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.

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)

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.

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.
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.