The reader view will display cross-origin content when CORS headers are set to prohibit the loading of cross-origin content by a site. This could allow access to content that should be restricted in reader view. This vulnerability affects Firefox < 58.
https://www.mozilla.org/security/advisories/mfsa2018-02/
https://usn.ubuntu.com/3544-1/
https://bugzilla.mozilla.org/show_bug.cgi?id=1420507