libfido2 can be fuzzed using AFL or libFuzzer, with or without
ASAN/MSAN/UBSAN.

AFL is more convenient when fuzzing the path from the authenticator to
libfido2 in an existing application. To do so, use preload-snoop.c with a real
authenticator to obtain an initial corpus, rebuild libfido2 with -DFUZZ=ON, and
use preload-fuzz.c to read device data from stdin.

libFuzzer is better suited for bespoke fuzzers; see fuzz_cred.c, fuzz_credman.c,
fuzz_assert.c, fuzz_hid.c, and fuzz_mgmt.c for examples. To build these
harnesses, use -DFUZZ=ON -DLIBFUZZER=ON.

If -DFUZZ=ON is enabled, symbols listed in wrapped.sym are wrapped in the
resulting shared object. The wrapper functions simulate failure according to a
deterministic RNG and probabilities defined in wrap.c. Harnesses wishing to
use this functionality should call prng_init() with a seed obtained from the
corpus. To mutate only the seed part of a libFuzzer harness's corpora,
use '-reduce_inputs=0 --fido-mutate=seed'.

To run under ASAN/MSAN/UBSAN, libfido2 needs to be linked against flavours of
libcbor and OpenSSL built with the respective sanitiser. In order to keep
memory utilisation at a manageable level, you can either enforce limits at
the OS level (e.g. cgroups on Linux), or patch libcbor with the diff below.

diff --git src/cbor/internal/memory_utils.c src/cbor/internal/memory_utils.c
index aa049a2..e294b38 100644
--- src/cbor/internal/memory_utils.c
+++ src/cbor/internal/memory_utils.c
@@ -28,7 +28,10 @@ bool _cbor_safe_to_multiply(size_t a, size_t b) {
 
 void* _cbor_alloc_multiple(size_t item_size, size_t item_count) {
   if (_cbor_safe_to_multiply(item_size, item_count)) {
-    return _CBOR_MALLOC(item_size * item_count);
+    if (item_count > 1000) {
+      return NULL;
+    } else
+      return _CBOR_MALLOC(item_size * item_count);
   } else {
     return NULL;
   }
