Twentyfourteen

Different versions will have different nomenclatures (twentythirteen, twentyfifthteen...etc)

1)Appearance → Editor → Templates → 404.php

2)Paste a php reverse shell and update the file (use classic php-reverse shell)

3)Start a nc listener

4)IMPORTANT! Use gobuster or similar to enumerate /themes location

http://192.168.12.131/wordpress/wp-content/themes/twentyfourteen/404.php