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Agent sandbox assurance

Filesystem-access testing

Verify what files an agent execution environment can read or change

Workspace mounts or file tools expose more than the task requires. The decision is how to verify what files an agent execution environment can read or change. The work begins with your actual workflow and looks for the evidence that separates intended behaviour from a consequential exception. It is particularly relevant to platform engineers and data owners. A useful outcome is an explicit account of what was examined, what remains uncertain, and which decision the evidence can support.

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The runtime isolation relationship

  1. Starting evidenceMount maps

    Establish the basis for mount boundary.

  2. Decision boundaryPath resolution

    Check traversal and symbolic-link destinations.

  3. Decision artefactFilesystem access matrix

    Retain the scoped finding and its unresolved dependencies.

Proposed working model. Untested surfaces remain outside the conclusion.

Decision

Frame the decision around mount boundary

Questions and the evidence they need

Criterion / evidenceMount boundary
Inspect mount maps for the relevant constraint or observation.
Criterion / evidencePath resolution
Inspect path permissions for the relevant constraint or observation.
Criterion / evidenceWrite and cleanup
Inspect symlink rules for the relevant constraint or observation.
Read each criterion against the available evidence before deciding that additional work is needed.

The decision is how to verify what files an agent execution environment can read or change. This is relevant to platform engineers and data owners when workspace mounts or file tools expose more than the task requires. Name the affected workflow, the consequence of a wrong decision, and the owner who can act on the result. The initial discussion should establish which evidence is already available and which uncertainty is material enough to investigate.

The specific failure hypothesis is that a permitted workspace path resolves through a link to a protected location. It remains a hypothesis until examined in the deployment. Existing internal tests or operating records may already resolve it; identify the uncovered question before commissioning additional work.

Mechanism

Filesystem authority follows resolved paths

Relationships within runtime isolation

Source or conditionMount boundary
Separate permitted workspaces from protected fixtures.
Crossing or choicePath resolution
Check traversal and symbolic-link destinations.
Consequence or decisionWrite and cleanup
Inspect actual effects and retained files.
Compare effective controls with harmless test observations. Each relationship needs evidence in the actual scope.

What this specialty examines

Filesystem review should consider resolved paths, permissions and ownership, not just input strings. Enumerate read, write, create, delete and traversal behaviour where relevant. Include temporary directories, outputs and shared volumes that may fall outside the main workspace.

The review should test read, write, traversal, link and cleanup cases within an approved file fixture. Identify the normal route, relevant exceptions and the authoritative record of the outcome. Inspect the relevant business-system record, enforcement event or process evidence alongside any reported status. Keep incomplete cases visible, with an explanation of what their absence prevents the owner from concluding.

The tradeoff to make explicit

For this decision, shared workspaces ease collaboration while increasing the need for path and ownership controls. Compare the available design or operating choices against the same intended outcome. If one option removes a required control or changes the task boundary, describe that difference before treating its apparent simplicity, speed or lower effort as an improvement. The decision should expose the tradeoff instead of burying it in an aggregate score.

Preparation

Connect the working inputs to the boundary being examined

Evidence packet for mount boundary

Illustrative working format

BasisMount maps
Resolve source, target, and permitted access, including indirect path crossings.
ContextPath permissions
Resolve source, target, and permitted access, including indirect path crossings.
ObservationSymlink rules
Resolve source, target, and permitted access, including indirect path crossings.
DependencySynthetic sensitive-file markers
Keep expected behaviour and the limits of the constructed example explicit.
Illustrative preparation record. Availability, revision, and handling need confirmation.

Start with mount maps, path permissions, symlink rules and synthetic sensitive-file markers. Identify the owner, revision and operating context of each source. Sanitised examples may support initial design, but differences from actual conditions should remain explicit. Where direct access is unsuitable, a customer-controlled evidence or execution route may be considered, with agreed handling and retention.

The scope also needs the runtime owner, agreed expectations for a representative boundary test, and the authority to inspect the relevant boundary. Specify what evidence would distinguish the failure in which a permitted workspace path resolves through a link to a protected location from acceptable behaviour. If stakeholders disagree about intended behaviour, resolve the interpretation before classifying the observed difference as a defect.

How the work proceeds

Carry mount boundary through to an accountable finding

From the scoped material to a decision record

  1. Establish conditionsMount maps

    Declare the permitted environment and resources.

  2. Examine the relationshipPath resolution

    Apply the scoped check to write and cleanup.

  3. Retain the findingFilesystem access matrix

    Record the evidence, open dependencies, and the owner’s next decision.

Proposed sequence; resolve interpretation or access gaps before relying on the next step.

Prepare the runtime boundary and fixtures

Use mount maps, path permissions, symlink rules and synthetic sensitive-file markers to identify the exact runtime and boundary claimed by platform engineers and data owners. Mark permitted resources and protected resources in a controlled fixture, including shared services where they are relevant. Define observable success, denial and incomplete setup before execution. The preparation output is a boundary map and fixture manifest that lets reviewers distinguish effective isolation from a test that never reached its target.

Exercise and inspect the boundary

Use a fixture containing permitted files, denied markers and controlled links. Observe the actual target after resolution and any resulting modification. Distinguish denied access from missing files or failed setup, which can otherwise look deceptively similar in logs.

Choose containment or configuration changes

Translate the observations into a filesystem access matrix with path-resolution and permission findings. Locate each issue in the worker, shared service, policy rollout or cleanup path rather than attributing every symptom to the sandbox as a whole. Decide which configuration change, narrower resource exposure or specialist investigation is warranted. Retest the affected boundary and inspect residual state before treating the changed environment as equivalent to the intended design.

Indicative artefact

Make the filesystem access matrix usable by its owner

Filesystem access matrix

Illustrative working format

Annotated fieldFilesystem access matrix
Identify the assessed workflow, revision, and decision boundary.
Annotated fieldPath-resolution
Link supporting and contrary observations to inspectable evidence.
Annotated fieldPermission findings
Link observations and counterevidence to the case, configuration, and conclusion limit.
Illustrative artefact structure, not a completed engagement or an observed result.

A useful decision artefact is a filesystem access matrix with path-resolution and permission findings. Supporting material may include selected cases, configuration or process records, evidence references, a disagreement log and prioritised actions. Retain the case selection rationale and material disagreements alongside the result; otherwise the result may be mistaken for complete coverage.

Illustrative scenario

Examine write and cleanup through a constructed exception

A constructed case and its decision paths

Illustrative scenario

  1. SituationMount boundary

    A permitted path resolves through a link to an excluded file fixture.

  2. Question to examinePath resolution

    Resolve the permitted path through its symbolic-link target.

  3. Conditional next actionWrite and cleanup

    Apply the boundary to the resolved object rather than the path label alone.

Fictional example. No executed test, customer engagement, or measured outcome is implied.

Consider a fictional situation: a document-processing agent writes outputs to a shared directory and accidentally reaches another task’s files. The scenario is illustrative, with no client engagement, executed test or measured improvement implied.

For the shared output directory, create distinct synthetic task ownership. Check whether one task can overwrite or retrieve another’s artefact. The resulting decision may require separate namespaces or a mediated artefact service rather than prompt-level path instructions.

Scope and next action

Keep the conclusion within the authorised runtime boundary

Confirm fit before the next commitment

Included boundaryMount boundary
This review is limited to the authorised runtime boundary.
PrerequisiteMount maps
The relevant owner must confirm access and the intended operating conditions.
Claim limitFilesystem access matrix
Untested surfaces remain outside the conclusion.
Bring the triggering change and existing evidence; resolve the dependency that could alter the decision.

Testing is confined to explicitly authorised environments and non-destructive cases. A bounded assessment cannot establish that escape or cross-boundary access is impossible. Any engagement depends on an agreed scope, qualified team, authorised access and confirmed availability. Bring the affected workflow, its current configuration or process, the triggering change and the evidence already available. The immediate scope question is prompted by the situation in which workspace mounts or file tools expose more than the task requires; confirm which missing observation would change the owner’s next action.

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A little more clarity

Questions to consider.

Start with the questions that shape a better decision.

Should file access be checked before or after resolving a path?

Evaluate the resolved target and effective permissions. Checking only the submitted string can miss links, traversal or alternate access routes. Use controlled fixtures so both the requested path and actual resource can be verified.

Which mount and permission records define file access?

Start with mount maps, path permissions, symlink rules and synthetic sensitive-file markers. Record their revision, owner and relationship to the intended workflow. Missing context should be identified before interpreting outcomes; a sanitised fixture is useful only when its relevant differences from the operating case are understood.

How should path resolution and links be tested?

Test read, write, traversal, link and cleanup cases within an approved file fixture. Retain effective runtime configuration and observations from the relevant boundary. Distinguish an enforced denial from failed setup or an operation that never reached the resource. Use controlled markers and preserve residual-state evidence.

What belongs in a filesystem access matrix?

The proposed output is a filesystem access matrix with path-resolution and permission findings. Include the selected conditions, supporting observations, unresolved differences and next decision. A reviewer should be able to follow each important conclusion back to its evidence and understand the limits without depending on an undocumented explanation from the author.

When should artefact exchange replace shared directories?

Prefer mediated artefact exchange when shared directories expose unrelated task state or cannot enforce required ownership. Test whether the alternative supports needed reads and writes while preserving isolation.

What if existing evidence already answers the question?

Review its scope, provenance and configuration before proposing new work. Existing work from platform engineers and data owners may already be sufficient. In that case, capture the decision, unresolved limitations and the change that would require another review. Additional assessment should address a real gap.

What would make the findings inconclusive?

Missing mount maps, path permissions, symlink rules and synthetic sensitive-file markers can prevent a reliable interpretation, depending on the case. Record the specific missing observation and its implication. The owner can supply evidence, narrow the claim or defer the dependent decision; an incomplete case should not silently be counted as a pass.

Start with the decision you need to make.

Tell us about the agent workflow you want reviewed, and where you need greater clarity. Scope and availability are confirmed together.

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Connected disciplines

Network-egress control review

Define and validate where a sandbox may send network traffic. The review connects destination policies, DNS behaviour, proxy routes and connection logs to a bounded review of execution boundaries, giving the runtime owner a clearer basis for the next action.

Secret-exposure testing

Check whether sandboxed work can expose credentials or other secret material. The review connects secret-injection design, redaction rules, access paths and synthetic canary values to a bounded review of execution boundaries, giving the runtime owner a clearer basis for the next action.

Resource-exhaustion testing

Establish what happens when agent execution consumes excessive resources. The review connects resource quotas, scheduler settings, workload limits and telemetry to a bounded review of execution boundaries, giving the runtime owner a clearer basis for the next action.