The TinyCBOR library takes a `size_t *` length argument in many
functions which at function call contains the length of a buffer, and
at exit the actual size of the data. The FIDO-2 code however uses
`uint8_t` fields in `struct`s to store the data. Previously, a pointer
to that `uint8_t` filed was just casted to `size_t *`, resulting in
three neighboring bytes also being interpreted as being part of the
buffer size - which could result in undetected buffer overflows.
Similar, upon exit of the function not only the `uint8_t` sized length
`struct` member but also three neighboring bytes were written to.
I didn't care to investigate, but this really looks like crafted CBOR
payloads send to the FIDO2 implementation could result in arbitrary
code execution on the device.
