this tool can be sold, then the support of technologies will depend on requests. I think I’ll add Java immediately and after that maybe x86/x64. Again it depends.

Suggestions and comments are welcome.

Qt Internals & Reversing

Today I took a break from the larger article I’m currently writing. To relax, I wrote a smaller article about the Qt framework. I hope you enjoy.

Qt Internals & Reversing

Half of the text of this article comes from my larger paper “Dynamic C++ Proposal”. I decided that it was useful to take the part about Qt internals, put it into another article and extend it by adding a reversing part. Because of its nature, this is not the usual kind of article I write. In fact, I wrote the reversing part in less than a day. So, this is a very easy one. However, I think it is useful for people who need to reverse a Qt application and certainly wouldn’t consider reading my other paper about Dynamic C++, which doesn’t sound like a paper about Qt and, in fact, isn’t a paper about Qt: the paragraph about Qt is only one among many others. Moreover, I haven’t seen serious articles about this subject.

The first thing which needs to be considered when reversing Qt applications is what Qt brought to the C++ language. Events (inside the Qt framework) are just virtual functions, so nothing new there. This is not a C++ reversing guide. What is new in Qt are signals and slots, which rely on the dynamism of the Qt framework.

So, first thing I’m going to show how this dynamism works. The second part focus on reversing and, at that point, I will show how to obtain all the metadata one needs when disassembling a “Q_OBJECT” class.