The ArrayBufferBuilder::append function in Mozilla Firefox before 39.0, Firefox ESR 31.x before 31.8 and 38.x before 38.1, and Thunderbird before 38.1 accesses unintended memory locations, which has unspecified impact and attack vectors.
Exploitability
AV:NAC:LAu:NImpact
C:CI:CA:C10/AV:N/AC:L/Au:N/C:C/I:C/A:C