Discoverer:             Elias Dimopoulos
Linkedin:                 https://gr.linkedin.com/in/dimopouloselias

Vulnerability:            Reflected XSS
Affected plugin:       Yoast SEO plugin <  5.8.0
Active installations:  5+ million
URL: https://wordpress.org/plugins/wordpress-seo/#developers

Assigned CVE:          CVE-2017-16842

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Thanks Yoast for their immediate response.
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The vulnerability lies in the "tab" parameter and can cause reflected 
XSS vulnerability.

The vulnerability can be exploited against an administrator by using the 
following url:

https://victim/wp-admin/admin.php?page=wpseo_search_console&tab=settings'><script>alert(window.location)</script><!--

The victim has to have a valid profile under 
https://victim/wp-admin/admin.php?page=wpseo_search_console&tab=settings 
(example: profile.png)
If there is the message "No profiles found" under the above link, the 
plugin has not finished with the configuration and the vulnerability 
cannot be exploited.

A logged in Administrator, who will click on the above link, he will 
receive an alert box with the value of "window.location".

In this case, the javascript code is just an alert box, however any kind 
of malicious javascript code can be used.

Reproduce:

1. Login to your wordpress as an admin.
2. Ensure that a valid profile has been configured under 
https://victim/wp-admin/admin.php?page=wpseo_search_console&tab=settings 
(example: profile.png)
3. Access the following link:

https://victim/wp-admin/admin.php?page=wpseo_search_console&tab=settings'><script>alert(window.location)</script><!--

4. The alert box should be executed.

The vulnerability has been tested against:

  *      Yoast SEO Version: 5.7.1
  *      WordPress 4.8.3 running Twenty Seventeen theme.
  *      Firefox 56.0 (64-bit) - Mozilla Firefox for Ubuntu canonical - 1.0


I also tried to track down the issue in the source code:

file: admin/google_search_console/class-gsc-platform-tabs.php:

70         private function set_current_tab( array $platforms ) {
71                 $this->current_tab = key( $platforms );
72                 if ( $current_platform = filter_input( INPUT_GET, 
'tab' ) ) {
73                         $this->current_tab = $current_platform;
74                 }
75         }

file: wordpress-seo/admin/google_search_console/class-gsc-table.php:

361   echo "<input id='field_platform' type='hidden' name='platform' 
value='{$platform}' />";

the filter_input function has the following definition:

mixed filter_input ( int $type , string $variable_name [, int $filter = 
FILTER_DEFAULT [, mixed $options ]] )

If filter is omitted like in the code above, FILTER_DEFAULT will be 
used, which is equivalent to FILTER_UNSAFE_RAW. This will result in no 
filtering taking place by default. (ref: 
https://php.net/manual/en/function.filter-input.php)