By rewriting the Host: request headers using the webRequest API, a WebExtension can bypass domain restrictions through domain fronting. This would allow access to domains that share a host that are otherwise restricted. This vulnerability affects Firefox ESR < 60.3 and Firefox < 63.
www.securityfocus.com/bid/105718
www.securitytracker.com/id/1041944
access.redhat.com/errata/RHSA-2018:3005
access.redhat.com/errata/RHSA-2018:3006
bugzilla.mozilla.org/show_bug.cgi?id=1467523
lists.debian.org/debian-lts-announce/2018/11/msg00008.html
security.gentoo.org/glsa/201811-04
usn.ubuntu.com/3801-1/
www.debian.org/security/2018/dsa-4324
www.mozilla.org/security/advisories/mfsa2018-26/
www.mozilla.org/security/advisories/mfsa2018-27/