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.
Auswirkungen in der Praxis

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.

Wie Angreifer es ausnutzen

Angreiferpfad Schritt für Schritt

  1. 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. 2

    External Application

  3. 3

    Application URL Handler

  4. 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. 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.

Verwundbares Codebeispiel

Vulnerable Objective-C

Application URL Handler

Verwundbar 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;}
Angreifer-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.

Angreifer-Payload HTML 
<iframe src="appscheme://replaceFileText?file=Bookmarks.dat&text=listOfMaliciousWebsites">
Sicheres Codebeispiel

Secure Objective-C

External Application

Sicher 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.
Präventions-Checkliste

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.
Erkennungssignale

How to detect CWE-939

SAST High

Führe statische Analyse (SAST) auf der Codebasis aus und suche im Datenfluss nach dem unsicheren Muster.

DAST Moderate

Führe dynamische Application-Security-Tests gegen den Live-Endpoint aus.

Runtime Moderate

Beobachte Runtime-Logs auf ungewöhnliche Exception-Traces, fehlerhafte Eingaben oder Versuche, Autorisierung zu umgehen.

Code review Moderate

Code Review: Markiere jeden neuen Code, der Eingaben von dieser Oberfläche ohne validierte Framework-Helper verarbeitet.

Plexicus Auto-Fix

Plexicus erkennt CWE-939 automatisch und öffnet in unter 60 Sekunden einen Fix-PR.

Codex Remedium scannt jeden Commit, identifiziert genau diese Schwachstelle und liefert einen reviewer-ready Pull Request mit dem Patch. Keine Tickets. Keine Hand-offs.

Demo anfordern Plexicus kostenlos testen
Häufig gestellte Fragen

Frequently asked questions

Was ist 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.

Wie gravierend ist CWE-939?

MITRE hat für diese Schwachstelle keine Exploit-Wahrscheinlichkeit veröffentlicht. Behandle sie als mittlere Auswirkung, bis dein Threat Model anderes belegt.

Welche Sprachen oder Plattformen sind von CWE-939 betroffen?

MITRE lists the following affected platforms: Mobile.

Wie kann ich CWE-939 verhindern?

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.

Wie erkennt und behebt Plexicus CWE-939?

Die SAST-Engine von Plexicus erkennt die Datenfluss-Signatur von CWE-939 bei jedem Commit. Bei einem Treffer öffnet unser Codex-Remedium-Agent einen Fix-PR mit korrigiertem Code, Tests und einer einzeiligen Zusammenfassung für den Reviewer.

Wo erfahre ich mehr über CWE-939?

MITRE veröffentlicht die kanonische Definition unter https://cwe.mitre.org/data/definitions/939.html. Für ergänzende Hinweise kannst du auch die OWASP- und NIST-Dokumentation heranziehen.

Verwandte Schwachstellen

Weaknesses related to CWE-939

CWE-862 Parent

Missing Authorization

This vulnerability occurs when an application fails to verify whether a user has permission to access specific data or execute certain…

CWE-1314 Sibling

Missing Write Protection for Parametric Data Values

This vulnerability occurs when a hardware device fails to protect the scaling parameters used to convert raw sensor readings. Untrusted…

CWE-425 Sibling

Direct Request ('Forced Browsing')

This vulnerability occurs when a web application fails to verify user permissions for every protected page, file, or API endpoint,…

CWE-638 Sibling

Not Using Complete Mediation

This vulnerability occurs when software fails to verify access permissions every single time a user or process tries to use a resource.…

Ressourcen

Further reading

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