Security

Security model

MicroCS scripts are trusted less than firmware but run in the same address space. The goal is robustness against buggy or hostile scripts, not isolation against an attacker with physical/debug access.

Mechanism	What it guarantees	Status
Memory safety of the VM	scripts cannot access raw memory: bounds-checked arrays/strings, no pointers, unsafe rejected	tested under ASan/UBSan and with the source fuzzer; not formally verified
Heap cap (cfg.heap_limit) + pool heap	a script cannot exhaust system RAM; OOM ends the run with MCS_ERR_MEMORY (no leak since Phase 2)	tested
Stack/frame limits	deep recursion → catchable StackOverflowException	tested
Time / step budgets (mcs_set_limits)	runaway loops are aborted; scripts cannot catch the abort	tested
Hook + mcs_request_abort	watchdog feeding, Ctrl-C, external kill	tested
Module whitelisting	only modules the host opens exist in C#; HAL classes exist only for non-NULL ops; cfg.stdlib removes library groups	by construction
VFS confinement	normalised paths cannot leave mounted roots; RDONLY and NOEXEC mounts	tested
Atomic uploads	interrupted put never leaves a half-written script	tested (RAM, LittleFS)
Image validation	magic/version/bounds; constant, global, local-slot and upvalue operands; closure captures; branch targets on instruction boundaries. Not verified: stack balance and operand types	1.2; fuzzed with tools/fuzz.py --image (load + disassemble)
Signed images / authenticated shell	—	planned: the shell has no authentication; do not expose it on an untrusted link

Because stack balance is not verified, a hand-crafted image can still corrupt the VM when it runs: treat .mcsb files like firmware (build them with mcs, ship them over a trusted channel). Source scripts have no such caveat — the compiler only emits balanced code.

Recommendations: give each untrusted script a step/time budget, mount system scripts read-only, put user uploads on a NOEXEC mount if they should only be data, and disable the shell (MCS_ENABLE_SHELL=0) in production builds unless the link is trusted.