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joplin/readme/dev/spec/note_viewer_isolation.md

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Note viewer isolation

The desktop application's note viewer runs in an iframe with a different protocol from the main application's top-level frame.

Rationale

Running the note viewer from a different protocol and/or domain moves the iframe's content to a separate process. This has at least two benefits:

  1. Reduces the impact of bugs in Joplin's HTML sanitizer.
  2. Improves performance when rendering large notes (see issue #10424).

Why does a different protocol improve performance/security?

Using a different domain and/or protocol for the note viewer does the following:

  1. enables Chromium's site isolation and
  2. prevents the note viewer from accessing the top-level context through window.top.

The protocol

We use Electron's protocol.handle to register the joplin-content:// protocol.

We use a file:// URL to load the base electron application.

We use a joplin-content://note-viewer/ URL to load the note viewer and the resources it contains.

For compatibility with renderer plugins that use absolute resource paths joplin-content://note-viewer/ fetches files relative to the / directory.

Here's an example:

  • The note viewer loads with the URL joplin-content://note-viewer/tmp/.mount_JoplinA7E0A/path/to/the/note/viewer/here.html
  • The note renders and includes a resource with the path /home/user/.config/joplin-desktop/path/here.css
  • /home/user/.config/joplin-desktop/path/here.css is converted to joplin-content://note-viewer/home/user/.config/joplin-desktop/path/here.css by Electron.
  • Joplin checks to make sure the joplin-content:// protocol has access to /home/user/.config/joplin-desktop/path/here.css. If it does, it fetches and returns the file.

joplin-content:// only has access to specific directories

When handleCustomProtocols creates a handler for the joplin-content:// protocol, it returns an object that allows certain directories to be marked as readable.

By default, the list of readable directories includes:

  • The app bundle
  • Data directories for each of the enabled plugins
  • The resource directory
  • The profile directory

Why not the sandbox property?

Enabling sandbox disables Electron's PDF viewer (related HTML spec change) and prevents content within the note viewer from displaying/requesting resources.