When an "iframe" has a "sandbox" attribute and its content is specified using "srcdoc", that content does not inherit the containing page's Content Security Policy (CSP) as it should unless the sandbox attribute included "allow-same-origin". This vulnerability affects Firefox < 55.
https://www.mozilla.org/security/advisories/mfsa2017-18/
https://bugzilla.mozilla.org/show_bug.cgi?id=1073952