# 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. 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.