The PDF viewer does not sufficiently sanitize PostScript calculator functions, allowing malicious JavaScript to be injected through a crafted PDF file. This JavaScript can then be run with the permissions of the PDF viewer by its worker. This vulnerability affects Firefox ESR < 52.8 and Firefox < 60.
https://www.mozilla.org/security/advisories/mfsa2018-12/
https://www.mozilla.org/security/advisories/mfsa2018-11/
https://www.debian.org/security/2018/dsa-4199
https://usn.ubuntu.com/3645-1/
https://security.gentoo.org/glsa/201810-01
https://lists.debian.org/debian-lts-announce/2018/05/msg00007.html
https://bugzilla.mozilla.org/show_bug.cgi?id=1452075
https://access.redhat.com/errata/RHSA-2018:1415
https://access.redhat.com/errata/RHSA-2018:1414