During TLS 1.2 exchanges, handshake hashes are generated which point to a message buffer. This saved data is used for later messages but in some cases, the handshake transcript can exceed the space available in the current buffer, causing the allocation of a new buffer. This leaves a pointer pointing to the old, freed buffer, resulting in a use-after-free when handshake hashes are then calculated afterwards. This can result in a potentially exploitable crash. This vulnerability affects Firefox < 56, Firefox ESR < 52.4, and Thunderbird < 52.4.
https://www.debian.org/security/2017/dsa-4014
https://www.debian.org/security/2017/dsa-3998
https://www.debian.org/security/2017/dsa-3987
https://security.gentoo.org/glsa/201803-14
https://lists.debian.org/debian-lts-announce/2017/11/msg00000.html
https://access.redhat.com/errata/RHSA-2017:2832