There wasn't any validation on the key url parameter which allowed for some cross site scripting to potentially occur. The fix is to add in validation to ensure the key is of normal standard extension key pattern.
CiviCRM Versions prior to 4.7.26 and 4.6.33
CiviCRM Versions 4.7.26 and 4.6.33
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Sean Madsen of Left Join Labs for reporting and helping to fix the issue.
Seamus Lee of Australian Greens for fixing the issue.