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Do not mount untrusted React components in your application’s React tree. React rendering does not isolate code. If third-party or otherwise untrusted JavaScript must run, place it in a separately sandboxed iframe, preferably served from a distinct origin, and expose only a narrow, validated messaging interface. Sanitizing HTML and enforcing Trusted Types address injection into DOM sinks; they do not sandbox executable components.
First identify what you are trying to run
“Untrusted React component” can mean several different things, and they need different controls. Plain text, markup, and executable component code are not interchangeable security problems.
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- Plain text: Render it as ordinary React text or children. Do not turn it into HTML unnecessarily.
- HTML to display: Sanitize it with a maintained sanitizer before insertion, and consider enforcing Trusted Types for DOM injection sinks.
- Executable component or plugin code: Treat it as arbitrary JavaScript. Do not evaluate or mount it in the privileged host page; use a separately sandboxed browsing context.
Why React rendering does not sandbox a component
A component mounted into the host application runs as part of that page, not in a separate security boundary. React’s rendering APIs do not isolate its JavaScript from the host environment. The same distinction matters for dangerouslySetInnerHTML: React warns that untrusted markup passed to it can create an XSS vulnerability and recommends using only trusted, sanitized data. See React’s common components documentation.
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Choose the right boundary for the risk
| Input | Practical approach | What it does not provide |
|---|---|---|
| Text | Render as ordinary React text or children. | There is no reason to interpret the text as markup or code. |
| HTML preview | Sanitize before insertion; consider a Trusted Types-enforcing policy. | Sanitization and Trusted Types do not isolate arbitrary JavaScript. |
| Executable component code | Render a separate document in a sandboxed iframe and mediate communication. | An iframe is not automatically safe if its permissions or surrounding architecture are too broad. |
Run executable components in a sandboxed iframe
An iframe’s sandbox attribute restricts capabilities; each allow-* token lifts a restriction. Without allow-scripts, the embedded page cannot run scripts. Without allow-same-origin, it receives a special opaque origin rather than its normal origin. Consult the current MDN iframe reference before choosing tokens, because permissions affect both security and functionality.
- Put the component in a separate document. Do not mount it into the host application’s tree. Prefer serving the untrusted content from a separate origin, and keep host secrets, authenticated application data, and privileged APIs out of that context.
- Grant only required capabilities. If the component must execute, scripts may be necessary. Leave out navigation, popup, form, or download permissions unless the product genuinely needs them. Start with restrictions and add only permissions you can justify.
- Preserve origin separation. Where feasible, omit
allow-same-origin. MDN strongly discourages combiningallow-scriptsandallow-same-originwhen the iframe is same-origin with its parent: the embedded document can remove its sandbox attribute, defeating that protection. A separate origin also limits damage if content is displayed outside its intended frame. - Keep a recovery path. Test the component’s required behavior and failure modes in the browsers you support. Sandbox restrictions can break features, while each embedded document also consumes memory and computing resources.
Make iframe messaging a small, explicit API
Use postMessage() for communication across origins rather than giving the embedded code direct access to host objects. Treat every message as untrusted input, even when it comes from your own iframe.
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- Define a small set of supported operations and exact message shapes.
- Validate message structure, types, payload limits, and permitted operations before acting.
- Check the sender’s origin where it is stable and meaningful, and use a specific target origin instead of a wildcard whenever possible.
- Do not treat the serialized origin
nullas a unique trusted identity. Sandboxed opaque origins can serialize that way, so simple origin allowlists may not distinguish a trusted frame from other opaque-origin documents. - Do not send secrets or expose privileged host capabilities merely because a message passed basic validation.
These checks follow from the browser’s same-origin policy and the constraints of sandboxed frames. The origin check and the message schema solve different parts of the problem: one helps identify the sender, while the other limits what a valid message can ask the host to do.
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Use sanitization and Trusted Types for HTML, not as a JavaScript sandbox
If the requirement is a static HTML preview rather than a live component, sanitization may address the relevant injection risk without running arbitrary component code. React specifically warns about untrusted values passed to dangerouslySetInnerHTML; only use sanitized, trusted markup there. Trusted Types enforcement can make dangerous DOM sinks require typed values, but it is only as safe as the policy that creates those values.
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Neither sanitization nor Trusted Types provides the execution boundary needed for arbitrary JavaScript. For an executable component, isolate execution in the iframe; for markup, prevent unsafe markup from reaching an injection sink.
Consider CSP as an additional layer
Content Security Policy’s sandbox directive can apply restrictions similar to an iframe sandbox. It is a supporting browser policy, not a substitute for separating untrusted code, constraining iframe permissions, and protecting host data. See the W3C Content Security Policy Level 3 specification.
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Before shipping, verify the actual boundary
- Confirm whether the input is text, HTML, or executable code, and do not use a more privileged treatment than it requires.
- For executable code, verify that it runs in a separate document rather than the host React tree.
- Review every sandbox token and justify each capability the frame receives.
- Check that untrusted content cannot access host secrets, authenticated data, or privileged application APIs.
- Exercise the message protocol with malformed, unexpected, and unauthorized requests; verify origin handling for the browser contexts you support.
- Test both intended functionality and blocked behaviors across target browsers.
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