TP-LINK TL-WR849N 0.9.1 4.16 devices do not require authentication to replace the firmware via a POST request to the cgi/softup URI.
https://fireshellsecurity.team/hack-n-routers/
http://packetstormsecurity.com/files/156586/TP-Link-TL-WR849N-0.9.1-4.16-Authentication-Bypass.html