Description: Reflected Cross-Site Scripting(XSS)
Affected Plugin: NextScripts: Social Networks Auto-Poster
Plugin Slug: social-networks-auto-poster-facebook-twitter-g
Affected Versions: <= 4.3.20
CVE ID: CVE-2021-38356
CVSS Score: 6.1(Medium)
CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
Researcher/s: Ramuel Gall
Fully Patched Version: 4.3.21

The nxs_ReposterListTable::column_title function in inc/nxs_class_snap.php echoed out the value of $_REQUEST[‘page’] when an administrator was visiting the plugin administration page at wp-admin/admin.php?admin.php?page=nxssnap-post.
  
function column_post_title($item){        
    //Build row actions
    $actions = array(
        'edit'      => sprintf('<a href="?page=%s&action=%s&item=%s">Edit</a>',$_REQUEST['page'],'edit',$item->ID),
        'delete'    => sprintf('<a href="?page=%s&action=%s&item=%s">Delete</a>',$_REQUEST['page'],'delete',$item->ID),
    );        
    //Return the title contents
    return sprintf('%1$s <span style="color:silver">(id:%2$s)</span>%3$s',
        /*$1%s*/ $item->post_title,
        /*$2%s*/ $item->ID,
        /*$3%s*/ $this->row_actions($actions)
    );
}

WordPress uses the value of the $_GET[‘page’] parameter in order to determine which administrative page to serve content for. It is also common practice for developers to use $_REQUEST for values stored in either $_GET or $_POST, as the $_REQUEST superglobal contains everything set in both $_GET and $_POST. As such, $_REQUEST[‘page’] might be expected to be set to the same value as $_GET[‘page’].

However, thanks to a quirk of how PHP orders parameters that are present in multiple superglobal variables, it was possible to perform a reflected cross-site scripting attack.

In most PHP configurations, depending on the request_order (or the variables_order if request_order is not set), $_POST takes precedence over $_GET when populating $_REQUEST. In other words, if both $_GET[‘page’] and $_POST[‘page’] are set, $_REQUEST[‘page’] will be set to the contents of $_POST[‘page’], rather than $_GET[‘page’].

This meant that it was possible to execute JavaScript in the browser of a logged-in administrator by tricking them into visiting a self-submitting form that sent a POST request to their site, for example,  hxxps://victimsite.site/wp-admin/admin.php?page=nxssnap-post, with the $_POST[‘page’] parameter set to malicious JavaScript.

The $_GET[‘page’] parameter could be set to nxssnap-post, so that WordPress would route the victim to the correct page, and then the malicious JavaScript in $_POST[‘page’] would be echoed out on that page.

As with all XSS attacks, malicious JavaScript running in an administrator’s session could be used to add malicious administrative users or insert backdoors into a site, and thus be used for site takeover.
Timeline

August 19, 2021 – We finish our investigation and begin the disclosure process for NextScripts: Social Networks Auto-Poster.
August 20, 2021 – We send over full disclosure to the plugin developer.
September 27, 2021 – We follow up with the plugin developer as the plugin has not yet been patched.
October 4, 2021 – A patched version of the plugin, 4.3.21, becomes available.
Conclusion

In today’s post, we covered a reflected Cross-Site Scripting(XSS) vulnerability that relied on a relatively obscure quirk of how PHP handles superglobal variables.

All Wordfence users, including Wordfence Premium customers and free Wordfence users are protected by our firewall’s built-in XSS protection. Nonetheless, we strongly recommend updating to the latest version of the plugin available, which is 4.3.23 at this time.

If you know a friend or colleague who is using this plugin on their site, we highly recommend forwarding this advisory to them to help keep their sites protected as this is a serious vulnerability that can lead to complete site takeover.