Advisory by Netsparker (now Invicti)
Name: Reflected Cross-site Scripting Vulnerability in WP-Polls 2.73
Affected Software: WP-Polls 2.73
Affected Versions: 2.73 and possibly below
Vendor Homepage: 
Vulnerability Type: Cross-site Scripting
Severity: Important
Status: Fixed
Invicti Advisory Reference: NS-16-009

Technical Details
Proof of Concept URL for XSS vulnerability in WP-Polls 2.73:

URL /wordpress/wp-admin/admin.php?page=wp-polls/polls-options.php
Parameter Name poll_bar_style
Parameter Type POST
Attack Pattern '" onmouseover= alert(0x000C5A)

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

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

Patch released by WP-Polls and announced here.

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

About Invicti
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.