Improper Authorization in Handler for Custom URL Scheme

Description

Mobile platforms and other architectures allow the use of custom URL schemes to facilitate communication between applications. In the case of iOS, this is the only method to do inter-application communication. The implementation is at the developer's discretion which may open security flaws in the application. An example could be potentially dangerous functionality such as modifying files through a custom URL scheme.

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 iOS application uses a custom URL scheme. The replaceFileText action in the URL scheme allows an external application to interface with the file incomingMessage.txt and replace the contents with the text field of the query string.

External Application

NSString *stringURL = @"appscheme://replaceFileText?file=incomingMessage.txt&text=hello";
NSURL *url = [NSURL URLWithString:stringURL];
[[UIApplication sharedApplication] openURL:url];

Application URL Handler

- (BOOL)application:(UIApplication *)application handleOpenURL:(NSURL *)url {
  if (!url) {
    return NO;
  }
  NSString *action = [url host];
  if([action isEqualToString: @"replaceFileText"]) {
    NSDictionary *dict = [self parseQueryStringExampleFunction:[url query]];
    //this function will write contents to a specified file
    FileObject *objectFile = [self writeToFile:[dict objectForKey: @"file"] withText:[dict objectForKey: @"text"]];
  }
  return YES;
}

The handler has no restriction on who can use its functionality. The handler can be invoked using any method that invokes the URL handler such as the following malicious iframe embedded on a web page opened by Safari.

<iframe src="appscheme://replaceFileText?file=Bookmarks.dat&text=listOfMaliciousWebsites">

The attacker can host a malicious website containing the iframe and trick users into going to the site via a crafted phishing email. Since Safari automatically executes iframes, the user is not prompted when the handler executes the iframe code which automatically invokes the URL handler replacing the bookmarks file with a list of malicious websites. Since replaceFileText is a potentially dangerous action, an action that modifies data, there should be a sanity check before the writeToFile:withText: function.

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