debugger: validate sec-websocket-accept response header

This addresses a TODO to validate that the sec-websocket-accept header
in the websocket handshake response is valid. To do this we need to
append the Websocket GUID to the original key sent in sec-websocket-key,
sha1 hash it, and then compare the base64 encoding with the value sent
in the sec-websocket-accept response header.

If they don't match, an error is thrown.

lib/internal/debugger/inspect_client.js

@@ -11,6 +11,7 @@ const {
 } = primordials;
 
 const Buffer = require('buffer').Buffer;
+const crypto = require('crypto');
 const { ERR_DEBUGGER_ERROR } = require('internal/errors').codes;
 const { EventEmitter } = require('events');
 const http = require('http');
@@ -35,13 +36,30 @@ const kTwoBytePayloadLengthField = 126;
 const kEightBytePayloadLengthField = 127;
 const kMaskingKeyWidthInBytes = 4;
 
+// This guid is defined in the Websocket Protocol RFC
+// https://tools.ietf.org/html/rfc6455#section-1.3
+const WEBSOCKET_HANDSHAKE_GUID = '258EAFA5-E914-47DA-95CA-C5AB0DC85B11';
+
 function unpackError({ code, message, data }) {
   const err = new ERR_DEBUGGER_ERROR(`${message} - ${data}`);
   err.code = code;
   ErrorCaptureStackTrace(err, unpackError);
   return err;
 }
 
+function validateHandshake(requestKey, responseKey) {
+  const expectedResponseKeyBase = requestKey + WEBSOCKET_HANDSHAKE_GUID;
+  const shasum = crypto.createHash('sha1');
+  shasum.update(expectedResponseKeyBase);
+  const shabuf = shasum.digest();
+
+  if (shabuf.toString('base64') !== responseKey) {
+    throw new ERR_DEBUGGER_ERROR(
+      `Websocket secret mismatch: ${requestKey} did not match ${responseKey}`
+    );
+  }
+}
+
 function encodeFrameHybi17(payload) {
   var i;
 
@@ -287,8 +305,8 @@ class Client extends EventEmitter {
   _connectWebsocket(urlPath) {
     this.reset();
 
-    const key1 = require('crypto').randomBytes(16).toString('base64');
-    debuglog('request websocket', key1);
+    const requestKey = crypto.randomBytes(16).toString('base64');
+    debuglog('request websocket', requestKey);
 
     const httpReq = this._http = http.request({
       host: this._host,
@@ -297,7 +315,7 @@ class Client extends EventEmitter {
       headers: {
         'Connection': 'Upgrade',
         'Upgrade': 'websocket',
-        'Sec-WebSocket-Key': key1,
+        'Sec-WebSocket-Key': requestKey,
         'Sec-WebSocket-Version': '13',
       },
     });
@@ -314,7 +332,7 @@ class Client extends EventEmitter {
     });
 
     const handshakeListener = (res, socket) => {
-      // TODO: we *could* validate res.headers[sec-websocket-accept]
+      validateHandshake(requestKey, res.headers['sec-websocket-accept']);
       debuglog('websocket upgrade');
 
       this._socket = socket;