Heresy is a simple experimental utility that allows you to inspect and instrument React Native applications at runtime.

It utilizes frida to inject javascript into the Hermes Engine global context, and a small suite of helper scripts to make this easier.

Blog Post + Tutorial: https://pilfer.github.io/mobile-reverse-engineering/react-native/heresy-inspect-and-instrument-react-native-apps/.

It currently only supports Android - if anyone wants to PR a CatalystImpl frida snippet for iOS then I'll happily accept it!

Disclaimer: This specific repository will only function as a proof-of-concept, as I'll be rewriting it into something more robust.

To get started with Heresy, simply rename the .heresy.example directory to .heresy and change package_name in the JSON file to the package name of your target app.

The only other configuration value you'll need to change is the rpc URL.

{
  "package_name": "com.your.app",
  "rpc_port": 1337,
  "hermes_before": ".heresy/_before.js",
  "hermes_hook": ".heresy/_hook.js",
  "heresy_config": {
    "http": false,
    "react_native_elements": false,
    "rpc": "wss://<your_serveo_subdomain>.serveo.net"
  }
}

If you need a quick tls tunnel (a requirement for properly configured RN apps), Serveo works just fine:

ssh -R 80:localhost:1337 serveo.net

From here, simply run the following:

npm install && npm run build

Assuming everything is configured, we can now start the app!

❯ npm run start

You'll be greeted by the following output and be given a shiny little JS REPL to start hacking on your app with!

> heresy@1.0.0 start
> node dist/app/index.js

[*] Starting Heresy...
[*] Waiting 1s before resuming app...
[*] hermes_before was loaded!
[*] hermes_hook was loaded!
[*] Hermes hook was loaded successfully
[*] Got connection from RN client!
[*] Waiting for user input - start typing whenever!

Note: You can modify .heresy/_hook.js and reload the app without needing to run npm rebuild. Any TypeScript file you modify will require a rebuild.

View Hermes runtime logs with:

adb logcat "*:S" ReactNativeJS:I -v raw

Root Config

Heresy Config

If your app doesn't have a files directory under /data/data/{package_name}/ directory, then make one - otherwise this tool won't work.

The Hermes agent consists of a collection of scripts that introduce novel functionality.

The Hermes VM gives us the usual stuff we might expect out of a JavaScript engine:

• https://github.com/facebook/hermes/blob/main/include/hermes/VM/CellKinds.def
• https://github.com/facebook/hermes/blob/main/include/hermes/VM/PredefinedStrings.def

An example .heresy/_hook.js file might look like:

/**
 * If you want to get typehints, you can use JSDoc comments like so:
 * 
 * @typedef {import('../src/hermes_agent/heresy').Heresy} Heresy
 * @typedef {import('../src/hermes_agent/heresy').HeresyEvent} HeresyEvent
 */

/** @type {Heresy} */
let h = globalThis.heresy

// Log full HTTP requests and responses
h.http_callback = (event) => {
  let req = event.request;
  let res = event.response;
  
  if (req.method === 'POST') {
    console.log('POST request was logged by heresy');

    Object.keys(req.headers).forEach((key) => {
      console.log(key + ': ' + req.headers[key]);
    });
    
    console.log('The response status code: ' + res.status);
    console.log('Body=', res.body);
  }
};

// Show an alert!
h.alert('Hello, world!')

// Log the contents of this.process, which contains NODE_ENV
h.dump_env();

Heresy does its best to make use of these goodies with the following:

The RPC system is just a simple typed WebSocket channel.

The WebSocket server has to be secure, otherwise the Hermes VM will throw an error. Set up a local tunnel to get around this, or simply use Caddy or nginx or similar.

When you run this tool and the rpc server is configured, you'll get a free REPL that will eval() anything you put in it and return the result.

Included are a few commands:

This functionality is made possible by hooking the Map.set method, which is used for the RN renderer under the hood.

The entire JSX Element stack ends up being utilized here, so with a little bit of massaging we can actually inspect, hook, and modify the state and properties of applications.

PoC

let origSet = Map.prototype.set;
Map.prototype.set = function(key, value) {

  // Find the element value you want to modify.
  // This is just an example - you will almost certainly need to print the whole
  // data structure a few times to find what you're looking for.
  if (key === 'RCTView' || value.return.return.return.type.displayName === 'View') {

    const headerProps = value.return.return.return.; // placeholder
    
    // The updater pushes changes to the Fiber renderer
    const updater = headerProps.stateNode.updater;
    
    // Get child elements
    const children = headerProps.stateNode.props.children;
    
    // Grab the elements we want to play with...
    let view = children[1];
    let blink = view.props.children[0];
    let button = view.props.children[1];
    
    // Modify the props of the elements
    button.props.title = 'Hello from heresy!';
    view.props.backgroundColor = '#2d2d2d';
    button.props.backgroundColor = '#ff0000';
    blink.props.text = 'Some new text value :)';
    
    updater.enqueueForceUpdate(headerProps.stateNode);
  }


  return origSet.call(this, key, value);
};

If you run into any problems or have something cool you'd like to share, please feel free to open an issue.