CWE-939 Base Incompleto

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.

Definición

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.
Impacto en el mundo real

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.

Cómo lo explotan los atacantes

Ruta del atacante paso a paso

  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.

Ejemplo de código vulnerable

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;}
Payload del atacante

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.

Payload del atacante HTML 
<iframe src="appscheme://replaceFileText?file=Bookmarks.dat&text=listOfMaliciousWebsites">
Ejemplo de código seguro

Secure Objective-C

External Application

Seguro 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.
Lista de prevención

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.
Señales de detección

How to detect CWE-939

SAST High

Ejecuta análisis estático (SAST) sobre el código buscando el patrón inseguro en el flujo de datos.

DAST Moderate

Ejecuta pruebas dinámicas de seguridad de aplicaciones (DAST) contra el endpoint en vivo.

Runtime Moderate

Vigila los logs en tiempo de ejecución para detectar trazas de excepción inusuales, entradas malformadas o intentos de bypass de autorización.

Code review Moderate

Revisión de código: marca cualquier código nuevo que maneje entrada desde esta superficie sin usar los helpers validados del framework.

Auto-corrección de Plexicus

Plexicus detecta automáticamente CWE-939 y abre un PR de corrección en menos de 60 segundos.

Codex Remedium escanea cada commit, identifica esta debilidad concreta y entrega un pull request listo para revisión con el parche. Sin tickets. Sin traspasos.

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Preguntas frecuentes

Frequently asked questions

¿Qué es 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.

¿Qué gravedad tiene CWE-939?

MITRE no ha publicado una calificación de probabilidad de explotación para esta debilidad. Trátala como de impacto medio hasta que tu modelo de amenazas demuestre lo contrario.

¿Qué lenguajes o plataformas se ven afectados por CWE-939?

MITRE lists the following affected platforms: Mobile.

¿Cómo puedo prevenir 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.

¿Cómo detecta y corrige Plexicus CWE-939?

El motor SAST de Plexicus detecta la firma de flujo de datos para CWE-939 en cada commit. Cuando hay coincidencia, nuestro agente Codex Remedium abre un PR de corrección con el código corregido, las pruebas y un resumen de una línea para el revisor.

¿Dónde puedo aprender más sobre CWE-939?

MITRE publica la definición canónica en https://cwe.mitre.org/data/definitions/939.html. También puedes consultar la documentación de OWASP y NIST para guías relacionadas.

Debilidades relacionadas

Weaknesses related to CWE-939

CWE-862 Padre

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CWE-1314 Hermano

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CWE-425 Hermano

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 Hermano

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.…

Recursos

Further reading

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