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.

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.

Potential Impact

Access Control, Other

Gain Privileges or Assume Identity, Varies by Context, Bypass Protection Mechanism

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

Good

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

Application URL Handler

Bad

- (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.

Attack

<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.

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:

Bad

// Android
                     @Overridepublic 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;}}
                     }

Bad

// 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:

Attack

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.

Mitigations & Prevention

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.

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 having to execute it. Typically, this is done by building a model of data flow and control flow, then sea

Real-World CVE Examples

CVE ID	Description
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.

Frequently Asked Questions

What is CWE-939?

CWE-939 (Improper Authorization in Handler for Custom URL Scheme) is a software weakness identified by MITRE's Common Weakness Enumeration. It is classified as a Base-level weakness. 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.

How can CWE-939 be exploited?

Attackers can exploit CWE-939 (Improper Authorization in Handler for Custom URL Scheme) to gain privileges or assume identity, varies by context, bypass protection mechanism. This weakness is typically introduced during the Implementation phase of software development.

How do I prevent CWE-939?

Key mitigations include: 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

What is the severity of CWE-939?

CWE-939 is classified as a Base-level weakness (Medium abstraction). It has been observed in 2 real-world CVEs.