An integer overflow during the parsing of XML using the Expat library. This vulnerability affects Firefox < 50.
https://www.mozilla.org/security/advisories/mfsa2016-89/
https://www.debian.org/security/2017/dsa-3898
https://bugzilla.mozilla.org/show_bug.cgi?id=1274777
http://www.securitytracker.com/id/1039427