Sandstorm Cap"n Proto before 0.5.3.1 allows remote crashes related to a compiler optimization. A remote attacker can trigger a segfault in a 32-bit libcapnp application because Cap"n Proto relies on pointer arithmetic calculations that overflow. An example compiler with optimization that elides a bounds check in such calculations is Apple LLVM version 8.1.0 . The attack vector is a crafted far pointer within a message.