Mozilla Firefox before 56.0, Firefox ESR before 52.4 or Thunderbird 52.4 :- 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.