Preventing Add-ons & Third-party Software From Loading DLLs Into Firefox

Loading a DLL or binary component into the Firefox process is a method employed by third-party software to enable low-level interactions between Firefox and the operating environment and/or applications that run within it. Binaries can be loaded via an add-on using JS-ctypes, or injected directly into the process using other methods to enable functionality not available natively. These techniques, however useful to the developer, do not always account for underlying changes to Firefox, and are frequently the root cause of startup crashes for users running a new version of Firefox for the first time.

Over the past year, these startup crashes have resulted in the delay or revision of four Firefox releases. They erode confidence in Firefox, and can render Firefox and the information it contains unusable. Users of Firefox need the confidence that their browser won’t crash on an update, and that they’ll have a positive experience using the newest and most secure version available. Firefox release managers need the confidence that they can release new versions of Firefox without worrying about crashes brought on by third-party software.

With the introduction of the Native Messaging API in WebExtensions in Firefox 50—released on November 8, 2016—extensions are able to communicate directly with a host executable running in a separate process. These executables are installed separately, and provide low-level interactions outside of the Firefox process without the possibility of crashing it. The use of Native Messaging with extensions is the supported method for third-party developers to perform interactions that are not available natively, and other methods will be actively discouraged.

Starting with Firefox 53, to be released on April 18, Mozilla will introduce changes designed to prevent the loading of third-party binaries into the Firefox process(es) that include:

  • Updating our add-on policies and enforcement methods to reject add-ons that load any binaries using JS-ctypes or other methods, and applying them to new and existing add-ons.
  • Product changes to better protect Firefox from DLL injection
  • Blocklisting software that loads or attempts to load DLLs into the Firefox process(es) using any method

These changes will also prepare us for wider adoption of the 64-bit version of Firefox on Windows in the near future, as some existing DLLs that are injected or loaded will not be compatible.

Add-on developers who are currently using JS-ctypes should begin immediately transitioning to the Native Messaging API using WebExtensions. Documentation and examples for Native Messaging are available on MDN, and you can ask questions or share concerns about these changes in these communication channels.

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