1 ====================== 2 Nanopb: Security model 3 ====================== 4 5 .. include :: menu.rst 6 7 .. contents :: 8 9 10 11 Importance of security in a Protocol Buffers library 12 ==================================================== 13 In the context of protocol buffers, security comes into play when decoding 14 untrusted data. Naturally, if the attacker can modify the contents of a 15 protocol buffers message, he can feed the application any values possible. 16 Therefore the application itself must be prepared to receive untrusted values. 17 18 Where nanopb plays a part is preventing the attacker from running arbitrary 19 code on the target system. Mostly this means that there must not be any 20 possibility to cause buffer overruns, memory corruption or invalid pointers 21 by the means of crafting a malicious message. 22 23 Division of trusted and untrusted data 24 ====================================== 25 The following data is regarded as **trusted**. It must be under the control of 26 the application writer. Malicious data in these structures could cause 27 security issues, such as execution of arbitrary code: 28 29 1. Callback, pointer and extension fields in message structures given to 30 pb_encode() and pb_decode(). These fields are memory pointers, and are 31 generated depending on the message definition in the .proto file. 32 2. The automatically generated field definitions, i.e. *pb_field_t* lists. 33 3. Contents of the *pb_istream_t* and *pb_ostream_t* structures (this does not 34 mean the contents of the stream itself, just the stream definition). 35 36 The following data is regarded as **untrusted**. Invalid/malicious data in 37 these will cause "garbage in, garbage out" behaviour. It will not cause 38 buffer overflows, information disclosure or other security problems: 39 40 1. All data read from *pb_istream_t*. 41 2. All fields in message structures, except: 42 43 - callbacks (*pb_callback_t* structures) 44 - pointer fields (malloc support) and *_count* fields for pointers 45 - extensions (*pb_extension_t* structures) 46 47 Invariants 48 ========== 49 The following invariants are maintained during operation, even if the 50 untrusted data has been maliciously crafted: 51 52 1. Nanopb will never read more than *bytes_left* bytes from *pb_istream_t*. 53 2. Nanopb will never write more than *max_size* bytes to *pb_ostream_t*. 54 3. Nanopb will never access memory out of bounds of the message structure. 55 4. After pb_decode() returns successfully, the message structure will be 56 internally consistent: 57 58 - The *count* fields of arrays will not exceed the array size. 59 - The *size* field of bytes will not exceed the allocated size. 60 - All string fields will have null terminator. 61 62 5. After pb_encode() returns successfully, the resulting message is a valid 63 protocol buffers message. (Except if user-defined callbacks write incorrect 64 data.) 65 66 Further considerations 67 ====================== 68 Even if the nanopb library is free of any security issues, there are still 69 several possible attack vectors that the application author must consider. 70 The following list is not comprehensive: 71 72 1. Stack usage may depend on the contents of the message. The message 73 definition places an upper bound on how much stack will be used. Tests 74 should be run with all fields present, to record the maximum possible 75 stack usage. 76 2. Callbacks can do anything. The code for the callbacks must be carefully 77 checked if they are used with untrusted data. 78 3. If using stream input, a maximum size should be set in *pb_istream_t* to 79 stop a denial of service attack from using an infinite message. 80 4. If using network sockets as streams, a timeout should be set to stop 81 denial of service attacks. 82 5. If using *malloc()* support, some method of limiting memory use should be 83 employed. This can be done by defining custom *pb_realloc()* function. 84 Nanopb will properly detect and handle failed memory allocations. 85
