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Improper Authorization in Handler for Custom URL Scheme

Incomplete Base
Structure: Simple
Description

This vulnerability occurs when an app implements a custom URL scheme handler but fails to properly verify which other apps or sources are allowed to trigger it.

Extended Description

Modern mobile and desktop platforms use custom URL schemes (like myapp://) as a bridge for apps to communicate and pass data. However, if the receiving app doesn't enforce strict authorization checks, any malicious or untrusted app on the same device can invoke this handler. This effectively creates an unintended public interface into your app's internal functions. Without proper validation, attackers can exploit this open handler to trigger dangerous actions. For example, a vulnerable handler might allow unauthorized file deletion, data export, or changes to app settings, simply by crafting a malicious URL. The security of the entire mechanism depends entirely on the developer implementing explicit checks to verify the calling source's identity and permissions.
Potential Mitigations 1
Phase: 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 an allowlist of applications to interface with is appropriate.
Demonstrative Examples 2
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

Code Example:

Good
Objective-C
objective-c
Application URL Handler

Code Example:

Bad
Objective-C
objective-c

//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.

Code Example:

Attack
HTML
html
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.

ID : DX-109

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:

Code Example:

Bad
Java


// 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;
}
}
}


Code Example:
Bad
Objective-C


// 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"])
{


objective-c
A call into native code can then be initiated by passing parameters within the URL:
Code Example:
Attack
JavaScript
javascript
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.
  
Observed Examples 2
CVE-2013-5725URL scheme has action replace which requires no user prompt and allows remote attackers to perform undesired actions.
CVE-2013-5726URL scheme has action follow and favorite which allows remote attackers to force user to perform undesired actions.
  
References 1
Scheming for Privacy and Security
Guillaume Ross
11-11-2013
https://brooksreview.net/2013/11/guest-post_scheming-for-privacy-and-security/(2023-04-07)
ID: REF-938
 
  
    
 
 
Applicable Platforms 
 
 
 
  
  
 
 
Technologies:
 
 
 Mobile : Undetermined 
 
 
 
  
 
 
Modes of Introduction 
 
 
 
 
 
  Implementation  
 
 
 
          
 
 
Related Weaknesses 
 
 
 
 
  ChildOf:
 Missing Authorization (CWE-862)