Normandy is a system that allows Mozilla to instruct Firefox clients to perform a variety of actions to aid the user and gather feedback to guide Mozilla in developing a better browser for them. To do this, we need a way to instruct Firefox to perform certain actions without having to download an update or wait for a new release.

The system is made of two main components, a server and client. These components are known simply as the Normandy client and Normandy server. The server provides information that the client acts on.

Normandy’s execution is built around two main concepts: Actions and Recipes. This document describes those concepts in detail.


An action is one of the capabilities provided by Normandy. As of Firefox 66, actions are implemented entirely within the browser as native code (though previously they were served by the server and retrieved at runtime). Actions are identified in recipes by name, such as “preference-experiment” or “show-heartbeat”.

Actions can accept a configuration object that is specified within the Normandy control interface. This configuration, which is part of a recipe, contains data required by the action, such as translated strings or boolean options for controlling behavior.

For example, a “survey” action could consist of code within Firefox to check whether we have shown a survey to the user recently, to determine which variant of a survey to show, and the code to actually display the survey itself. When issued to clients, this action is paired with configuration for the survey action would contain the text of the survey prompt and the URL to redirect the user to after answering.


A recipe is a model that represents a single instance of an action we want to perform. Recipes contain three important pieces of information:

  1. Client filtering data that determines which users the recipe should be run for. This takes the form of a JEXL expression that is evaluated on the client and has access to several pieces of data about the client, such as the data collected by Telemetry.

  2. The name of the action that this recipe should perform when run.

  3. The configuration data to pass to the Action code when it is executed, called the recipe’s arguments.

Recipes are stored in the Normandy server’s database and the Normandy client fetches them from the service during execution. All active recipes are sent to every client, and filtering is done locally on the client.

Here’s an example of a recipe in JSON format, as fetched by the Normandy client:

   "id": 1,
   "name": "Console Log Test",
   "revision_id": 12,
   "action": "console-log",
   "arguments": {
      "message": "It works!"
   "filter_expression": "normandy.version >= '65.0' && stableSample(0.5)"


The data retrieved by the Normandy client is the minimum subset of data that is needed execute the action. Other API end points return more information about the recipe, including it’s history and approval status.

Normandy Client

The Normandy client is a Firefox component that reads and executes instructions from the server. Upon activation (typically every few hours), the client:

  1. Downloads recipes from the Normandy service.

  2. Verifies the signature of the recipes.

  3. Evaluates the recipe filters and removes recipes that do not match the client.

  4. Executes the corresponding action code using the recipe’s arguments.

Security Considerations

Since the goal of SHIELD is to allow Mozilla to perform certain privileged actions quickly without shipping full updates to Firefox, it is a tempting target for compromising Firefox users. Normandy includes several security controls to help mitigate this risk.

Action and Recipe Signing

Recipes that are downloaded by the client are signed according to the Content-Signature protocol as provided by the Autograph service. The client verifies the signature upon downloading the recipes, ensuring that only recipes that have been signed with a Mozilla-controlled key are executed.

This helps prevent Man-in-the-Middle attacks where an adversary pretends to be the remote Normandy service.

Peer Approval

Recipes cannot be enabled in the Normandy admin interface without going through an approval process </user/peer_approval>. One user must submit the recipe for approval, and a separate user must approve the recipe before it can be distributed by the service.

This helps prevent compromise of a single account from compromising the entire service, since two accounts need to be compromised to publish a recipe.

Safe Instructions

Recipe data is provided in either a non-executable format (JSON) or a safe executable format designed specifically to be weak and without enough access to exfiltrate data (JEXL filters). In technical terms, the JEXL filters are not turing complete, by design. This means that even if an attacker could run a malicious recipe in Firefox, they are limited to the actions implemented in the existing client.

Configurable Admin

The admin interface for Normandy can be disabled via a Django setting, which allows for disabling the admin interface on public-facing web servers and running them with read-only privileges. The writable admin interface is then deployed behind a VPN to restrict access to authorized users.