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Do not mount untrusted React components in your application’s normal React tree. React rendering does not sandbox JavaScript. If the code must execute, place it in a sandboxed iframe, preferably on a separate origin, and expose only a small, validated message interface. Sanitizing HTML and using Trusted Types help prevent injection through HTML sinks; neither isolates arbitrary component code.
First identify what you are treating as untrusted
The right control depends on whether you have plain text, HTML markup, or executable JavaScript. A third-party React component is executable code, even if it arrives as a component file or a plugin rather than a script string.
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| Input | Appropriate approach | What it does not provide |
|---|---|---|
| Plain text | Render it as ordinary React text or children. | It does not authorize treating the text as HTML or code. |
| HTML markup | Sanitize it with a maintained sanitizer before insertion; consider enforcing Trusted Types for DOM injection sinks. | Sanitization and Trusted Types do not isolate arbitrary JavaScript. |
| Executable component or plugin code | Run it in a separate, sandboxed browsing context, usually an iframe, with deliberate origin separation. | An iframe is not automatically safe if you grant excessive capabilities or expose privileged host data. |
React warns that passing untrusted content to dangerouslySetInnerHTML can introduce XSS. Use that API only with trusted, sanitized data. See React’s common component reference.
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A component mounted in the host application’s tree executes in the host page’s JavaScript environment. React’s component model and rendering APIs organize UI; they do not create a separate origin or isolate a component’s code from the application. A component that is not trusted should therefore not receive the privileges of the host page merely because it is rendered by React.
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HTML defenses address a different risk. React’s dangerouslySetInnerHTML is an explicit injection point. Trusted Types can require typed values at protected DOM sinks, and React documents passing a TrustedHTML value through without string coercion. The policy that creates that value still has to ensure the markup is safely sanitized. These measures are defenses for HTML injection, not a sandbox for JavaScript. See React 19.3’s announcement.
Run executable components in a sandboxed iframe
An iframe creates a separate document, and its sandbox attribute restricts what that document may do. Add only the permissions the component needs. Without allow-scripts, scripts in the embedded document cannot run. Without allow-same-origin, the embedded document is treated as having a special opaque origin rather than inheriting its usual origin.
- Serve the component in a separate document. Do not mount its code into the privileged application tree. Prefer a separate origin for untrusted content so the browser’s same-origin policy also limits direct access to the host application.
- Start with a restrictive sandbox. For example,
<iframe sandbox="allow-scripts" src="https://preview.example/" title="Untrusted component preview"></iframe>permits scripts while leaving other sandbox restrictions in place. The example origin is illustrative; use an origin you control and do not add permissions without a concrete need. - Grant capabilities selectively. Tokens such as
allow-same-origin,allow-forms,allow-popups, and navigation-related permissions lift restrictions. Avoid them unless a required feature depends on them. - Keep host privileges outside the frame. Do not place secrets, authenticated application data, or privileged APIs in the embedded document. Treat any capability deliberately exposed through messaging as something the untrusted component may try to misuse.
- Test the actual feature set and failure cases. Sandbox permissions change both security and behavior. Check what happens when a script, form, navigation, or requested host operation is denied in the browsers your product supports.
MDN strongly discourages combining allow-scripts and allow-same-origin when the iframe content is same-origin with its parent: the embedded document may be able to remove the sandbox attribute, defeating its protection. A separate origin is an additional boundary; it does not make careless permissions or exposed host capabilities harmless. See MDN’s iframe reference.
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Use a narrow message protocol between the frame and host
Cross-origin frame communication should go through postMessage(), not direct access to the other document’s JavaScript objects. Treat messages as an API boundary: define exactly which operations are allowed, what each message must contain, and how large or complex its payload may be.
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- Validate the message’s shape, types, and permitted operation before acting on it.
- Check the sender’s origin when it is stable and meaningful, and use an appropriate
targetOrigininstead of*when you can specify one. - Do not treat the serialized origin
nullas a unique trusted identity. Sandboxed documents with opaque origins can serialize that way, so a simple origin allowlist may not distinguish one such sender from another. - Keep the host’s response narrowly scoped. A frame should not receive general-purpose access to application state or privileged functions when a specific, constrained operation will do.
These precautions follow from the browser’s same-origin policy and iframe sandbox behavior. See MDN’s same-origin policy guide and its iframe reference.
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Content Security Policy’s sandbox directive can apply restrictions to a resource in a manner similar to an iframe sandbox. It can reinforce a browser policy, but it is not a substitute for keeping untrusted code out of the host tree, limiting iframe permissions, and separating origins where feasible. The W3C Content Security Policy Level 3 specification describes the directive.
Choose controls by the failure you need to prevent
- Prevent markup-based injection: render text as text; for HTML, sanitize it and consider Trusted Types enforcement. Do not mistake either control for JavaScript isolation.
- Allow arbitrary component code to execute: use a sandboxed iframe rather than a host-tree mount. Give scripts only if necessary, and avoid same-origin access where feasible.
- Limit damage from a compromised preview: keep secrets and privileged APIs out of it, prefer a separate origin, and expose only validated, constrained operations over messaging.
- Support a feature that needs additional browser capabilities: grant the narrowest required sandbox token and test its behavior. Every token trades some restriction for functionality.
Iframe isolation also has operational costs: embedded documents consume additional memory and computing resources, and restrictive settings may break features that rely on storage, navigation, forms, or other browser capabilities. Measure and test those requirements rather than loosening the sandbox preemptively.
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