Summary

A flaw in Tauri's is_local_url() function causes it to incorrectly classify remote URLs as trusted local origins on Windows and Android. On these systems, Tauri maps custom URI scheme protocols to http://<scheme>.localhost/ because those platforms' WebView implementations cannot serve custom URI schemes directly.

The issue is that Tauri's check to see if the origin is local, only checks the first subdomain of the URL. An attacker can abuse this by hosting a page on a domain whose subdomain matches the custom scheme of the application (e.g. http://app.attacker.com/)."

Example:

• Local URL: app://localhost/ → on Android/Windows: http://app.localhost/
• The check passes for any URL starting with http://app., including http://app.evil.com/

As a result, the attacker page can invoke backend commands that the developer intended to be accessible only to the app's own frontend and that are explicitly restricted from being called by external or remote origins.

Details

Vulnerable function:

#[cfg(any(windows, target_os = "android"))]
let local = {
  let protocol_url = self.manager().tauri_protocol_url(uses_https);
  let maybe_protocol = current_url
    .domain()
    .and_then(|d| d.split_once('.'))  // BUG: only splits on first dot
    .unwrap_or_default()
    .0;

  protocols.contains_key(maybe_protocol) && scheme == protocol_url.scheme()
};

Link: https://github.com/tauri-apps/tauri/blob/1ef6a119b1571d1da0acc08bdb7fd5521a4c6d52/crates/tauri/src/webview/mod.rs#L1680

split_once('.') discards everything after the first .. For http://app.evil.com/, the extracted label is app. If the application has registered a protocol named app, protocols.contains_key("app") returns true and the URL is classified as Origin::Local. The correct check must assert the full domain is exactly <protocol>.localhost.

PoC

We created a proof of concept app that can be found here. The app registers a custom app:// protocol and exposes a ping command restricted to local origins only. It provides a button to open a URL in a WebView, pre-filled with https://app.robbe-bc9.workers.dev/, an attacker-controlled page that invokes ping on load. Because the domain's first label matches the registered app protocol, is_local_url() classifies it as a local origin and the command succeeds.

capabilities/main.json contains the following code, which only exposes ping locally:

{
  "$schema": "../../../crates/tauri-schema-generator/schemas/capability.schema.json",
  "identifier": "main",
  "local": true,
  "windows": ["*"],
  "permissions": [
    "sample:allow-ping"
  ]
}

src/lib.rs contains the following code, to register a custom scheme:

tauri::Builder::default()
  .register_uri_scheme_protocol("app", |_ctx, _request| { ... })

Impact

The attacker page can invoke backend commands that the developer intended to be accessible only to the app's own frontend and that are explicitly restricted from being called by external or remote origins.

References

• GHSA-7gmj-67g7-phm9