Security aspects of the Remote Agent

The Remote Agent is not a web-facing feature and as such has different security characteristics than traditional web platform APIs. The primary consumers are out-of-process programs that connect to the agent via a remote protocol, but can theoretically be extended to facilitate browser-local clients communicating over IPDL.

Design considerations

The Remote Agent allows consumers to interface with Firefox through an assorted set of domains for inspecting the state and controlling execution of documents running in web content, injecting arbitrary scripts to documents, do browser service instrumentation, simulation of user interaction for automation purposes, and for subscribing to updates in the browser such as network- and console logs.

The remote interfaces are served over an HTTP wire protocol, by a server listener hosted in the Firefox binary. This can only be started by passing the --remote-debugging-port flag. Connections are by default restricted to loopback devices (such as localhost and, but this can be overridden with the remote.force-local preference.

Since the Remote Agent is not an in-document web feature, the security concerns we have for this feature are essentially different to other web platform features. The primary concern is that the HTTPD is not spun up without passing one of the command-line flags. It is out perception that if a malicious user has the capability to execute arbitrary shell commands, there is little we can do to prevent the browser being turned into an evil listening device.

User privacy concerns

There are no user privacy concerns beyond the fact that the offered interfaces will give the client access to all browser internals, and thereby follows all browser-internal secrets.

How the Remote Agent works

When the --remote-debugging-port flag is used, it spins up an HTTPD on the desired port, or defaults to localhost:9222. The HTTPD serves WebSocket connections via nsIWebSocket.createServerWebSocket that clients connect to in order to give the agent remote instructions.

The remote.force-local preference controls whether the HTTPD accepts connections from non-loopback clients. System-local loopback connections are the default:

    if (Preferences.get(FORCE_LOCAL) && !LOOPBACKS.includes(host)) {
      throw new Error("Restricted to loopback devices");

The Remote Agent implements a large subset of the Chrome DevTools Protocol (CDP). This protocol allows a client to:

  • take control over the user session for automation purposes, for example to simulate user interaction such as clicking and typing;

  • instrument the browser for analytical reasons, such as intercepting network traffic;

  • and extract information from the user session, including cookies and local strage.

There are no web-exposed features in the Remote Agent whatsoever.

Security model

It shares the same security model as DevTools and Marionette, in that there is no other mechanism for enabling the Remote Agent than by passing a command-line flag.

It is our assumption that if an attacker has shell access to the user account, there is little we can do to prevent secrets from being accessed or leaked.

The Remote Agent is available on all release channels. The security review was completed in November 2019.