diff --git a/lib/net/imap.rb b/lib/net/imap.rb
index 0d7e4db3..e2e17ae6 100644
--- a/lib/net/imap.rb
+++ b/lib/net/imap.rb
@@ -1440,9 +1440,11 @@ def logout!
     #
     def starttls(**options)
       @ssl_ctx_params, @ssl_ctx = build_ssl_ctx(options)
+      handled = false
       error = nil
       ok = send_command("STARTTLS") do |resp|
         if resp.kind_of?(TaggedResponse) && resp.name == "OK"
+          handled = true
           clear_cached_capabilities
           clear_responses
           start_tls_session
@@ -1454,6 +1456,13 @@ def starttls(**options)
         disconnect
         raise error
       end
+      unless handled
+        disconnect
+        raise InvalidResponseError,
+              "STARTTLS handler was bypassed, although server responded %p" % [
+                ok.raw_data.chomp
+              ]
+      end
       ok
     end
 
@@ -3165,6 +3174,7 @@ def idle(timeout = nil, &response_handler)
 
       synchronize do
         tag = Thread.current[:net_imap_tag] = generate_tag
+        guard_against_tagged_response_skipping_handler!(tag, "IDLE")
         put_string("#{tag} IDLE#{CRLF}")
 
         begin
@@ -3629,21 +3639,29 @@ def send_command(cmd, *args, &block)
           put_string(" ")
           send_data(i, tag)
         end
-        put_string(CRLF)
-        if cmd == "LOGOUT"
-          @logout_command_tag = tag
-        end
-        if block
-          add_response_handler(&block)
-        end
+        @logout_command_tag = tag if cmd == "LOGOUT"
+        guard_against_tagged_response_skipping_handler!(tag, cmd)
+        add_response_handler(&block) if block
         begin
-          return get_tagged_response(tag, cmd)
+          put_string(CRLF)
+          get_tagged_response(tag, cmd)
         ensure
-          if block
-            remove_response_handler(block)
-          end
+          remove_response_handler(block) if block
         end
       end
+    rescue InvalidResponseError
+      disconnect
+      raise
+    end
+
+    def guard_against_tagged_response_skipping_handler!(tag, cmd)
+      return unless (resp = @tagged_responses[tag])&.name&.upcase == "OK"
+      raise InvalidResponseError, format(
+        "Received tagged 'OK' to incomplete %s command (tag=%s).  " \
+        "This could indicate a malicious server, a man-in-the-middle, or " \
+        "client-side command injection.  Disconnecting.",
+        cmd, tag
+      )
     end
 
     def generate_tag
diff --git a/test/net/imap/test_imap.rb b/test/net/imap/test_imap.rb
index e82d08cc..a9745fc3 100644
--- a/test/net/imap/test_imap.rb
+++ b/test/net/imap/test_imap.rb
@@ -138,15 +138,67 @@ def test_starttls
       end
     end
 
-    def test_starttls_stripping
+    def test_starttls_stripping_not_ok
       imap = nil
-      starttls_stripping_test do |port|
+      server = create_tcp_server
+      port = server.addr[1]
+      start_server do
+        sock = server.accept
+        begin
+          sock.print("* OK test server\r\n")
+          sock.gets
+          sock.print("RUBY0001 BUG unhandled command\r\n")
+        ensure
+          sock.close
+          server.close
+        end
+      end
+      begin
         imap = Net::IMAP.new("localhost", :port => port)
         assert_raise(Net::IMAP::InvalidResponseError) do
           imap.starttls(:ca_file => CA_FILE)
         end
         assert imap.disconnected?
-        imap
+      ensure
+        imap.disconnect if imap && !imap.disconnected?
+      end
+
+      assert_equal false, imap.tls_verified?
+      assert_equal({ca_file: CA_FILE},        imap.ssl_ctx_params)
+      assert_equal(CA_FILE,                   imap.ssl_ctx.ca_file)
+      assert_equal(OpenSSL::SSL::VERIFY_PEER, imap.ssl_ctx.verify_mode)
+    end
+
+    def test_starttls_stripping_ok_sent_before_response
+      # to coordinate between threads (better than sleep)
+      server_to_client, client_to_server = Queue.new, Queue.new
+      imap = nil
+      server = create_tcp_server
+      port = server.addr[1]
+      start_server do
+        sock = server.accept
+        begin
+          sock.print("* OK test server\r\n")
+          assert_equal :send_malicious_response, client_to_server.pop
+          sock.print("RUBY0001 OK hahaha, fooled you!\r\n")
+          server_to_client << :malicious_response_sent
+          sock.gets
+        ensure
+          sock.close
+          server.close
+        end
+      end
+      begin
+        imap = Net::IMAP.new("localhost", :port => port)
+        client_to_server << :send_malicious_response
+        assert_equal :malicious_response_sent, server_to_client.pop
+        sleep 0.010 # to be sure the network buffers have flushed, etc
+        assert_raise(Net::IMAP::InvalidResponseError) do
+          imap.starttls(:ca_file => CA_FILE)
+        end
+        assert imap.disconnected?
+      ensure
+        imap.disconnect if imap && !imap.disconnected?
       end
       assert_equal false, imap.tls_verified?
       assert_equal({ca_file: CA_FILE},        imap.ssl_ctx_params)
@@ -963,27 +1015,6 @@ def starttls_test
     end
   end
 
-  def starttls_stripping_test
-    server = create_tcp_server
-    port = server.addr[1]
-    start_server do
-      sock = server.accept
-      begin
-        sock.print("* OK test server\r\n")
-        sock.gets
-        sock.print("RUBY0001 BUG unhandled command\r\n")
-      ensure
-        sock.close
-        server.close
-      end
-    end
-    begin
-      imap = yield(port)
-    ensure
-      imap.disconnect if imap && !imap.disconnected?
-    end
-  end
-
   def create_tcp_server
     return TCPServer.new(server_addr, 0)
   end