Crypto · C

How to fuzz wolfSSL

A FIPS-certified TLS library for RTOS and automotive — correctness claims demand verification.

wolfSSL's design prioritises a small footprint for RTOS and automotive targets, meaning its TLS handshake and certificate parsing take many manual shortcuts that can introduce length-check omissions. Its FIPS boundary and claimed correctness make fuzzing especially valuable as an independent verification layer.

Common bug classes

•Heap buffer overflow in TLS ClientHello extension parsing
•Integer underflow in Certificate message length check
•Out-of-bounds read in DTLS fragment reassembly buffer
•Null dereference in session ticket key rotation
•Use-after-free in TLS 1.3 key schedule error path

Recommended setup

Fuzzers

→ AFL++
→ libFuzzer
→ Honggfuzz

Sanitizers

→ ASan
→ UBSan

Harness scaffold

#include <stdint.h>
#include <stddef.h>
#include <wolfssl/ssl.h>

int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
  WOLFSSL_CTX *ctx = wolfSSL_CTX_new(wolfTLSv1_3_client_method());
  if (!ctx) return 0;
  WOLFSSL *ssl = wolfSSL_new(ctx);
  if (!ssl) { wolfSSL_CTX_free(ctx); return 0; }
  /* Inject fuzz data as the incoming record stream */
  wolfSSL_SetIORecv(ssl, NULL);
  wolfSSL_MemoryFeed(ssl, data, (int)size);
  wolfSSL_connect(ssl);
  wolfSSL_free(ssl);
  wolfSSL_CTX_free(ctx);
  return 0;
}

Save this as fuzz_target.cc, build with your compiler + sanitizer flags, and you have a working starting point.

Notable CVEs found by fuzzing

→ CVE-2022-38153

Start fuzzing wolfSSL on Fuzze.rs →

Push the harness above + a Dockerfile. First month 50% off.