Code review can help establish whether an extension behaves as intended. It cannot, by itself, stop that extension from performing an operation it is able to invoke. To answer “What is this extension actually entitled to have?”, define its permitted capabilities separately from its behavioral contract, then enforce those permissions at the host boundary.
Correct behavior and permitted consequences are different questions
Behavioral verification asks whether an implementation meets its contract: for given inputs, does it produce the expected result? Authority asks what consequences that implementation is permitted to create. Passing tests answers the first question; it does not establish the second.
As Ken W Alger puts it, “Code review is evidence about implementation. It should not be mistaken for enforcement of authority.” Review remains valuable for finding logic errors, vulnerabilities, unsuitable dependencies, and race conditions. Its limit is enforcement: reading code does not make an unauthorized operation unavailable when the code runs.
Why a sandbox may still leave an extension overpowered
A sandbox can constrain where code executes without adequately restricting what the code can do. If the host exposes powerful operations, sandboxed code may still be able to invoke them. The host interface is therefore a critical place to enforce authority: operations outside the extension’s grants must be denied there, rather than left to reviewer judgment or developer intent.
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What the invoice example demonstrates
Alger describes a small illustrative test bed, not a production study. Two invoice-reconciliation implementations produce the same expected report, but one also attempts to issue a refund. Its manifest permits invoice and payment reads but does not grant refunds, so the host denies that operation. The expected report alone would not reveal the difference in authority.
The example is a test of a mediated host interface, not a WebAssembly runtime. Alger describes the host as about 200 lines of Python with one dependency and four scenarios: correctness, authority, discover, and verify. These details characterize his demonstration; they are not independent benchmarks or evidence about how often real extensions misbehave.
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Why observed behavior is not a complete permission policy
A discovery run can record which capabilities an implementation exercised, but a single run cannot establish every capability the task legitimately requires. In Alger’s credit-note example, discovery missed the need for payments.write. The resulting manifest denied a legitimate adjusting write.
That failure exposes the weakness in deriving least privilege directly from observed behavior. Observation tells you what a component did, not what it may need. A missing capability can indicate an implementation reaching beyond policy, or that the policy omitted a valid path; the contract and surrounding context must help distinguish them.
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Separate the task contract from grants and enforcement
A more reliable design treats three things as related but distinct: the task’s behavioral contract, the policy that grants capabilities, and the runtime control that enforces those grants. Alger summarizes the distinction this way: “Verification asks whether an implementation satisfies its behavioral contract. Authority asks what consequences that implementation is permitted to create.”
- Specify the task contract: describe required behavior and valid cases, including uncommon paths such as a credit-note adjustment.
- Have policy own the grants: the implementation or a model may propose needed capabilities, but should not decide its own permissions. “The model can participate without owning the boundary,” Alger writes.
- Enforce permissions at the host: an operation without a grant should be technically unavailable or denied, not merely flagged after code review.
- Retain execution records: audit records can help explain which operations were attempted and whether policy allowed them.
- Revisit denials against the contract: determine whether the attempted effect was unauthorized or whether a legitimate capability was missing from the policy.
These are practical implications of Alger’s examples, not a universal operating procedure validated across systems. The core requirement is that authority be checked independently of behavioral correctness and remain intact if the implementation is regenerated.
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Direct grants do not settle indirect authority
A manifest listing direct permissions is not necessarily a complete account of what a component can cause. If an allowed component can invoke another component, effects may be reachable indirectly. Alger explicitly notes that his small Python host does not model the full reference graph, so the demonstration does not prove complete capability security.
In a real design, assess not only which operations a component can call directly, but also what other components it can cause to act and under what grants. The demonstration establishes the value of host-side denial for the paths it models; it does not establish how to scope, audit, or revise every grant over a system’s lifecycle.
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Use review and authority controls for what each can do
Code review and tests provide evidence about implementation behavior. A capability policy and runtime enforcement constrain the consequences available to that implementation. Neither replaces the other: review helps assess whether the code is correct and safe, while an independently owned, enforced boundary limits what it can affect even when its behavior falls short of expectations.
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