Unlock JavaScript Reverse Engineering with JsRpc: A Step‑by‑Step Guide

Introduction

JsRpc is a tool that opens a WebSocket connection between a browser and a Go server, allowing the browser to execute predefined JavaScript code via HTTP‑style API calls. It can be used for JS reverse‑engineering, calling encrypted functions, or directly fetching data.

Code Structure

-- main.go (server main code)
-- resources/JsEnv.js (client‑side JS injection)

Basic Usage

Run the server program and the JS script; they communicate through the WebSocket, enabling you to invoke interfaces that execute JS and return the desired values (e.g., encryption/decryption).

Implementation Details

The browser creates a WebSocket client that connects to the server. The server sends a request, the client runs the specified JS method, and returns the result back to the server, which then displays it. The method works over HTTPS and supports wss.

HTTPS / TLS Notes

If the site uses HTTPS, a plain ws connection may cause security errors. Using wss or a local IP (127.0.0.1) avoids the issue. Without a certificate, compile main.go and connect to ws on port 12080. With a certificate, enable RunTls in main.go and use port 12443.

Running the Pre‑built Binary

Download the compiled binary from the GitHub releases page and double‑click to start the service.

API Overview

/list

: view currently connected WebSocket services /ws: endpoint for the browser to inject the WebSocket connection /result: retrieve data in JSON format, e.g.

{"group":"hhh","hello":"好困啊yes","name":"baidu","status":"200"}

Calls use ?group and ?name query parameters, for example:

ws://127.0.0.1:12080/ws?group={}&name={}
http://127.0.0.1:12080/go?group={}&name={}&action={}¶m={}

Injecting JS and Building the Communication Environment

Paste the contents of JsEnv.js into the browser console (breakpoints may be needed).

Registering a Method

// Connect
var demo = new Hlclient("ws://127.0.0.1:12080/ws?group=hhh&name=baidu");
// Register method "hello"
demo.regAction("hello", function (resolve, param) {
    var c = "好困啊" + param;
    resolve(c);
});

Accessing the Interface

http://127.0.0.1:12080/go?group=hhh&name=baidu&action=hello¶m=yes
// Response
{
  "group":"hhh",
  "hello":"好困啊yes",
  "name":"baidu",
  "status":"200"
}

Conclusion

JsRpc enables a browser‑to‑Go WebSocket bridge that can execute arbitrary JavaScript and return results, which is useful for reverse engineering, crawling, or automating encrypted functions.