🍒 pick 24d5c77: 🔒🥅 Handle tagged "OK" to incomplete command [backport #664]

Taking a "belt-and-suspenders" approach:

This is a potential problem for any command which registers a response
handler: a malicious server can easily guess what the next tag will be,
and send an `OK` response _before_ the client the response handler is
attached.

`STARTTLS` is an extreme example of this issue: if the `STARTTLS`
handler does not run, then `#starttls` will not start the TLS session,
and the connection is not secured, _but no error is raised._

We should _also_ attach the response handler before sending the `CRLF`,
but that is neither necessary (the response handler will added before
the `synchronize` mutex is unlocked) nor sufficient (the fake `OK` can
be sent _much_ earlier).

On the other hand, it _is_ okay for the server to send an error tagged
response (`NO` or `BAD`), before the sending the command has completed.

Author: nevans

lib/net/imap.rb

@@ -2311,6 +2311,7 @@ def send_command(cmd, *args, &block)
           put_string(" ")
           send_data(i, tag)
         end
+        guard_against_tagged_response_skipping_handler!(tag)
         put_string(CRLF)
         if cmd == "LOGOUT"
           @logout_command_tag = tag
@@ -2326,6 +2327,17 @@ def send_command(cmd, *args, &block)
           end
         end
       end
+    rescue InvalidResponseError
+      disconnect
+      raise
+    end
+
+    def guard_against_tagged_response_skipping_handler!(tag)
+      return unless (resp = @tagged_responses[tag])&.name&.upcase == "OK"
+      raise(InvalidResponseError,
+            "Server sent tagged 'OK' before command was finished: %p. " \
+            "This could indicate a malicious server or client-side " \
+            "command injection. Disconnecting." % [resp.raw_data.chomp])
     end
 
     def generate_tag