An out-of-bounds write in \ClearKeyDecryptor\ while decrypting some Clearkey-encrypted media content. The \ClearKeyDecryptor\ code runs within the Gecko Media Plugin (GMP) sandbox. If a second mechanism is found to escape the sandbox this vulnerability allows for the writing of arbitrary data within memory resulting in a potentially exploitable crash. This vulnerability affects Firefox ESR < 45.9 Firefox ESR < 52.1 and Firefox < 53.