diff --git a/spec/std/http/server/handlers/websocket_handler_spec.cr b/spec/std/http/server/handlers/websocket_handler_spec.cr
index db226502ce36..dc7e852e936e 100644
--- a/spec/std/http/server/handlers/websocket_handler_spec.cr
+++ b/spec/std/http/server/handlers/websocket_handler_spec.cr
@@ -22,9 +22,10 @@ describe HTTP::WebSocketHandler do
     io = IO::Memory.new
 
     headers = HTTP::Headers{
-      "Upgrade"           => "WS",
-      "Connection"        => "Upgrade",
-      "Sec-WebSocket-Key" => "dGhlIHNhbXBsZSBub25jZQ==",
+      "Upgrade"               => "WS",
+      "Connection"            => "Upgrade",
+      "Sec-WebSocket-Key"     => "dGhlIHNhbXBsZSBub25jZQ==",
+      "Sec-WebSocket-Version" => "13",
     }
     request = HTTP::Request.new("GET", "/", headers: headers)
     response = HTTP::Server::Response.new(io)
@@ -47,6 +48,7 @@ describe HTTP::WebSocketHandler do
         "Upgrade" =>           "websocket",
         "Connection" =>        {{connection}},
         "Sec-WebSocket-Key" => "dGhlIHNhbXBsZSBub25jZQ==",
+        "Sec-WebSocket-Version" => "13",
       }
       request = HTTP::Request.new("GET", "/", headers: headers)
       response = HTTP::Server::Response.new(io)
@@ -63,16 +65,17 @@ describe HTTP::WebSocketHandler do
 
       response.close
 
-      io.to_s.should eq("HTTP/1.1 101 Switching Protocols\r\nUpgrade: websocket\r\nConnection: Upgrade\r\nSec-Websocket-Accept: s3pPLMBiTxaQ9kYGzzhZRbK+xOo=\r\n\r\n")
+      io.to_s.should eq("HTTP/1.1 101 Switching Protocols\r\nUpgrade: websocket\r\nConnection: Upgrade\r\nSec-WebSocket-Accept: s3pPLMBiTxaQ9kYGzzhZRbK+xOo=\r\n\r\n")
     end
   {% end %}
 
   it "gives upgrade response for case-insensitive 'WebSocket' upgrade request" do
     io = IO::Memory.new
     headers = HTTP::Headers{
-      "Upgrade"           => "WebSocket",
-      "Connection"        => "Upgrade",
-      "Sec-WebSocket-Key" => "dGhlIHNhbXBsZSBub25jZQ==",
+      "Upgrade"               => "WebSocket",
+      "Connection"            => "Upgrade",
+      "Sec-WebSocket-Key"     => "dGhlIHNhbXBsZSBub25jZQ==",
+      "Sec-WebSocket-Version" => "13",
     }
     request = HTTP::Request.new("GET", "/", headers: headers)
     response = HTTP::Server::Response.new(io)
@@ -89,6 +92,67 @@ describe HTTP::WebSocketHandler do
 
     response.close
 
-    io.to_s.should eq("HTTP/1.1 101 Switching Protocols\r\nUpgrade: websocket\r\nConnection: Upgrade\r\nSec-Websocket-Accept: s3pPLMBiTxaQ9kYGzzhZRbK+xOo=\r\n\r\n")
+    io.to_s.should eq("HTTP/1.1 101 Switching Protocols\r\nUpgrade: websocket\r\nConnection: Upgrade\r\nSec-WebSocket-Accept: s3pPLMBiTxaQ9kYGzzhZRbK+xOo=\r\n\r\n")
+  end
+
+  it "returns bad request if Sec-WebSocket-Key is missing" do
+    io = IO::Memory.new
+
+    headers = HTTP::Headers{
+      "Upgrade"               => "websocket",
+      "Connection"            => "Upgrade",
+      "Sec-WebSocket-Version" => "13",
+    }
+    request = HTTP::Request.new("GET", "/", headers: headers)
+    response = HTTP::Server::Response.new(io)
+    context = HTTP::Server::Context.new(request, response)
+
+    handler = HTTP::WebSocketHandler.new { }
+    handler.call context
+
+    response.close
+
+    io.to_s.should eq("HTTP/1.1 400 Bad Request\r\nContent-Length: 0\r\n\r\n")
+  end
+
+  it "returns upgrade required if Sec-WebSocket-Version is missing" do
+    io = IO::Memory.new
+
+    headers = HTTP::Headers{
+      "Upgrade"           => "websocket",
+      "Connection"        => "Upgrade",
+      "Sec-WebSocket-Key" => "dGhlIHNhbXBsZSBub25jZQ==",
+    }
+    request = HTTP::Request.new("GET", "/", headers: headers)
+    response = HTTP::Server::Response.new(io)
+    context = HTTP::Server::Context.new(request, response)
+
+    handler = HTTP::WebSocketHandler.new { }
+    handler.call context
+
+    response.close
+
+    io.to_s.should eq("HTTP/1.1 426 Upgrade Required\r\nSec-WebSocket-Version: 13\r\nContent-Length: 0\r\n\r\n")
+  end
+
+  it "returns upgrade required if Sec-WebSocket-Version is invalid" do
+    io = IO::Memory.new
+
+    headers = HTTP::Headers{
+      "Upgrade"               => "websocket",
+      "Connection"            => "Upgrade",
+      "Sec-WebSocket-Key"     => "dGhlIHNhbXBsZSBub25jZQ==",
+      "Sec-WebSocket-Version" => "12",
+    }
+    request = HTTP::Request.new("GET", "/", headers: headers)
+    response = HTTP::Server::Response.new(io)
+    context = HTTP::Server::Context.new(request, response)
+
+    handler = HTTP::WebSocketHandler.new { }
+    handler.call context
+
+    response.close
+
+    io.to_s.should eq("HTTP/1.1 426 Upgrade Required\r\nSec-WebSocket-Version: 13\r\nContent-Length: 0\r\n\r\n")
   end
 end
diff --git a/spec/std/http/web_socket_spec.cr b/spec/std/http/web_socket_spec.cr
index 81cd6a460970..06209567448c 100644
--- a/spec/std/http/web_socket_spec.cr
+++ b/spec/std/http/web_socket_spec.cr
@@ -374,6 +374,61 @@ describe HTTP::WebSocket do
     ws2.run
   end
 
+  it "handshake fails if server does not switch protocols" do
+    port_chan = Channel(Int32).new
+    spawn do
+      http_ref = nil
+      http_server = http_ref = HTTP::Server.new(0) do |context|
+        context.response.status_code = 200
+      end
+
+      http_server.bind
+      port_chan.send(http_server.port)
+      http_server.listen
+
+      http_ref.not_nil!.close
+    end
+
+    listen_port = port_chan.receive
+
+    expect_raises(Socket::Error, "Handshake got denied. Status code was 200.") do
+      HTTP::WebSocket::Protocol.new("127.0.0.1", port: listen_port, path: "/")
+    end
+  end
+
+  it "handshake fails if server does not verify Sec-WebSocket-Key" do
+    port_chan = Channel(Int32).new
+    spawn do
+      http_ref = nil
+      has_been_called = false
+
+      http_server = http_ref = HTTP::Server.new(0) do |context|
+        response = context.response
+        response.status_code = 101
+        response.headers["Upgrade"] = "websocket"
+        response.headers["Connection"] = "Upgrade"
+        if has_been_called
+          response.headers["Sec-WebSocket-Accept"] = "foobar"
+          http_ref.not_nil!.close
+        else
+          has_been_called = true
+        end
+      end
+
+      http_server.bind
+      port_chan.send(http_server.port)
+      http_server.listen
+    end
+
+    listen_port = port_chan.receive
+
+    2.times do
+      expect_raises(Socket::Error, "Handshake got denied. Server did not verify WebSocket challenge.") do
+        HTTP::WebSocket::Protocol.new("127.0.0.1", port: listen_port, path: "/")
+      end
+    end
+  end
+
   typeof(HTTP::WebSocket.new(URI.parse("ws://localhost")))
   typeof(HTTP::WebSocket.new("localhost", "/"))
   typeof(HTTP::WebSocket.new("ws://localhost"))
diff --git a/src/http/server/handlers/websocket_handler.cr b/src/http/server/handlers/websocket_handler.cr
index 3d7ebc58b948..0e25ddd6a516 100644
--- a/src/http/server/handlers/websocket_handler.cr
+++ b/src/http/server/handlers/websocket_handler.cr
@@ -15,20 +15,28 @@ class HTTP::WebSocketHandler
 
   def call(context)
     if websocket_upgrade_request? context.request
-      key = context.request.headers["Sec-Websocket-Key"]
+      response = context.response
+
+      version = context.request.headers["Sec-WebSocket-Version"]?
+      unless version == WebSocket::Protocol::VERSION
+        response.status_code = 426
+        response.headers["Sec-WebSocket-Version"] = WebSocket::Protocol::VERSION
+        return
+      end
 
-      accept_code =
-        {% if flag?(:without_openssl) %}
-          Digest::SHA1.base64digest("#{key}258EAFA5-E914-47DA-95CA-C5AB0DC85B11")
-        {% else %}
-          Base64.strict_encode(OpenSSL::SHA1.hash("#{key}258EAFA5-E914-47DA-95CA-C5AB0DC85B11"))
-        {% end %}
+      key = context.request.headers["Sec-WebSocket-Key"]?
+
+      unless key
+        response.status_code = 400
+        return
+      end
+
+      accept_code = WebSocket::Protocol.key_challenge(key)
 
-      response = context.response
       response.status_code = 101
       response.headers["Upgrade"] = "websocket"
       response.headers["Connection"] = "Upgrade"
-      response.headers["Sec-Websocket-Accept"] = accept_code
+      response.headers["Sec-WebSocket-Accept"] = accept_code
       response.upgrade do |io|
         ws_session = WebSocket.new(io)
         @proc.call(ws_session, context)
diff --git a/src/http/web_socket/protocol.cr b/src/http/web_socket/protocol.cr
index 4a4dfb9a7ebb..8ed2c2e2df47 100644
--- a/src/http/web_socket/protocol.cr
+++ b/src/http/web_socket/protocol.cr
@@ -26,7 +26,7 @@ class HTTP::WebSocket::Protocol
   end
 
   MASK_BIT = 128_u8
-  VERSION  =     13
+  VERSION  = "13"
 
   record PacketInfo,
     opcode : Opcode,
@@ -257,11 +257,13 @@ class HTTP::WebSocket::Protocol
       end
     {% end %}
 
+    random_key = Base64.strict_encode(StaticArray(UInt8, 16).new { rand(256).to_u8 })
+
     headers["Host"] = "#{host}:#{port}"
     headers["Connection"] = "Upgrade"
     headers["Upgrade"] = "websocket"
-    headers["Sec-WebSocket-Version"] = VERSION.to_s
-    headers["Sec-WebSocket-Key"] = Base64.strict_encode(StaticArray(UInt8, 16).new { rand(256).to_u8 })
+    headers["Sec-WebSocket-Version"] = VERSION
+    headers["Sec-WebSocket-Key"] = random_key
 
     path = "/" if path.empty?
     handshake = HTTP::Request.new("GET", path, headers)
@@ -269,7 +271,12 @@ class HTTP::WebSocket::Protocol
     socket.flush
     handshake_response = HTTP::Client::Response.from_io(socket)
     unless handshake_response.status_code == 101
-      raise Socket::Error.new("Handshake got denied. Status code was #{handshake_response.status_code}")
+      raise Socket::Error.new("Handshake got denied. Status code was #{handshake_response.status_code}.")
+    end
+
+    challenge_response = Protocol.key_challenge(random_key)
+    unless handshake_response.headers["Sec-WebSocket-Accept"]? == challenge_response
+      raise Socket::Error.new("Handshake got denied. Server did not verify WebSocket challenge.")
     end
 
     new(socket, masked: true)
@@ -285,4 +292,12 @@ class HTTP::WebSocket::Protocol
 
     raise ArgumentError.new("No host or path specified which are required.")
   end
+
+  def self.key_challenge(key)
+    {% if flag?(:without_openssl) %}
+      Digest::SHA1.base64digest("#{key}258EAFA5-E914-47DA-95CA-C5AB0DC85B11")
+    {% else %}
+      Base64.strict_encode(OpenSSL::SHA1.hash("#{key}258EAFA5-E914-47DA-95CA-C5AB0DC85B11"))
+    {% end %}
+  end
 end