The injected data is located after the Authenticode signature, which is located in the The Attribute Certificate Table of the file (PE Format). Here is some sample code to read it from the file:

#include <windows.h>
#include <wintrust.h>

void ReadAuthenticodeTail(PCWSTR path, LPBYTE* tailData, LPDWORD tailSize)
{
  *tailData = nullptr;
  *tailSize = 0;

  // open file & map it in memory
  auto file = CreateFile(path, GENERIC_READ, FILE_SHARE_READ, nullptr, OPEN_EXISTING, 0, nullptr);
  if (file != INVALID_HANDLE_VALUE)
  {
    auto mapping = CreateFileMapping(file, nullptr, PAGE_READONLY, 0, 0, nullptr);
    if (mapping)
    {
      auto map = (LPBYTE)MapViewOfFile(mapping, FILE_MAP_READ, 0, 0, 0);
      if (map)
      {
        // do some checks
        auto dosHeader = (PIMAGE_DOS_HEADER)map;
        if (dosHeader->e_magic == IMAGE_DOS_SIGNATURE)
        {
          auto headers = (PIMAGE_NT_HEADERS)(map + dosHeader->e_lfanew);
          if (headers->Signature == IMAGE_NT_SIGNATURE)
          {
            // get to Attribute Certificate Table
            auto va = headers->OptionalHeader.DataDirectory[IMAGE_DIRECTORY_ENTRY_SECURITY].VirtualAddress;
            auto size = headers->OptionalHeader.DataDirectory[IMAGE_DIRECTORY_ENTRY_SECURITY].Size;
            if (va && size)
            {
              // va is in fact a file offset
              // cf https://learn.microsoft.com/en-us/windows/win32/debug/pe-format#the-attribute-certificate-table-image-only
              auto cert = (LPWIN_CERTIFICATE)(map + va);

              // check it's authenticode signature and there's a tail
              if (cert->wRevision == WIN_CERT_REVISION_2_0 &&
                cert->wCertificateType == WIN_CERT_TYPE_PKCS_SIGNED_DATA &&
                cert->dwLength < size)
              {
                // read what's after certificate (the tail)
                *tailSize = size - cert->dwLength;
                *tailData = new BYTE[*tailSize];
                CopyMemory(*tailData, map + va + cert->dwLength, *tailSize);
              }
            }
          }
        }
        UnmapViewOfFile(map);
      }
      CloseHandle(mapping);
    }
    CloseHandle(file);
  }
}

int main()
{
  LPBYTE tail;
  DWORD size;
  ReadAuthenticodeTail(L"c:\\somepath\\SomeSignedFileWithInjectedData.exe", &tail, &size);
  if (tail)
  {
    // TODO: do something with it...

    delete[] tail;
  }
  return 0;
}