Another de-facto insecurity we need to fix in ES5

Mark S. Miller erights at
Thu Jun 18 09:25:15 PDT 2009

Wonderful. +1. Thanks!

On Thu, Jun 18, 2009 at 9:15 AM, Allen Wirfs-Brock <
Allen.Wirfs-Brock at> wrote:

> >-----Original Message-----
> >From: Brendan Eich [mailto:brendan at]
> >
> >> BTW, I haven't yet perceived that we have consensus on putting this
> >> into ES5.  My interpretation of  Brendan's initial comments on the
> >> matter was that he was opposed to it for ES5.  (I'm sure he'll let
> >> us know whether or not that is correct).
> >
> >It's late in the game, but a sentence about [[Prototype]] seems
> >doable. I defer to you on this, since you're Editor and it'll fall
> >upon you to draft the change.
> >
> OK, I'll put it in.  Here is what I am inserting into section 8.6.2
> following the paragraph about [[Prototype]] that comes immediately after
> table 4:
> Every ECMAScript object has a Boolean-valued [[Extensible]] internal
> property that controls whether or not named properties may be added to the
> object. If the value of the [[Extensible]] internal property is false then
> additional named properties may not be added to the object.  In addition, if
> [[Extensible]] is false the value of the [[Prototype]] internal property of
> the object may not be modified.  Once the value of an [[Extensible]]
> internal property has been set to false if may not be subsequently changed
> to true.
> This specification defines no ECMAScript language operators or built-in
> functions that permit a program to modify an object's [[Prototype]] internal
> property or to change the value of [[Extensible]] from false to true.
> Implementation specific extensions that modify [[Prototype]] or
> [[Extensible]] must not violate the invariants defined in the preceding
> paragraph.
> Notice that I also made sure that [[Extensible]] cannot go from false to
> true which was also previously unstated in the specification.
> Allen

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