Advisory by Netsparker
Name: Self XSS Vulnerability in Yoast SEO 3.3.2
Affected Software : Yoast SEO
Affected Versions: 3.3.2 and possibly below
Vendor Homepage : 
Vulnerability Type : Cross-site Scripting
Severity : Important
Status : Fixed
Netsparker Advisory Reference : NS-16-007

Technical Details
Proof of Concept URLs for XSS vulnerabilities in Yoast SEO 3.3.2:

URL /wordpress/wp-admin/admin-ajax.php
Parameter Name new_value
Parameter Type POST
Attack Pattern ><iMg src=N onerror=alert(9)>

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

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

Patch released by Yoast SEO and annocunced here.

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

About Netsparker
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 Enterprise. Visit our website for more information.