cryptography is a package designed to expose cryptographic primitives and recipes to Python developers. In affected versions Cipher.update_into would accept Python objects which implement the buffer protocol, but provide only immutable buffers. This would allow immutable objects (such as bytes) to be mutated, thus violating fundamental rules of Python and resulting in corrupted output. This now correctly raises an exception. This issue has been present since update_into was originally introduced in cryptography 1.8.