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Re: FW: Feedback on the Strict-Transport-Security specification

From: Adam Barth <w3c@adambarth.com>
Date: Tue, 27 Oct 2009 22:20:28 -0700
Message-ID: <7789133a0910272220l4fba201cu181447cc00f41b63@mail.gmail.com>
To: Eric Lawrence <ericlaw@exchange.microsoft.com>
Cc: "public-webapps@w3.org" <public-webapps@w3.org>
Thanks for your feedback.  Comments inline.  (I've skipped the
editorial comments.)

On Tue, Oct 27, 2009 at 5:01 PM, Eric Lawrence
<ericlaw@exchange.microsoft.com> wrote:
> I am a bit concerned that the spec doesn’t mandate behavior for
> mixed-content; I know such requirements would be controversial and
> non-trivial, but without the behavior being mandated by the spec, I think
> we’re likely to see divergent and incompatible behavior on STS sites.

There's a tension about what to put in STS and what is more
appropriate for a more general policy delivery mechanism, like
Content-Security-Policies <https://wiki.mozilla.org/Security/CSP>.
The main reason not to include STS in CSP that the browser needs to
know the STS policy before it receives the CSP header because the
browser needs to hand errors during the SSL / TLS handshake.

In the case of mixed content, we can wait until we receive an HTTP
header, so we don't need to play tricks with time scoping (i.e.,
Max-Age) or URL scoping (i.e., includeSubDomains).  I'd like to see
browser vendors expose policy levers for controlling mixed content,
but I'm not sure whether STS or CSP is a better home for that
directive.

> Hopefully this is still the latest draft?
> http://lists.w3.org/Archives/Public/www-archive/2009Sep/att-0051/draft-hodges-strict-transport-sec-05.plain.html

I believe it is.

> [Section 2.4.2: Detailed Core Requirements]: 4.UAs need to re-write all
> insecure UA "http" URI loads to use the "https" secure scheme for those web
> sites for which secure policy is enabled.  This requirement is
> insufficiently specific and does not really explain what “rewrite” means?
> Does this mean that the HTML parser will detect any insecure-but-should-be
> URIs and rewrite them within the markup, such that JavaScript could observe
> the change in the HREF attribute?

This is how our original prototype worked, but I don't think that's
how the real implementations should work.

> Or does it simply mean that upon
> de-reference the URI is automatically “upgraded” to HTTPS with no notice to
> the caller?

What I'd recommend here is to treat the HTTP-to-HTTPS "rewrite" as a
simulated 307 redirect, like the one the site is supposed to provide
if we actually retrieved the HTTP URL.

> [Section 2.4.2: Detailed Core Requirements]: Requirements #5 and #6 are
> problematic because browsers (generally speaking) often don’t have rock
> solid knowledge of where the proper “private domain” / “public suffix”
> transition occurs.

I think there might be some confusion about what "higher-level" means
in this context.  The intent is that:

1) both example.com and foo.example.com could set policy for
bar.foo.example.com.
2) Neither bar.foo.example.com nor foo.example.com could set policy
for example.com.
3) bar.foo.example.com cannot set policy for foo.example.com.
4) foo.example.com cannot set policy for qux.example.com.

etc.

I don't think we need a notion of a public suffix to enforce these rules.

> [Section 5.1: Syntax] Are the tokens intended to be interpreted
> case-sensitively?

Yes.  I think this is implied but the grammar style Jeff using, but it
might be worth noting for us non-ABNF experts.

> [Section 5.1: Syntax] What should be done if the server has multiple
> Strict-Transport-Security response header fields of different values?

My opinion is we should honor the most recently received header, both
within a request and between requests.

> [Section 6.1: HTTP-over-Secure-Transport Request Type] Why must the server
> include this header on every response?  This seems likely to prove
> prohibitively difficult across, say, Akamai load balancers for images, etc.
> What happens if the server fails to include the response header on a given
> response?

I think that's a server conformance requirement.  The UA conformance
requirements are set up so that this doesn't matter too much.  As long
as you get your entry in the STS cache, you'll be fine.

> [Section 6.2] A STS Server must not include the Strict-Transport-Security
> HTTP Response Header in HTTP responses conveyed over a non-secure
> transport.  Why not?  It seems harmless to include if the UA doesn’t respect
> it.

Again, this is a server conformance requirement that doesn't affect
UAs.  It doesn't make sense to send the header here.  We might as well
prohibit servers from sending it.

> [Section 7.1] What if the STS response header is present but contains no
> tokens?  7.1 suggests that the header alone indicates an STS server.

That sounds like a bug.  An empty header should be a no-op.

> [Section 7.1.1; Design Decision #4] I know there are reasons to avoid using
> secure protocols to IP-literal addressed servers, but in Intranet
> environments this may be expected and desirable. Why forbid it here?

I don't think there's any way to provide security in this case.  My
understanding is that anyone can get these certificates.  Is there
some benefit to supporting these cases?  Maybe CAs might change their
policies in the future?

> [Section 7.1.2] While I understand the restrictions imposed here, it is
> something of a shortfall that https://www.example.com cannot enforce STS for
> requests to http://example.com.  The threat here is obvious: the user
> typically visits https://www.paypal.com and gets STS applied, but in a
> coffee shop or untrusted network, inadvertently types just “paypal.com” in
> the address bar.  Because STS isn’t applied cached for that server, possible
> exploit occurs.

The thought is that https://www.paypal.com/ can load an image from
https://paypal.com/ to enable STS for the root domain.  Letting
www.paypal.com opt in for paypal.com is going to lead to a bunch of
unhappy people who type "paypal.com" and reach an hard blocking page
if there is a CN mismatch.

> [Section 10: UA Implementation Advice; Section 2.4.3: Ancillary
> Requirements;] This portion of the spec troubles me the most. I was looking
> forward to this spec settling things once and for all and requiring mixed
> content to be treated as a fatal error. However, the spec doesn’t require
> that, and thus I think it’s missing out on an absolutely critical
> opportunity.  If UAs differ in behavior (e.g. IEvN silently blocks “without
> recourse” mixed content but Firefox does not) then it’s likely that users
> and developers will erroneously conclude that the more secure UA is “broken”
> or “buggy.”

I responded to this at the top of the email.  There seems to be some
amount of support for making STS imply blocking mixed content.  If you
think this is what we should do, then we can do it.  One concern I
have here is that browser's mixed content detection is notoriously
buggy, but maybe this requirement will motivate us to get it right.

> Having noted this, I do need to observe that controlling mixed content is
> harder for IE than for any other browsers, because IE requires add-ons to go
> directly to the network stack (WinINET/URLMon) while competitive browsers
> typically expect that the add-on will use NPAPIs to request that the host
> browser collect data on their behalf.

This picture is actually even less rosy.  Some popular NPAPI plug-ins
use a mix of browser-provided and OS-provided networking services
because the NPAPI network APIs lack some basic functionality (like
setting headers on GET requests).

> Other cases of “mixed” content: the WebSocket specification, which supports
> both secure and insecure modes.  Ditto for FTP/FTPS.

and CORS.  There's a lot of complexity to mixed content.

> [Section 10] I was disappointed not to see any mention of the privacy
> implications of STS hostname storage, and/or recommendations on how such
> storage should interact with browser “private modes” and/or cleanup
> features.

We should add this discussion.  The implementation in Chrome stores
only hashes of host names and clears the cache when the user resets
browser data.  In "private mode", Chrome makes a fresh STS cache and
store the directives in memory only (which is relatively useless).

> Other thoughts: Should STS offer a flag such that all cookies received from
> the STS server would be automatically upgraded to “SECURE” cookies?

I think this is a good idea for an new token in a future version.  I'm
not sure whether Jeff has updated the grammar in the spec yet, but the
plan is to use a forward-compatible syntax that lets vendors
experiment with more tokens.

> One threat not mentioned is cross-component interactions.  This spec appears
> to primarily concern browsers, while the real-world environment is
> significantly more complex.  For instance, there are a number of file types
> which will automatically open in applications other than the browser when
> installed; those other applications may perform network requests to an STS
> host using a network stack other than that provided by the browser. That
> network stack may not support STS, or may not have previously cached STS
> entries for target servers. Thus a threat exists that out-of-browser
> requests could be induced that circumvent STS.

For Internet Explorer, I would recommend coupling the STS cache with
the WinInet cookie jar.  That way, Secure cookies in Internet Explorer
would be protected by STS even in external applications.

Thanks for your detailed comments.

Adam
Received on Wednesday, 28 October 2009 05:21:28 GMT

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