Code review can help answer whether an implementation is correct. It cannot, by itself, prevent that implementation from performing an operation it was never supposed to perform. To stop an extension from misbehaving, ask two separate questions: Does it meet its behavioral contract? and What is it actually entitled to affect?
Correct behavior and permitted authority are different questions
Behavioral tests check whether a program produces expected results for the cases exercised. Code review can help uncover logic errors, vulnerabilities, questionable dependency choices, and race conditions. Both are valuable evidence about an implementation. Neither makes an unauthorized operation unavailable at runtime.
Ken W Alger puts the distinction plainly: “Verification asks whether an implementation satisfies its behavioral contract. Authority asks what consequences that implementation is permitted to create.” A passing test suite therefore does not establish least privilege. It shows that tested behavior matched expectations; it does not prove the implementation cannot cause other effects.
Why a sandbox may still leave an extension overpowered
A sandbox can constrain where code runs without sufficiently limiting what it can do. The practical boundary depends on the operations exposed by the host: if an extension can call a host operation that issues refunds, for example, isolation alone does not establish whether that call is authorized.
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Alger’s proposed answer is an independent authority boundary: define narrowly scoped capabilities in policy, then have the host runtime enforce them. Reviewers can assess the implementation, while the runtime prevents an ungranted operation from succeeding. As Alger writes, “Code review is evidence about implementation. It should not be mistaken for enforcement of authority.”
What the invoice example demonstrates—and what it does not
Alger describes a small demonstration with four scenarios—correctness, authority, discover, and verify—implemented in about 200 lines of Python with one dependency. It tests a mediated host interface, not a WebAssembly runtime, and is an illustration rather than a production study or independent benchmark.
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Matching output does not rule out an excess operation
In the authority scenario, two invoice-reconciliation implementations produce the same expected report. One also attempts to issue a refund. The manifest allows invoice and payment reads but does not grant the refund capability, so the host denies that operation. The example separates the visible behavioral result from the additional consequence the implementation attempts to create.
Observed behavior is not a complete permission contract
The discovery scenario exposes a flaw in an appealing shortcut: infer the least-privilege manifest from operations seen in a run. A discovery run misses a legitimate credit-note path that needs payments.write. A manifest built only from that observation then denies the valid adjusting write.
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Observation records what happened in the exercised run; it does not establish every operation a valid implementation may need. As Alger puts it, “Observation tells you what a component did, not what it may need.” Discovery can inform policy, but it cannot decide the complete contract on its own.
How to separate the task contract from the authority policy
- Specify expected behavior. Describe the task’s inputs, outputs, and valid cases, including less common paths such as credit-note adjustments. Use tests to check that implementations meet this behavioral contract.
- List permitted effects separately. Define which capabilities the extension may use, such as invoice reads, payment reads, or payment writes. Do not treat a successful test run as proof that the list is complete.
- Keep grant decisions with policy. A model or implementation author may propose capabilities, but should not thereby own the decision to grant them. Alger summarizes the intended separation: “The model can participate without owning the boundary.”
- Enforce grants at the host boundary. Make denied operations technically unavailable through the mediated host interface, rather than relying only on review or on the extension choosing not to call them.
- Keep execution audit records. Records of attempted and denied operations help distinguish an implementation reaching beyond policy from a contract that omitted a legitimate case. That interpretation is a practical implication of Alger’s examples, not a universal operating rule established by the demonstration.
- Revise policy deliberately. When a denial reveals a valid missing case, update the capability contract through policy rather than silently broadening permissions based solely on observed behavior.
Direct capabilities do not settle every authority question
A manifest that names direct permissions may not describe all the effects an extension can cause. If a permitted component can invoke another component, authority may pass through that relationship and lead to effects not apparent from the first component’s direct grants.
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Alger explicitly notes that his small Python host does not model the full reference graph. The demonstration therefore illustrates enforcement at a mediated interface, but does not prove complete capability security for systems with indirect authority. The scope of grants, their audit, and how they change over time also require design beyond what this example establishes.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What to take away when evaluating an extension
- Behavioral evidence: Do tests show that this implementation meets its expected contract?
- Authority scope: Which operations and effects can the runtime permit?
- Enforcement: Can the host technically deny an excess operation, or is it only flagged in review?
- Contract completeness: Does policy account for valid but unobserved paths?
- Indirect authority: Could the extension cause effects through another permitted component?
Keep the behavior contract and authority boundary separately specified and checked, so that regenerating or replacing an implementation does not silently change what it is allowed to affect.
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