Array.prototype.slice web-compat issue?

Allen Wirfs-Brock allen at
Thu Aug 29 14:21:47 PDT 2013

On Aug 29, 2013, at 1:39 PM, Kevin Reid wrote:

> On Thu, Aug 29, 2013 at 12:56 PM, Allen Wirfs-Brock <allen at> wrote:
> On Aug 29, 2013, at 10:51 AM, Kevin Reid wrote:
>> This is a hazardous change for SES-style security. For example, I've just taken a quick look at our (Caja) codebase and found a place where is used to obtain a “no funny business” array (i.e. doesn't have side effects when you read it) and another where it's used to obtain an array which must be in the caller's realm. These would be easy enough to replace with a more explicit operation, but I wanted to point out that this is not a harmless change.
> In the use case what is foo? Is it known to be an Array?  Are you saying this is how you clone an Array?
> Sorry, both are of that form, if I was unclear. When we want to simply clone an existing array, belonging to a secured realm, I think we generally use slice as a method, and there is no security property there.
> Of the two cases I refer to, one is a function (the trademarking stamp()) which takes a list of objects as a parameter and needs to ensure that successive stages of processing operate on exactly the same set of objects and do not trigger any side effects in the list's implementation. Here, realm is irrelevant but the list's implementation must be relied on, so in practice we want an Array from stamp()'s own realm.
> The other case is one where it is a cross-frame protocol and we specifically want an object which belongs to 'our own' realm because its prototypes are frozen and specially extended, whereas the calling realm's prototypes notably are not frozen (it's outside of the shiny happy sandbox) and therefore constitute a risk to least-authority programming which we want to stop at the boundaries. (Note for MarkM: It's actually a little bit more complicated than this, but the details are irrelevant to the principle.)

for both cases, are you using Array.isArray to determine that you are operating upon an array?

what would be the appropriate thing to happen (all things considered) in a world where subclasses of Array exist?


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