Common Weakness Enumeration

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

Weakness ID: 939
Abstraction: Base
Structure: Simple
Status: Incomplete
Presentation Filter:
+ Description
The software 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.
+ Relationships

The table(s) below shows the weaknesses and high level categories that are related to this weakness. These relationships are defined as ChildOf, ParentOf, MemberOf and give insight to similar items that may exist at higher and lower levels of abstraction. In addition, relationships such as PeerOf and CanAlsoBe are defined to show similar weaknesses that the user may want to explore.

+ Relevant to the view "Research Concepts" (CWE-1000)
ChildOfClassClass - a weakness that is described in a very abstract fashion, typically independent of any specific language or technology. More specific than a Pillar Weakness, but more general than a Base Weakness. Class level weaknesses typically describe issues in terms of 1 or 2 of the following dimensions: behavior, property, and resource.862Missing Authorization
+ Relevant to the view "Software Development" (CWE-699)
MemberOfCategoryCategory - a CWE entry that contains a set of other entries that share a common characteristic.1212Authorization Errors
+ Relevant to the view "Architectural Concepts" (CWE-1008)
MemberOfCategoryCategory - a CWE entry that contains a set of other entries that share a common characteristic.1011Authorize Actors
+ Modes Of Introduction

The different Modes of Introduction provide information about how and when this weakness may be introduced. The Phase identifies a point in the life cycle at which introduction may occur, while the Note provides a typical scenario related to introduction during the given phase.

ImplementationREALIZATION: This weakness is caused during implementation of an architectural security tactic.
+ Applicable Platforms
The listings below show possible areas for which the given weakness could appear. These may be for specific named Languages, Operating Systems, Architectures, Paradigms, Technologies, or a class of such platforms. The platform is listed along with how frequently the given weakness appears for that instance.


Class: Mobile (Undetermined Prevalence)

+ Demonstrative Examples

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

(good code)
Example Language: Objective-C 
NSString *stringURL = @"appscheme://replaceFileText?file=incomingMessage.txt&text=hello";
NSURL *url = [NSURL URLWithString:stringURL];
[[UIApplication sharedApplication] openURL:url];

Application URL Handler

(bad code)

- (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 code)
Example Language: HTML 
<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:

(bad code)
Example Language: Java 
// Android
public boolean shouldOverrideUrlLoading(WebView view, String url){
if (url.substring(0,14).equalsIgnoreCase("examplescheme:")){
writeDataToView(view, UserData);
return false;
return true;
(bad code)
Example Language: 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"])

// 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 code)
Example Language: JavaScript 
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.

+ Observed Examples
URL scheme has action replace which requires no user prompt and allows remote attackers to perform undesired actions.
URL scheme has action follow and favorite which allows remote attackers to force user to perform undesired actions.
+ Potential Mitigations

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.

+ References
[REF-938] Guillaume Ross. "Scheming for Privacy and Security". 2013-11-11. <>.
+ Content History
+ Submissions
Submission DateSubmitterOrganization
2014-01-14CWE Content TeamMITRE
+ Modifications
Modification DateModifierOrganization
2017-01-19CWE Content TeamMITRE
updated Relationships
2017-11-08CWE Content TeamMITRE
updated Modes_of_Introduction, References, Relationships
2020-02-24CWE Content TeamMITRE
updated Applicable_Platforms, Relationships
2020-06-25CWE Content TeamMITRE
updated Potential_Mitigations
2021-03-15CWE Content TeamMITRE
updated Demonstrative_Examples
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Page Last Updated: March 15, 2021