In a number of locations within the CiviCRM code base there were potentially un-escaped variables passed into html link attributes such as alt and title. One such example was in event registration pages where administrators were able to set the button text and also the title attribute to anything they chose. This fixes it by properly escaping the content of those attributes.
CiviCRM Versions prior to 4.7.26 and 4.6.33
CiviCRM Versions 4.7.26 and 4.7.33
Either Upgrade to the latest version of CiviCRM
If you cannot upgrade apply the following patch
Sean Madsen of Left Join Labs for reporting the issue
Seamus Lee of Australian Greens for fixing the issue.