[webkit-dev] Best way to disable JavaScript

[Adam Barth]

Unfortunately, you can't implement CSP that way.  Consider the case of
two same-origin iframes A and B.  Suppose A has a restrictive CSP
policy (say that bans everything) and B doesn't.  If B uses the DOM to
insert a script element into A, then CSP should block that script
element from executing.  Stripping script tags at parse time won't.

Adam


On Sun, Mar 17, 2013 at 8:26 PM, Geoffrey Garen <ggaren at apple.com> wrote:
> Hi folks.
>
> Currently, we have two different ways to disable JavaScript execution:
>
> (1) Paste / Drag n Drop / editing: Remove script elements and script attributes from untrusted source markup at parse time.
>
> (2) JavaScript disabled setting / Content Security Policy: Check settings and/or CSP at runtime.
>
> There are problems with mode (2):
>
> * It breaks features that are implemented in JavaScript.
>
> The Web Inspector, bookmarklets, extensions, Safari Reader, and Safari autofill all run JavaScript. This means that they break when users disable JavaScript.
>
> As a defense against phishing attacks, mail clients and other web content readers disable JavaScript. This means that they can't implement pieces of their UI in JavaScript.
>
> (FWIW, WebKit violates the CSP specification in this regard: "Enforcing a CSP policy should not interfere with the operation of user-supplied scripts such as third-party user-agent add-ons and JavaScript bookmarklets.")
>
> * It subjects users to XSS attacks.
>
> Runtime checking mode leaves inert JavaScript in the untrusted document. This is a risky proposition. Operations that clone or adopt nodes from the untrusted document unwittingly re-vivify that inert JavaScript, subjecting the user to attack. Experience shows that this is a difficult programming model to get right.
>
> * It's hard to verify.
>
> We have 18 different call sites to canExecuteScripts() in WebKit, not counting the call sites that pertain to plug-ins. Are you confident we've caught all the right places? Do you know if the feature you just added needs to call canExecuteScripts()?
>
> * It's two different ways to do the same thing.
>
> Simplicity is a goal of the WebKit project.
>
>
> Proposal:
>
> If -- for any reason -- JavaScript is disabled in a given document, the document parser will elide all JavaScript. This means that runtime checks can be removed.
>
> One potential downside to this proposal is that it changes the document's internal structure. Since the changes are not generally observable, since they only take place when we're already making much bigger changes by preventing whole scripts from running, and since we haven't seen any compatibility problems from our paste / drag n drop / editing behavior in this regard, I think this downside is acceptable.
>
> Another potential downside is that CSP errors will be reported at parse time instead of runtime. FWIW, some authors might see this as an upside.
>
> Any objections?
>
> Thanks,
> Geoff
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