The sink function is ajax_run_package() in class-admin-upgrade.php
This function is bound to "wp_ajax_um_run_package" and will be executed in um_run_upgrade() in class-admin-upgrade.php if admin press the upgrade "Run" button.
The 'pack' parameter can be controlled by attacker.
The url is /wp-admin/admin-ajax.php, action and nonce is known, attacker can capture packet or directly send malicious packet whose 'pack' parameter is carefully designed.
Then we turn to the class-admin-upgrade.php
The core part is variable $_POST['pack'] only filtered by sanitize_text_field() which connot filter '.' or '/'
$this->packages_dir . sanitize_text_field( $_POST['pack'] ) . DIRECTORY_SEPARATOR . 'init.php'
in ajax_run_package() has three parts: $this->packages_dir, $_POST['pack'] and /init.php
The variable packages_dir is assigned in __construct() and should be /var/www/html/wordpress/wp-content/plugins/packages/
if $_POST['pack'] is "../../.."
$this->packages_dir . sanitize_text_field( $_POST['pack'] ) . DIRECTORY_SEPARATOR . 'init.php'
in ajax_run_package() should be /var/www/html/wordpress/wp-content/plugins/packages/../../../init.php
which is /var/www/html/wordpress/init.php
Of course $_POST['pack'] can be any directory attacker want, which means attacker can include malicious "init.php" in any directory he want and excute malicious PHP code.
That is to say, if attacker create a malicious "init.php", he can use this vulnerability to execute any PHP code he want. If a method could be discovered that allows uploading arbitrary PHP code, this could be used to execute that code.
Thus, there is the Directory Traversal and Local File Inclusion vulnerability.