A readonly proxy is a wrapper that prevents direct property assignment while still allowing access to forwarded methods. It can reduce accidental modification, but it does not stop a method from changing host state once the call reaches the underlying object. That distinction is central in sandbox containment failures.
Expanded Definition
A readonly proxy is a defensive programming construct that intercepts access to an underlying object and blocks direct property assignment, while still forwarding reads and callable methods. In security terms, that means it can limit casual mutation of an object’s surface area, but it does not automatically constrain the behaviour of the object’s methods once they execute. The distinction matters because authority can still flow through the call boundary even when the wrapper appears immutable.
Usage in the industry is still evolving, especially in JavaScript sandboxing, plugin isolation, and agentic AI tooling where objects may be passed across trust boundaries. A readonly proxy is best understood as a narrow integrity guard, not a full containment mechanism. Guidance from the NIST Cybersecurity Framework 2.0 is relevant here because access control, integrity, and misuse-resistant design all depend on understanding what is actually enforced versus what is only implied by a wrapper. The most common misapplication is treating a readonly proxy as a sandbox boundary, which occurs when developers assume blocked assignment also prevents forwarded methods from altering host state.
Examples and Use Cases
Implementing readonly proxies rigorously often introduces compatibility and performance constraints, requiring organisations to weigh safer object handling against the risk of breaking legitimate mutations or method-driven workflows.
- Browser or Node.js plugin systems use a readonly proxy to expose configuration objects without allowing direct reassignment, while still letting plugins read permitted values.
- Agentic AI tool wrappers may present a readonly proxy to an agent so it can inspect metadata but cannot overwrite policy fields before invoking an action.
- Shared state objects in application code can be wrapped to prevent accidental writes during refactoring, reducing the chance of unintended side effects.
- Security testing teams use readonly proxies to demonstrate that blocking property assignment alone does not stop a method from modifying hidden or internal state.
- Sandbox prototypes may rely on proxies to constrain object surfaces, but they must be paired with stronger isolation controls, such as compartmentalisation and explicit capability limits, as discussed by OWASP and other security guidance.
Why It Matters for Security Teams
Security teams need to recognise that readonly proxies are about partial integrity control, not complete trust enforcement. If a design review mistakes “cannot assign properties” for “cannot change state,” then hidden method side effects can become a route for policy bypass, data tampering, or sandbox escape. That is especially relevant in agentic AI and NHI-adjacent architectures, where wrappers may be used to expose tools, config objects, or runtime state to autonomous code while preserving a sense of safety that may not exist.
The control lesson is straightforward: if a system depends on an object remaining unchanged, the design must prevent mutation at the object, method, and authority layers, not just at the property assignment layer. Identity and access assumptions should also be explicit when proxies are used to mediate privileged operations, because a forwarded method can still act with the privileges of the host context. The OWASP Cheat Sheet Series remains useful for secure design thinking around trust boundaries and input handling. Organisations typically encounter the practical impact only after a plugin, script, or agent changes host state despite “read-only” wrapping, at which point readonly proxy limitations become operationally unavoidable to address.
Standards & Framework Alignment
This section maps relevant standards and security frameworks to the operational risks and controls described in this guidance.
OWASP Agentic AI Top 10, CSA MAESTRO and OWASP Non-Human Identity Top 10 address the attack and risk surface, while NIST CSF 2.0 and NIST AI RMF set the governance and control requirements practitioners need to meet.
| Framework | Control / Reference | Relevance |
|---|---|---|
| NIST CSF 2.0 | PR.AC-4 | Access restrictions must be enforced, not implied, which fits proxy-based trust boundaries. |
| OWASP Agentic AI Top 10 | Agentic systems often expose tools and state through wrappers that can mislead about true containment. | |
| CSA MAESTRO | MAESTRO addresses control boundaries and trust separation in agentic AI systems using mediated objects. | |
| NIST AI RMF | AI RMF governance applies where wrappers are used to manage trustworthy AI system behaviour. | |
| OWASP Non-Human Identity Top 10 | NHI controls are relevant when non-human workloads access objects through proxy-mediated boundaries. |
Treat proxy wrappers as partial controls and verify actual authorization before granting object access.
