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by cxr 38 days ago
> <iframe sandbox=""> - which really is designed to be used as a sandbox

Not for untrusted content living on the same origin to prevent it from exercising any of the powers that it would ordinarily have to be able to access sensitive data. It's a misleading name and shouldn't have been chosen. There is no combination of CSP or the iframe sandbox attribute that can be relied upon for that purpose. This is a fundamental limitation of the way the specs were written.

(There needs to be a big warning about this on MDN, but moving from the old wiki to a wiki with GitHub for login to the GitHub-based pull request process really didn't help the there's-a-problem-on-this-page-but-limited-resources-to-make-things-better problem.)
1 comments
simonw 38 days ago
That's why I'm careful not to include allow-same-origin in the sandbox attribute - without that the iframe content is treated as a separate origin from the parent.

And I serve the content in srcdoc= to ensure there's no URL a user can visit which would directly execute the content outside of that iframe sandbox.

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cxr 38 days ago
> That's why I'm careful not to include allow-same-origin in the sandbox attribute

It doesn't matter. I just said there is no combination of CSP or the iframe sandbox attribute that can be relied upon here.

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simonw 37 days ago
If that's true then my project is fatally flawed and I need to stop distributing it.

I'm not convinced it's true - I've been thinking about this for months, and building experimental prototypes to help me get to the combination that I think makes sense.

Can you describe an exploit that the combination I'm using of iframe sandbox= srcdoc= with an injected meta CSP tag doesn't handle?

Would moving the untrusted content to be served from a separate domain entirely close the hole?

(In case it's not clear the iframe sandbox= is the bit that's doing most of the work here - the CSP stuff is there mainly to protect against malicious apps that deliberately exfiltrate stolen private data.)

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cxr 37 days ago
> Would moving the untrusted content to be served from a separate domain entirely close the hole?

Yes.

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