Information
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Advisory by Netsparker
Name: XSS Vulnerability in Mailpoet Newsletters WordPress plugin
Affected Software : Mailpoet Newsletters
Affected Versions: v2.6.19 and possibly below
Vendor Homepage : http://www.mailpoet.com/ 
Vulnerability Type : Cross-site Scripting
Severity : Important
CVE-ID : TBA
Status : Fixed
Netsparker Advisory Reference : NS-16-002

Technical Details
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Proof of Concept URL for XSS in Mailpoet Newsletters v2.6.19:

wordpress-4.4.1/?wysija-page=1&controller=confirm&wysija-key='"--></style></scRipt><scRipt>alert(0x0034B4)</scRipt>&action=unsubscribe&demo=1&wysijap=subscriptions

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

Advisory Timeline
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18 Jan 2016 - First Contact
29 Jan 2016 - Vendor Fixed
02 Feb 2016 - Advisory Released

Solution
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Patch released by MailPoet and annocunced here: https://wordpress.org/plugins/wysija-newsletters/changelog/. Download the latest version.

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.