Inspecting WebSocket frames in an undetectable way

Intercepting the WebSocket data is easy. Simply execute the following script before the page constructs the WebSocket. This snippet monkey-patches the WebSocket constructor: When a new WebSocket constructor is created, the snippet subscribes to the message event, from where you can do whatever you want with the data.

This snippet is designed to be indistinguishable from native code so the modification cannot easily be detected by the page (however, see the remarks at the end of this post).

(function() {
    var OrigWebSocket = window.WebSocket;
    var callWebSocket = OrigWebSocket.apply.bind(OrigWebSocket);
    var wsAddListener = OrigWebSocket.prototype.addEventListener;
    wsAddListener = wsAddListener.call.bind(wsAddListener);
    window.WebSocket = function WebSocket(url, protocols) {
        var ws;
        if (!(this instanceof WebSocket)) {
            // Called without 'new' (browsers will throw an error).
            ws = callWebSocket(this, arguments);
        } else if (arguments.length === 1) {
            ws = new OrigWebSocket(url);
        } else if (arguments.length >= 2) {
            ws = new OrigWebSocket(url, protocols);
        } else { // No arguments (browsers will throw an error)
            ws = new OrigWebSocket();
        }

        wsAddListener(ws, 'message', function(event) {
            // TODO: Do something with event.data (received data) if you wish.
        });
        return ws;
    }.bind();
    window.WebSocket.prototype = OrigWebSocket.prototype;
    window.WebSocket.prototype.constructor = window.WebSocket;

    var wsSend = OrigWebSocket.prototype.send;
    wsSend = wsSend.apply.bind(wsSend);
    OrigWebSocket.prototype.send = function(data) {
        // TODO: Do something with the sent data if you wish.
        return wsSend(this, arguments);
    };
})();

In a Chrome extension, the snippet can be run via a content script with run_at:'document_start', see Insert code into the page context using a content script.

Firefox also supports content scripts, the same logic applies (with contentScriptWhen:'start').

Note: The previous snippet is designed to be indistinguishable from native code when executed before the rest of the page. The only (unusual and fragile) ways to detect these modifications are:

• Pass invalid parameters to the WebSocket constructor, catch the error and inspecting the implementation-dependent (browser-specific) stack trace. If there is one more stack frame than usual, then the constructor might be tampered (seen from the page’s perspective).

• Serialize the constructor. Unmodified constructors become function WebSocket() { [native code] }, whereas a patched constructor looks like function () { [native code] } (this issue is only present in Chrome; in Firefox, the serialization is identical).

• Serialize the WebSocket.prototype.send method. Since the function is not bound, serializing it (WebSocket.prototype.send.toString()) reveals the non-native implementation. This could be mitigated by overriding the .toString method of .send, which in turn can be detected by the page by a strict comparison with Function.prototype.toString. If you don’t need the sent data, do not override OrigWebSocket.prototype.send.