Inside Flash Player Protected Mode for Firefox

Today, we launched Flash Player Protected Mode for Firefox on Windows. Our Protected Mode implementation allows Flash Player to run as a low integrity process with several additional restrictions that prohibit the runtime from accessing sensitive resources. This approach is based on David LeBlanc’s Practical Windows Sandbox design and builds upon what Adobe created for the Adobe Reader X sandbox. By running the Flash Player as a restricted process, Adobe is making it more difficult for an attacker to turn a simple bug into a working exploit. This blog post will provide an overview of the technical implementation of the design.

When you first navigate to a page with Flash (SWF) content, you will notice that there are now three processes for Flash Player. This may seem odd at first, but there is a good reason for this approach. One of the goals for the design was to minimize the number of changes that were necessary in Firefox to support the sandbox. By keeping Flash Player and Firefox loosely coupled, both organizations can make changes to their respective code without the complexity of coordinating releases.

The first process under the Firefox instance is called “plugin-container.exe.” Firefox has run plugins in this separate process for quite some time, and we did not want to re-architect that implementation. With this design, the plugin container itself is only a thin shim that allows us to proxy NPAPI requests to the browser. We also use this process as our launching point for creating the broker process. Forking the broker as a separate process allows us to be independent of the browser and gives us the freedom to restrict the broker process in the future. From the broker process, we will launch the fully sandboxed process. The sandboxed process has significant restrictions applied to it. It is within the sandbox process that the Flash Player engine consumes and renders Web content.

The restrictions we apply to this sandboxed process come from the Windows OS. Windows Vista and Windows 7 provide the tools necessary to properly sandbox a process. For the Adobe Reader and Acrobat sandbox implementation introduced in 2010, Adobe spent significant engineering effort trying to approximate those same controls on Windows XP. Today, with Windows 8 just around the corner and Windows XP usage rapidly decreasing, it did not make sense for the Flash Player team to make that same engineering investment for Windows XP. Therefore, we’ve focused on making Protected Mode for Firefox available on Windows Vista and later.

For those operating systems, we take advantage of three major classes of controls:

The first control is that we run the sandboxed process at low integrity. By default, processes started by the user are executed at medium integrity. Running the process at a low integrity level prevents the process from writing to most areas of the user’s local profile and the registry which require a medium integrity level to access. This also allows us to take advantage of User Interface Privilege Isolation (UIPI) which prevents low integrity processes from sending windows messages to higher integrity processes.

The second class of controls applied to the sandboxed process is to restrict the capabilities of the access token. A process will inherit a list of available Security Identifiers (SIDs) from the user’s security profile. These SIDs represent the different OS groups to which the user belongs. The access token contains this list of SIDs along with a set of controls for those SIDs. The Windows OS will compare the SIDs in the access tokens to the group permissions of the target object (e.g a file) to determine if access is allowed. The Windows OS allows us to define how the process SIDs are used in that comparison.

In general, a sandboxed process will need to be able to access resources directly owned by the user. However, in most cases it is unlikely that the sandbox will need the extended set of resources available to the user via group permissions. As a concrete example, your company may have a contact list on a network share that is available to everyone within your “Employees” group. It isn’t your file but you can access it because you are in the “Employees” group for your company. The Flash Player sandbox process doesn’t need to be able to directly access that file.

We identified that our sandbox process will need to access OS resources using the following SIDs: BUILTIN\Users, Everyone, the User’s Logon SID, and NTAUTHORITY\INTERACTIVE. For any other SIDs that are inherited from the user, we set the deny-only attribute to prohibit the process from accessing the resource based solely on that SID. To continue the example of the contact list on the file share, the sandboxed process would not be able to access the contact list because the file is not owned by you and the deny-only attribute on the “Employees” group SID would prevent access using your group permission. Process privileges are also limited to only the SeChangeNotifyPrivilege, which is required for the process to be notified of file system changes and for certain APIs to work correctly. The graphic below shows the permissions applied to the sandboxed process.

The third control applied to the sandboxed process are job restrictions. As one example, we can prevent the sandboxed process from launching other processes by setting Active Processes to 1. We can also limit the sandbox’s ability to communicate with other processes by restricting access to USER Handles and Administrator Access. The USER Handles restriction complements UIPI by preventing the process from accessing user handles created by processes not associated with our job. Finally, we can limit the sandboxes ability to interfere with the OS by limiting access to System Parameters, Display Settings, Exit Windows and Desktop.

More information on job limits, privilege restrictions and UIPI can be found in Part 2 of Inside Adobe Reader Protected Mode.

Once you get past OS-provided controls, the next layer of defense is Flash Player broker controls.

The OS broker process runs at medium integrity and acts as a gatekeeper between the untrusted sandbox process and the operating system. The sandbox process must ask the OS broker process for access to sensitive resources that it may legitimately need. Some examples of resources that are managed by the broker include file system access, camera access, print access and clipboard access. For each resource request, the sandbox contains policies which define what can and cannot be accessed. For instance, the sandbox process can request file system access through the broker. However, the policy within the broker will limit access to the file system so that the sandbox can only write to a predetermined, specific set of file system paths. This prevents the sandbox from writing to arbitrary locations on the file system. As another example, the sandbox cannot launch applications directly. If Flash Player needs to launch the native control panel, the Flash Player engine must forward the request to the broker process. The broker will then handle the details of safely launching the native control panel. Access to other OS resources such as the camera are similarly controlled by the broker. This architecture ensures that the sandboxed process cannot directly access most parts of the operating system without that access first being verified by the broker.

Overall, the Flash Player sandbox process has been a journey of incremental improvements with each step bringing end-users a more secure environment. We started by supporting Protected Mode within Internet Explorer, which enabled Flash Player to run as a low integrity process with limited write capabilities. From there, we worked with Google on building the Chrome sandbox, which converted Flash Player to using a more robust broker implementation. This release of Flash Player Protected Mode for Firefox on Windows takes the Chrome implementation one step further by changing Flash Player to run with job limits on the process. With Flash Player Protected Mode being based on the same technology as Adobe Reader X, we are confident that this implementation will be a significant barrier and help prevent exploits via Flash Player for Firefox users. Going forward, we plan to continue to build on this infrastructure with more sandbox projects, such as our upcoming release of Flash Player for Chrome Pepper. As we combine these efforts with other efforts, such as the background updater, we are making it increasingly more difficult for Flash Player to be targeted for malicious purposes.

Peleus Uhley, Platform Security Strategist, ASSET
Rajesh Gwalani, Security Engineering Manager, Flash Runtime