# Nanopb: Security model

Importance of security in a Protocol Buffers library
----------------------------------------------------

In the context of protocol buffers, security comes into play when
decoding untrusted data. Naturally, if the attacker can modify the
contents of a protocol buffers message, he can feed the application any
values possible. Therefore the application itself must be prepared to
receive untrusted values.

Where nanopb plays a part is preventing the attacker from running
arbitrary code on the target system. Mostly this means that there must
not be any possibility to cause buffer overruns, memory corruption or
invalid pointers by the means of crafting a malicious message.

Division of trusted and untrusted data
--------------------------------------

The following data is regarded as **trusted**. It must be under the
control of the application writer. Malicious data in these structures
could cause security issues, such as execution of arbitrary code:

1.  Callback, pointer and extension fields in message structures given
    to pb_encode() and pb_decode(). These fields are memory pointers,
    and are generated depending on the message definition in the .proto
    file.
2.  The automatically generated field definitions, i.e.
    `pb_msgdesc_t`.
3.  Contents of the `pb_istream_t` and `pb_ostream_t` structures
    (this does not mean the contents of the stream itself, just the
    stream definition).

The following data is regarded as **untrusted**. Invalid/malicious data
in these will cause "garbage in, garbage out" behaviour. It will not
cause buffer overflows, information disclosure or other security
problems:

1.  All data read from `pb_istream_t`.
2.  All fields in message structures, except:
    -   callbacks (`pb_callback_t` structures)
    -   pointer fields and `_count` fields for pointers
    -   extensions (`pb_extension_t` structures)

Invariants
----------

The following invariants are maintained during operation, even if the
untrusted data has been maliciously crafted:

1.  Nanopb will never read more than `bytes_left` bytes from
    `pb_istream_t`.
2.  Nanopb will never write more than `max_size` bytes to
    `pb_ostream_t`.
3.  Nanopb will never access memory out of bounds of the message
    structure.
4.  After `pb_decode()` returns successfully, the message structure will
    be internally consistent:
    -   The `count` fields of arrays will not exceed the array size.
    -   The `size` field of bytes will not exceed the allocated size.
    -   All string fields will have null terminator.
    -   bool fields will have valid true/false values (since
        nanopb-0.3.9.4)
    -   pointer fields will be either `NULL` or point to valid data
5.  After `pb_encode()` returns successfully, the resulting message is a
    valid protocol buffers message. (Except if user-defined callbacks
    write incorrect data.)
6.  All memory allocated by `pb_decode()` will be released by a subsequent
    call to `pb_release()` on the same message.

Further considerations
----------------------

Even if the nanopb library is free of any security issues, there are
still several possible attack vectors that the application author must
consider. The following list is not comprehensive:

1.  Stack usage may depend on the contents of the message. The message
    definition places an upper bound on how much stack will be used.
    Tests should be run with all fields present, to record the maximum
    possible stack usage. For recursive messages, define
    `PB_MESSAGE_NESTING_MAX` to limit the recursion depth.
2.  Callbacks can do anything. The code for the callbacks must be
    carefully checked if they are used with untrusted data.
3.  If using stream input, a maximum size should be set in
    `pb_istream_t` to stop a denial of service attack from using an
    infinite message.
4.  If using network sockets as streams, a timeout should be set to stop
    denial of service attacks.
5.  If using `malloc()` support, some method of limiting memory use
    should be employed. This can be done by defining custom
    `pb_realloc()` function. Nanopb will properly detect and handle
    failed memory allocations.
