Example - 1

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

return NO;

// //this function will write contents to a specified file// 
NSDictionary *dict = [self parseQueryStringExampleFunction:[url query]];FileObject *objectFile = [self writeToFile:[dict objectForKey: @"file"] withText:[dict objectForKey: @"text"]];
if (!url) {}NSString *action = [url host];if([action isEqualToString: @"replaceFileText"]) {}return YES;- (BOOL)application:(UIApplication *)application handleOpenURL:(NSURL *)url {}

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 - 2

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// 

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

// // iOS// 



// // Make data available back in webview.// 
UIWebView *webView = [self writeDataToView:[URL query]];
NSString *functionString = [URL resourceSpecifier];if ([functionString hasPrefix:@"specialFunction"]){}return NO;
NSURL *URL = [exRequest URL];if ([[URL scheme] isEqualToString:@"exampleScheme"]){}return YES;-(BOOL) webView:(UIWebView *)exWebView shouldStartLoadWithRequest:(NSURLRequest *)exRequest navigationType:(UIWebViewNavigationType)exNavigationType{}

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.