FIX: Sending a POST HTTP Request May Cause an Access Violation

• The Microsoft Foundation Classes (MFC), included with: Microsoft Visual C++, 32-bit Edition, version 4.1

SYMPTOMS

When sending a POST HTTP request to an MFC ISAPI DLL, you may see an access violation occur in the debugger or get a message about a failed request from the Web browser.

CAUSE

There is a bug in CHttpServer::GetQuery() which is located in ISAPI.CPP in the MFC source code directory (MSDEV\MFC\SRC). On line 206 of ISAPI.CPP, you can see the following code:

   _tcscpy(lpszQuery, (LPCTSTR) pCtxt->m_pECB->lpbData);

RESOLUTION

To resolve this problem, change ISAPI.CPP and rebuild the MFC ISAPI Library.

First, modify ISAPI.CPP by changing the line on 206 from:

   _tcscpy(lpszQuery, (LPCTSTR) pCtxt->m_pECB->lpbData);

   memcpy(lpszQuery, (LPCTSTR) pCtxt->m_pECB->lpbData, pCtxt->m_pECB-
   >cbAvailable);
   lpszQuery[pCtxt->m_pECB->cbAvailable] = '\0';

      LPTSTR CHttpServer::GetQuery(CHttpServerContext* pCtxt,
            LPTSTR lpszQuery, DWORD cbQuery)
         {
            DWORD cbRemaining;

            //
            // If the request is a GET, then the lpszQueryString member of
            // the ECB contains the query string.
            //
            // If the request is a POST, then you have to get all of the
            // data,both from the lpbData member, and then read the rest of
            // the data via the ReadClient() call.
            //
            if (cbQuery < pCtxt->m_pECB->cbTotalBytes)
               return NULL;

            memcpy(lpszQuery,
                               (LPCTSTR) pCtxt->m_pECB->lpbData,
                                pCtxt->m_pECB->cbAvailable);
            lpszQuery[pCtxt->m_pECB->cbAvailable] = '\0';

            if ((cbRemaining = pCtxt->m_pECB->cbTotalBytes
                                           - pCtxt->m_pECB->cbAvailable) >
      0)
                  {
              pCtxt->ReadClient((LPVOID) (lpszQuery
                                           + pCtxt->m_pECB->cbAvailable),
                                  &cbRemaining);
                  }
            return lpszQuery;
         }

   NMAKE /f MFCISAPI.MAK DEBUG=1

If you are building your ISAPI DLL linking to the MFC DLL (using EAFXIS(D).LIB), you can build the MFC ISAPI library by going to the \MFC\SRC directory and typing:

   NMAKE /f MFCISAPI.MAK DEBUG=1 DLL=2

STATUS

Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article. This bug was corrected in Visual C++ 32- bit Edition version 4.2.