Working securely with untrusted content



Content not assigned to the application sandbox can provide additional scripting functionality to your application, but only if it meets the security criteria of the runtime. This topic explains the AIR security contract with non-application content.

Security.allowDomain()

AIR applications restrict scripting access for non-application content more stringently than the Flash Player browser plug-in restricts scripting access for untrusted content. For example, in Flash Player in the browser, when a SWF file that is assigned to the local-trusted sandbox calls the System.allowDomain() method, scripting access is granted to any SWF loaded from the specified domain. The analogous approach is not permitted from application content in AIR applications, since it would grant unreasonable access unto the non-application file into the user's file system. Remote files cannot directly access the application sandbox, regardless of calls to the Security.allowDomain() method.

Scripting between application and non-application content

AIR applications that script between application and non-application content have more complex security arrangements. Files that are not in the application sandbox are only allowed to access the properties and methods of files in the application sandbox through the use of a sandbox bridge. A sandbox bridge acts as a gateway between application content and non-application content, providing explicit interaction between the two files. When used correctly, sandbox bridges provide an extra layer of security, restricting non-application content from accessing object references that are part of application content.

The benefit of sandbox bridges is best illustrated through example. Suppose an AIR music store application wants to provide an API to advertisers who want to create their own SWF files, with which the store application can then communicate. The store wants to provide advertisers with methods to look up artists and CDs from the store, but also wants to isolate some methods and properties from the third-party SWF file for security reasons.

A sandbox bridge can provide this functionality. By default, content loaded externally into an AIR application at runtime does not have access to any methods or properties in the main application. With a custom sandbox bridge implementation, a developer can provide services to the remote content without exposing these methods or properties. Consider the sandbox bridge as a pathway between trusted and untrusted content, providing communication between loader and loadee content without exposing object references.

For more information on how to securely use sandbox bridges, see Scripting between content in different domains.

Protection against dynamically generating unsafe SWF content

The Loader.loadBytes() method provides a way for an application to generate SWF content from a byte array. However, injection attacks on data loaded from remote sources could do severe damage when loading content. This is especially true when loading data into the application sandbox, where the generated SWF content can access the full set of AIR APIs.

There are legitimate uses for using the loadBytes() method without generating executable SWF code. You can use the loadBytes() method to generate an image data to control the timing of image display, for example. There are also legitimate uses that do rely on executing code, such as dynamic creation of SWF content for audio playback. In AIR, by default the loadBytes() method does not let you load SWF content; it only allows you to load image content. In AIR, the loaderContext property of the loadBytes() method has an allowLoadBytesCodeExecution property, which you can set to true to explicitly allow the application to use loadBytes() to load executable SWF content. The following code shows how to use this feature:

var loader:Loader = new Loader(); 
var loaderContext:LoaderContext = new LoaderContext(); 
loaderContext.allowLoadBytesCodeExecution = true; 
loader.loadBytes(bytes, loaderContext);

If you call loadBytes() to load SWF content and the allowLoadBytesCodeExecution property of the LoaderContext object is set to false (the default), the Loader object throws a SecurityError exception.

Note: In a future release of Adobe AIR, this API may change. When that occurs, you may need to recompile content that uses the allowLoadBytesCodeExecution property of the LoaderContext class.