Web Site Blocker

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Introduction

This article describes how a BHO (Browser Helper Object) could be used to block a particular web-site.

Background

BHO is a simple ATL COM object that Internet Explorer will load each time it runs, i.e. for every instance of Internet Explorer. BHOs run in Internet Explorer’s address space and can perform any operations on available objects (windows, modules etc.). BHO instantiates and gets destructed with browser’s instance as it is tied to a browser’s main window.

If your system has got active desktop enabled the BHO gets instantiated along with the windows explorer as well. To disable the BHO for windows explorer, you can add following code snippet to DllMain,

TCHAR strLoader[MAX_PATH];

::GetModuleFileName (NULL, strLoader, MAX_PATH);
if(stricmp("explorer.exe", strLoader) == 0)
 return FALSE;

BHO’s COM Server must implement IObjectWithSite that will help our object to hook on browser’s events. Internet Explorer will pass a pointer to its IUnknown interface by the means of IObjectWithSite. Only SetSite method of IObjectWithSite will need to be implemented as follows, 

STDMETHODIMP CBhoApp::SetSite(IUnknown *pUnkSite)
{
  // Retrieve and store the IWebBrowser2 pointer 
  m_spWebBrowser2 = pUnkSite; 
  if (m_spWebBrowser2 == NULL)
   return E_INVALIDARG;

  // Retrieve and store the IConnectionPointerContainer pointer 
  m_spCPC = m_spWebBrowser2;
  if (m_spCPC == NULL)
   return E_POINTER;

   // Connect to the container for receiving event notifications
  return Connect();
}

Here connect function would look like,

HRESULT CBhoApp::Connect()
{
  HRESULT hr;
  CComPtr<IConnectionPoint> spCP;

  // Receives the connection point for WebBrowser events
  hr = m_spCPC->FindConnectionPoint(DIID_DWebBrowserEvents2, &spCP);
  if (FAILED(hr))
   return hr;

  // Pass our event handlers to the container. Each time an event occurs
  // the container will invoke the functions of the IDispatch interface 
  // we implemented.
  hr = spCP->Advise(reinterpret_cast<IDispatch*>(this),&m_dwCookie);
  
  return hr; 
}

By calling Advise method we tell the browser that BHO would be eager to receive notifications about events. By this, what we mean is, BHO will provide the browser with the pointer to IDispatch (this is due to Component’s event handling). Browser then calls IDispatch’s Invoke method and passes it the ID of an event as an argument. So our BHO must implement Invoke method to handle the events.

STDMETHODIMP CBhoApp::Invoke(DISPID dispidMember,REFIID riid,LCID lcid,WORD wFlags,DISPPARAMS *pDispParams,VARIANT   
    *pvarResult, EXCEPINFO *pExcepInfo, UINT *puArgErr)
{
 USES_CONVERSION; //This macro should be called when using ATL string conversion 
     //macros to avoid compile time errors (here we are using OLE2T)
 
 if(dispidMember == DISPID_BEFORENAVIGATE2)
 {
  BSTR bstrUrlName;
  HRESULT hr = m_spWebBrowser2->get_LocationURL(&bstrUrlName);
  if(FAILED(hr))
   return hr;

  LPTSTR psz = new TCHAR[SysStringLen(bstrUrlName)];
  lstrcpy(psz, OLE2T(bstrUrlName));

  // Here, I am directly comparing with xyz.com. You can maintain a list of all sites to 
  // be blocked and then compare. Or you can also keep this data in a database, but I guess
  // that might affect the performance. (Experts! please comment on this.)


<pre>  if(stricmp("<a href="http://www.xyz.com/%22,(const%20char%20*)psz">http://www.xyz.com/",(const%20char%20*)psz</a>) == 0) 

  // Here you can also use strstr instead of stricmp
  // which will help to allow all domain originating from xyz. 
  {

     VARIANT vFlags = {0},vTargetFrameName = {0};
     // Instead of “about:blank”, you can redirect user to some page saying site has been blocked. :-)
     m_spWebBrowser2->Navigate(SysAllocString(L"about:blank"),&vFlags,&vTargetFrameName,NULL,NULL);
     m_spWebBrowser2->put_Visible(VARIANT_TRUE);
     return S_FALSE;
  }
  return S_OK;
 }
 else if(dispidMember == DISPID_NAVIGATECOMPLETE2) 

 // This checking is done to avoid post-navigation
 // loading of a page. 
 {

  BSTR bstrUrlName;
  HRESULT hr = m_spWebBrowser2->get_LocationURL(&bstrUrlName);
  if(FAILED(hr))
   return hr;

  // Convert the text from Unicode to ANSI
  LPTSTR psz = new TCHAR[SysStringLen(bstrUrlName)];
  lstrcpy(psz, OLE2T(bstrUrlName));
  ::OutputDebugString("In Navigate Complete");
  ::OutputDebugString(psz);
  if(stricmp("<a href="http://www.xyz.com/%22,psz">http://www.xyz.com/",psz</a>) == 0)
  {
   VARIANT vFlags = {0},vTargetFrameName = {0};
   m_spWebBrowser2->Navigate(SysAllocString(L"about:blank"),&vFlags,&vTargetFrameName,NULL,NULL);
   m_spWebBrowser2->put_Visible(VARIANT_TRUE);
  }
  return S_OK;
 }
 return S_FALSE;
}

You also need to change .rgs file of your project. Add following code snippet to it,

HKLM
{
   SOFTWARE
   {
      Microsoft
      {
         Windows
         {
            CurrentVersion
            {
               Explorer
               {
                  'Browser Helper Objects'
                  {
                     ForceRemove {0CB66BA8-5E1F-4963-93D1-E1D6B78FE9A2}
                  }
               }
            }
         }
      }
   }
}

Using the code

Once you are done with the compilation, register your component using regsvr32. Whenever you want to disable the BHO, simply use regsvr32 with /u option. One can also provide a simple UI to do this.

Improvements

• A UI element could be added to add a URL to the list of “sites to be blocked”.

References