# Common API used in Malware

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## Generic

### Networking

| Raw Sockets   | WinAPI Sockets |
| ------------- | -------------- |
| socket()      | WSAStratup()   |
| bind()        | bind()         |
| listen()      | listen()       |
| accept()      | accept()       |
| connect()     | connect()      |
| read()/recv() | recv()         |
| write()       | send()         |
| shutdown()    | WSACleanup()   |

### TLS pinning and chunked transport

Many loaders wrap their TCP stream in `SslStream` and pin the server’s leaf certificate against an embedded copy (certificate pinning). Bot info/tasks are compressed (e.g., GZip). When responses exceed a threshold (~1 MB), data is fragmented into small chunks (e.g., 16 KB segments) to avoid size-based heuristics and reduce memory spikes during deserialisation.


### Persistence

| Registry         | File          | Service                      |
| ---------------- | ------------- | ---------------------------- |
| RegCreateKeyEx() | GetTempPath() | OpenSCManager                |
| RegOpenKeyEx()   | CopyFile()    | CreateService()              |
| RegSetValueEx()  | CreateFile()  | StartServiceCtrlDispatcher() |
| RegDeleteKeyEx() | WriteFile()   |                              |
| RegGetValue()    | ReadFile()    |                              |

### Encryption

| Name                  |
| --------------------- |
| WinCrypt              |
| CryptAcquireContext() |
| CryptGenKey()         |
| CryptDeriveKey()      |
| CryptDecrypt()        |
| CryptReleaseContext() |

### Anti-Analysis/VM

| Function Name                                             | Assembly Instructions |
| --------------------------------------------------------- | --------------------- |
| IsDebuggerPresent()                                       | CPUID()               |
| GetSystemInfo()                                           | IN()                  |
| GlobalMemoryStatusEx()                                    |                       |
| GetVersion()                                              |                       |
| CreateToolhelp32Snapshot \[Check if a process is running] |                       |
| CreateFileW/A \[Check if a file exist]                    |                       |

### Emulator API fingerprinting & sleep evasion

Malware often fingerprints sandbox emulators by searching for Defender’s virtualised exports (seen in the Malware Protection Emulator). If any of these symbols are present (case-insensitive scan of the process), execution is delayed for 10–30 minutes and re-checked to waste analysis time.

Examples of API names used as canaries:
- `MpVmp32Entry`, `MpVmp32FastEnter`, `MpCallPreEntryPointCode`, `MpCallPostEntryPointCode`, `MpFinalize`, `MpReportEvent*`, `MpSwitchToNextThread*`
- `VFS_*` family: `VFS_Open`, `VFS_Read`, `VFS_MapViewOfFile`, `VFS_UnmapViewOfFile`, `VFS_FindFirstFile/FindNextFile`, `VFS_CopyFile`, `VFS_DeleteFile`, `VFS_MoveFile`
- `ThrdMgr_*`: `ThrdMgr_GetCurrentThreadHandle`, `ThrdMgr_SaveTEB`, `ThrdMgr_SwitchThreads`

Typical delay primitive (user-land):
```cmd
cmd /c timeout /t %RANDOM_IN_[600,1800]% > nul
```

Argument gatekeeping
- Operators sometimes require a benign-looking CLI switch to be present before running the payload (e.g., `/i:--type=renderer` to mimic Chromium child processes). If the switch is absent, the loader exits immediately, hindering naive sandbox execution.


### Stealth

| Name                     |                                                                            |
| ------------------------ | -------------------------------------------------------------------------- |
| VirtualAlloc             | Alloc memory (packers)                                                     |
| VirtualProtect           | Change memory permission (packer giving execution permission to a section) |
| ReadProcessMemory        | Injection into external processes                                          |
| WriteProcessMemoryA/W    | Injection into external processes                                          |
| NtWriteVirtualMemory     |                                                                            |
| CreateRemoteThread       | DLL/Process injection...                                                   |
| NtUnmapViewOfSection     |                                                                            |
| QueueUserAPC             |                                                                            |
| CreateProcessInternalA/W |                                                                            |

### Execution

| Function Name    |
| ---------------- |
| CreateProcessA/W |
| ShellExecute     |
| WinExec          |
| ResumeThread     |
| NtResumeThread   |

### Miscellaneous

- GetAsyncKeyState() -- Key logging
- SetWindowsHookEx -- Key logging
- GetForeGroundWindow -- Get running window name (or the website from a browser)
- LoadLibrary() -- Import library
- GetProcAddress() -- Import library
- CreateToolhelp32Snapshot() -- List running processes
- GetDC() -- Screenshot
- BitBlt() -- Screenshot
- InternetOpen(), InternetOpenUrl(), InternetReadFile(), InternetWriteFile() -- Access the Internet
- FindResource(), LoadResource(), LockResource() -- Access resources of the executable

## Malware Techniques

### DLL Injection

Execute an arbitrary DLL inside another process

1. Locate the process to inject the malicious DLL: CreateToolhelp32Snapshot, Process32First, Process32Next
2. Open the process: GetModuleHandle, GetProcAddress, OpenProcess
3. Write the path to the DLL inside the process: VirtualAllocEx, WriteProcessMemory
4. Create a thread in the process that will load the malicious DLL: CreateRemoteThread, LoadLibrary

Other functions to use: NTCreateThreadEx, RtlCreateUserThread

### Reflective DLL Injection

Load a malicious DLL without calling normal Windows API calls.\
The DLL is mapped inside a process, it will resolve the import addresses, fix the relocations and call the DllMain function.

### Thread Hijacking

Find a thread from a process and make it load a malicious DLL

1. Find a target thread: CreateToolhelp32Snapshot, Thread32First, Thread32Next
2. Open the thread: OpenThread
3. Suspend the thread: SuspendThread
4. Write the path to the malicious DLL inside the victim process: VirtualAllocEx, WriteProcessMemory
5. Resume the thread loading the library: ResumeThread

### PE Injection

Portable Execution Injection: The executable will be written in the memory of the victim process and it will be executed from there.

### Process Hollowing (a.k.a **RunPE**)

`Process Hollowing` is one of the favourite **defence-evasion / execution** tricks used by Windows malware.  The idea is to launch a *legitimate* process in the **suspended** state, remove (hollow) its original image from memory and copy an **arbitrary PE** in its place.  When the primary thread is finally resumed the malicious entry-point executes under the guise of a trusted binary (often signed by Microsoft).

Typical workflow:

1. Spawn a benign host (e.g. `RegAsm.exe`, `rundll32.exe`, `msbuild.exe`) **suspended** so that no instructions run yet.
   ```c
   STARTUPINFOA  si = { sizeof(si) };
   PROCESS_INFORMATION pi;
   CreateProcessA("C:\\Windows\\Microsoft.NET\\Framework32\\v4.0.30319\\RegAsm.exe",
                 NULL, NULL, NULL, FALSE, CREATE_SUSPENDED, NULL, NULL, &si, &pi);
   ```
2. Read the malicious payload into memory and parse its PE headers to obtain `SizeOfImage`, sections and the new `EntryPoint`.
3. **NtUnmapViewOfSection** / **ZwUnmapViewOfSection** – unmap the original image base of the suspended process.
4. **VirtualAllocEx** – reserve RWX memory of `SizeOfImage` inside the remote process.
5. **WriteProcessMemory** – copy the `Headers` first, then iterate over sections copying their raw data.
6. **SetThreadContext** – patch the value of `EAX/RAX` (`RCX` on x64) or `Rip` in the context structure so that `EIP` points to the payload’s `EntryPoint`.
7. **ResumeThread** – the thread continues, executing the attacker-supplied code.

Minimal proof-of-concept (x86) skeleton:
```c
void RunPE(LPCSTR host, LPVOID payload, DWORD payloadSize){
  // 1. create suspended process
  STARTUPINFOA si = {sizeof(si)}; PROCESS_INFORMATION pi;
  CreateProcessA(host, NULL,NULL,NULL,FALSE,CREATE_SUSPENDED,NULL,NULL,&si,&pi);

  // 2. read remote PEB to get ImageBaseAddress
  CONTEXT ctx; ctx.ContextFlags = CONTEXT_FULL;
  GetThreadContext(pi.hThread,&ctx);
  PVOID baseAddr;
  ReadProcessMemory(pi.hProcess,(PVOID)(ctx.Ebx+8),&baseAddr,4,NULL);

  // 3. unmap original image & allocate new region at same base
  NtUnmapViewOfSection(pi.hProcess,baseAddr);
  PVOID newBase = VirtualAllocEx(pi.hProcess,baseAddr,pHdr->OptionalHeader.SizeOfImage,
                                 MEM_COMMIT|MEM_RESERVE,PAGE_EXECUTE_READWRITE);
  // 4-5. copy headers & sections …
  // 6. write new image base into PEB and set Eip
  WriteProcessMemory(pi.hProcess,(PVOID)(ctx.Ebx+8),&baseAddr,4,NULL);
  ctx.Eax = (DWORD)(newBase) + pHdr->OptionalHeader.AddressOfEntryPoint;
  SetThreadContext(pi.hThread,&ctx);
  // 7. run!
  ResumeThread(pi.hThread);
}
```

Practical notes observed in the **DarkCloud Stealer** campaign:

* The loader picked `RegAsm.exe` (part of the .NET Framework) as host – a signed binary unlikely to draw attention.
* The decrypted VB6 stealer (`holographies.exe`) is *not* dropped on disk; it only ever exists inside the hollowed process making static detection harder.
* Sensitive strings (regexes, paths, Telegram credentials) are **RC4-encrypted** per-string and only decrypted at runtime, further complicating memory scanning.

Detection ideas:
* Alert on `CREATE_SUSPENDED` processes that never create GUI/console windows before a memory region is allocated as **RWX** (rare for benign code).
* Look for a call sequence `NtUnmapViewOfSection ➜ VirtualAllocEx ➜ WriteProcessMemory` across different processes.



## Hooking

- The **SSDT** (**System Service Descriptor Table**) points to kernel functions (ntoskrnl.exe) or GUI driver (win32k.sys) so user processes can call these functions.
  - A rootkit may modify these pointer to addresses that he controls
- **IRP** (**I/O Request Packets**) transmit pieces of data from one component to another. Almost everything in the kernel uses IRPs and each device object has its own function table that can be hooked: DKOM (Direct Kernel Object Manipulation)
- The **IAT** (**Import Address Table**) is useful to resolve dependencies. It's possible to hook this table in order to hijack the code that will be called.
- **EAT** (**Export Address Table**) Hooks. This hooks can be done from **userland**. The goal is to hook exported functions by DLLs.
- **Inline Hooks**: This type are difficult to achieve. This involve modifying the code of the functions itself. Maybe by putting a jump at the beginning of this.


## References

- [Unit42 – New Infection Chain and ConfuserEx-Based Obfuscation for DarkCloud Stealer](https://unit42.paloaltonetworks.com/new-darkcloud-stealer-infection-chain/)
- [Check Point Research – Under the Pure Curtain: From RAT to Builder to Coder](https://research.checkpoint.com/2025/under-the-pure-curtain-from-rat-to-builder-to-coder/)

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