CWE-939: Improper Authorization in Handler for Custom URL Scheme

Description:
The product uses a handler for a custom URL scheme, but it does not properly restrict which actors can invoke the handler using the scheme.

Extended 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.
Affected technologies: Mobile
Parent weaknesses: CWE-862, CWE-940

Consequences:
  - Access Control: Gain Privileges or Assume Identity

Detection methods:
  - Automated Static Analysis: Automated static analysis, commonly referred to as Static Application Security Testing (SAST), can find some instances of this weakness by analyzing source code (or binary/compiled code) without havin

Mitigations:
  - [Architecture and Design] Utilize a user prompt pop-up to authorize potentially harmful actions such as those modifying data or dealing with sensitive information. 
                   When designing functionality of actions in the URL scheme, consider whether the action should be accessible to all mobile applications, or if 

Demonstrative examples:
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];
- (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 
              

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

Real-world examples:
  - CVE-2013-5725: URL scheme has action replace which requires no user prompt and allows remote attackers to perform undesired actions.
  - CVE-2013-5726: URL scheme has action follow and favorite which allows remote attackers to force user to perform undesired actions.