Hooked Filetype Pdf

[Melissa Russian]

Ive followed the How to Create Data Handlers tutorial on how to register one, just with one minor change: I've added the DataHandler under HKCR\*\ShellEx\DataHandler with a custom GUID and a corresponding CLSID entry and linked it to a dll that does nothing. And now it was loaded every time I've clicked or "entered" a file. I didn't go as far as implementing all the required interfaces but according to the documentation:

So basically you want to do what an antivirus does, right? intercepting all I/O calls? You want something called filters (Last time I used it was on my old XP machine). Filters are the way to go if you don't want to write your own driver!

A bit out of my depth, but here goes. I read something ages ago that explorer calls SHELL32!CExecuteApplication::Execute each time an application is double-clicked. No real documentation about it exists though. If you trace the call, it ends up calling CreateProcess. I suppose you could hook that. However, CreateProcess would not work if the application is already running and it is a single-instance application, like MS Outlook for example. This function only gets invoked if explorer needs to create a process.

I faced similar problem when i try to hook all the execution calls to a particular drive letter. Initially i hooked shellexecuteex with detour library. This failed in windows 7 as the explorer started using direct kernel32.dll or ntdll equivalent calls skipping the shell API layer.Better way to handle custom handling on the application execution is by implementing a shell folder. I hide the actual drive letter by adding a registry key. Then i implemented a shell folder which displays the content of the hidden drive letter content. By this method, i was able to get better control of the items displayed in the explorer eventually capture the double click. Eventhough you need to write a significant amount of code to the make the UI identical to explorer, this is the more appropriate & proper documented approach for your requirement.

Im using acf/validate_valuepassing wp_check_filetype to make sure images uploaded via a front end form are image types, the code below is working perfectly on the front end, however when I go to publish the post created by the form in the backend (as its automatically saved as a draft) I get the error message for when it is not a accpeted file type , when it is.

@tararotten In that case the code I posted should work for the front end basic uploader, and the admin media gallery. If you upload using the basic uploader, you will get the filename (not numeric) but if you use the media gallery you will get the attachment id (numeric). By checking to see if the $value is numeric you can determine how to get the mime type so you can validate it.

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If you are writing a program, independent from the programming language, you will most likely use a compiler to build the program from the coresponding source code. The source code snippets are basically translated to Machine Language, which is in the very end binary code like 01010011 00110011 01100011 01110101 01110010 00110011, which can be directly executed by a CPU:

Some compilers, like gcc for example, produce assembler code before translating to Machine Code. Assembler Code instructions actually have an 1-to-1 mapping with Machine Code. So this is the closest to Machine Code and looks for example like that:

The Windows OS has two different privilege levels, that were implemented to protect the Operating System from for example crashes caused by installed applications. All applications installed on a Windows System run in the so called User-mode. The kernel and device drivers run in the so called Kernel-mode. Applications in the User-mode cannot access or manipulate memory sections in the Kernel-mode. AV/EDR systems can only monitor application behaviour in the User-mode, due to the Kernel Patch Protection. And the very last instance in the User-mode are the Windows API functions from NTDLL.dll. If any function from NTDLL.dll is called, the CPU switches to Kernel-mode next, which cannot be monitored by AV/EDR vendors anymore. The single functions of NTDLL.dll are called Syscalls.

Where do we, as simulated attackers, need or use the Windows API? If we, for example, want to write specific bytes, such as shellcode, into a process we can import WriteProcessMemory from the file kernel32.dll with the following C#-Code snippet:

As of the NTDLL.dll functions are the last intance, that can be monitored for suspicious activities from attackers or malware by AV/EDR vendors, they are typically doing exactly that. They inject a custom DLL-file into every new process. You can find DLL files, loaded into a process from AV/EDR Vendors via for example Sysinternals procexp64.exe. You need to check the Show Lower Pane button in the View menu and afterwards check the button to show DLLs loaded:

If a program loads a function like NtWriteVirtualMemory from kernel32.dll, a copy of kernel32.dll is placed into memory. The AV/EDR vendors typically manipulate the in memory copy of this file and add their own code into specific functions, like NtWriteVirtualMemory. When the function is called by the program, the AV/EDRs additional code is executed first, which does in the case of NtWriteVirtualMemory for example an analysis of the bytes, which shell be written into the remote process. By using this technique, they can see the cleartext shellcode bytes, because they are already decrypted in this moment. The AV/EDR vendors technique of embedding their own code in memory by patching API functions is called Userland-Hooking.

By loading a custom Invoke-Mimikatz version like I did in my seccond blog post Bypass AMSI by manual modification part II with defender enabled on a system, the in-memory-scanner catches Mimikatz from memory after decryption and PE-loading. If you take a look at the code again - the decryption is done first, and the PE-Loader runs afterwards. We know now, that the PE-Loader calls several potentially suspicious Windows API calls. Those calls trigger the in-memory-scanner. So avoiding the calls will result in no memory scan at all.

If your implant or tool loads some functions from kernel32.dll or NTDLL.dll, a copy of the library file is loaded into memory. The AV/EDR vendors typically patch some of the functions from the in memory copy and place a JMP assembler instruction at the beginning of the code to redirect the Windows API function to some inspecting code from the AV/EDR software itself. So before calling the real Windows API function code, an analysis is done. If this analysis results in no suspicious/malicious behaviour and returns a clean result, the original Windows API function is called afterwards. If something malicious is found, the Windows API call is blocked or the process will be killed. I stole a nice picture from ired.team, which may help for understanding the process:

Both blog posts focus on bypassing the EDR-software CylancePROTECT and build a PoC code for this specific software. By patching the additional JMP instruction from the manipulated NTDLL.dll in memory, the analysis code of Cylance will never be executed at all. Therefore no detections/blockings can take place:

One disadvantage for this technique is, that you may have to change the patch for every different AV/EDR vendor. It is not very likely, that they all place an additional JMP instruction in front of the same functions at the same point. They will most likely hook different functions and maybe use another location for their patch. If you already know, which AV/EDR solution is in place in your target environment, you can use this technique and you will be fine bypassing the protection by patching the patch.

One problem here is, that the assembler code is different at some points between Windows OS versions and sometimes even between service pack/built numbers. Google project Zero did some research about the differences, so that they can be looked up on the linked website. By embedding all different assembler code versions for all OS-Versions its possible to check for the underlying operating system on runtime and choose the correct assembler code for the needed Windows API function. Assembler code can be embeded in C-Projects via Visual Studio by using ASM-files. The Dumpert project is therefore using an ASM-file, which contains all nessesary Windows API functions in assembler code for each Windows version:

To use this technique you need to know the exact NTDLL.dll functions needed for your project and extract the corresponding assembler code for them via disassembling. Afterwards you need to build an ASM-file containing all different offsets for different Windows OS-Versions. Sounds complicated.

But using this technique will enable us to bypass Userland-Hooking in general. This technique is independent from different vendors. They all will not see any Windows API function imports or calls at all. No function imports -> no patch/hook by the AV/EDR software -> stealth/bypass.

With the release of the tool SysWhispers it became much easier to create custom ASM-files with the corresponding C-Header files. The manual overhead for disassembling ntdll.dll is left out. Building the ASM and Header-File became straight forward by executing a single python script:

Talking with @IKalendarov about NimlineWhispers, he found that Windows Defender with Cloud protection enabled executes the Shellcode successfully but throws an alert stating Defensive Evasion detected afterwards:

I found, that this detection can easily be bypassed by renaming the Windows API functions in the ASM-File and of course also in the shellcode injection code. NtAllocateVirtualMemory becomes NtAVM for example and so on. If your shellcode itself or the code behind it contains any Windows API function imports - this can be detected again. So the shellcode loader and the shellcode itself should use Syscalls to stay undetected from Userland-Hooks.

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