CWE-939 Base Incomplete

Improper Authorization in Handler for Custom URL Scheme

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.

Definition

What is CWE-939?

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.

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.

Real-world impact

Real-world CVEs caused by CWE-939

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.

How attackers exploit it

Step-by-step attacker path

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.
2
External Application
3
Application URL Handler
4
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.
5
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.

Vulnerable code example

Vulnerable Objective-C

Application URL Handler

Vulnerable Objective-C

- (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;}

Attacker payload

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.

Attacker payload HTML

<iframe src="appscheme://replaceFileText?file=Bookmarks.dat&text=listOfMaliciousWebsites">

Secure code example

Secure Objective-C

External Application

Secure Objective-C

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

What changed: the unsafe sink is replaced (or the input is validated/escaped) so the same payload no longer triggers the weakness.

Prevention checklist

How to prevent CWE-939

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 signals

How to detect CWE-939

SAST High

Run static analysis (SAST) on the codebase looking for the unsafe pattern in the data flow.

DAST Moderate

Run dynamic application security testing against the live endpoint.

Runtime Moderate

Watch runtime logs for unusual exception traces, malformed input, or authorization bypass attempts.

Code review Moderate

Code review: flag any new code that handles input from this surface without using the validated framework helpers.

CWE-939

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Frequently asked questions

What is CWE-939?

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.

How serious is CWE-939?

MITRE has not published a likelihood-of-exploit rating for this weakness. Treat it as medium-impact until your threat model proves otherwise.

What languages or platforms are affected by CWE-939?

MITRE lists the following affected platforms: Mobile.

How can I prevent CWE-939?

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.

How does Plexicus detect and fix CWE-939?

Plexicus's SAST engine matches the data-flow signature for CWE-939 on every commit. When a match is found, our Codex Remedium agent opens a fix PR with the corrected code, tests, and a one-line summary for the reviewer.

Where can I learn more about CWE-939?

MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/939.html. You can also reference OWASP and NIST documentation for adjacent guidance.

Related weaknesses

Weaknesses related to CWE-939

CWE-862 Parent

Improper Authorization in Handler for Custom URL Scheme

What is CWE-939?

Real-world CVEs caused by CWE-939

Step-by-step attacker path

Vulnerable Objective-C

Secure Objective-C

How to prevent CWE-939

How to detect CWE-939

Don't catalog this weakness. Prove it's reachable.

Frequently asked questions

Weaknesses related to CWE-939

Missing Authorization

Missing Write Protection for Parametric Data Values

Direct Request ('Forced Browsing')

Not Using Complete Mediation

Further reading

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Sample evidence handover

Server-Side Request Forgery in `webhooks/receiver`