Improper Verification of Source of a Communication Channel

Description

When an attacker can successfully establish a communication channel from an untrusted origin, the attacker may be able to gain privileges and access unexpected functionality.

Demonstrations

The following examples help to illustrate the nature of this weakness and describe methods or techniques which can be used to mitigate the risk.

Note that the examples here are by no means exhaustive and any given weakness may have many subtle varieties, each of which may require different detection methods or runtime controls.

Example One

This Android application will remove a user account when it receives an intent to do so:

IntentFilter filter = new IntentFilter("com.example.RemoveUser");
MyReceiver receiver = new MyReceiver();
registerReceiver(receiver, filter);

public class DeleteReceiver extends BroadcastReceiver {
  @Override
  public void onReceive(Context context, Intent intent) {
    int userID = intent.getIntExtra("userID");
    destroyUserData(userID);
  }
}

This application does not check the origin of the intent, thus allowing any malicious application to remove a user. Always check the origin of an intent, or create an allowlist of trusted applications using the manifest.xml file.

Example Two

These Android and iOS applications intercept URL loading within a WebView and perform special actions if a particular URL scheme is used, thus allowing the Javascript within the WebView to communicate with the application:

// Android
@Override
public boolean shouldOverrideUrlLoading(WebView view, String url){

  if (url.substring(0,14).equalsIgnoreCase("examplescheme:")){
    if(url.substring(14,25).equalsIgnoreCase("getUserInfo")){
      writeDataToView(view, UserData);
      return false;
    }
    else{
      return true;
    }
  }

}

// iOS
-(BOOL) webView:(UIWebView *)exWebView shouldStartLoadWithRequest:(NSURLRequest *)exRequest navigationType:(UIWebViewNavigationType)exNavigationType
{

  NSURL *URL = [exRequest URL];
  if ([[URL scheme] isEqualToString:@"exampleScheme"])
  {

    NSString *functionString = [URL resourceSpecifier];
    if ([functionString hasPrefix:@"specialFunction"])
    {


      // Make data available back in webview.
      UIWebView *webView = [self writeDataToView:[URL query]];

    }
    return NO;

  }
  return YES;

}

A call into native code can then be initiated by passing parameters within the URL:

window.location = examplescheme://method?parameter=value

Because the application does not check the source, a malicious website loaded within this WebView has the same access to the API as a trusted site.

See Also