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Re: [SRI] Comments on Subresource Integrity spec

From: Joel Weinberger <jww@chromium.org>
Date: Tue, 12 May 2015 03:34:04 +0000
Message-ID: <CAHQV2KmJgQE6q8_1EnrKUWY5sPo0eqxyByMQR5_Oe6+RZErPgw@mail.gmail.com>
To: Gervase Markham <gerv@mozilla.org>, public-webappsec@w3.org
FYI, I just uploaded https://github.com/w3c/webappsec/pull/348 to add a
note about Mixed Content. Please feel free to comment there about if you
feel that the comment is sufficient or not.

On Tue, May 12, 2015 at 1:16 PM Joel Weinberger <jww@chromium.org> wrote:

> On Mon, May 11, 2015 at 11:34 PM Gervase Markham <gerv@mozilla.org> wrote:
>> On 08/05/15 22:15, Joel Weinberger wrote:
>> > In
>> >
>> https://w3c.github.io/webappsec/specs/subresourceintegrity/#the-integrity-attribute-1
>> ,
>> > in particular the production for hash-algo, which references the CSP
>> > spec and its definition of hash algorithms, so I believe it implicitly
>> > does.
>> Well, OK, but that seems a bit obscure. I think there would be no harm
>> in being explicit about it - either with a list, or by saying yuo are
>> deferring to the CSP spec.
> As Dan mentioned, I think it's been pretty clear on the list. I don't
> particularly feel like the wording needs to be changed to make this any
> more explicit, but if any of the other editors (or anyone else) wants to
> make a pull request to add in wording to clarify, I'd certainly merge it.
>> >     * The spec does not say how to form the identifying tag for a hash
>> >     function - e.g. converting "SHA-256" into "sha256". This may seem
>> >     simple, but the possible variants of SHA-3 are still not defined
>> (AIUI),
>> >     and so there may be ambiguity here. The CSP document punts on this
>> too,
>> >     simply defining 3 values - "sha256", "sha384" and "sha512". Is this
>> >     their problem, or something to be addressed by the spec?
>> >
>> > I don't think I understand the question. In the CSP spec,
>> > https://w3c.github.io/webappsec/specs/CSP2/#source-list-valid-hashes
>> > defines a conversion of the hash algorithms in the syntax into explicit
>> > hash functions.
>> Yes, you don't understand the question :-) Let me try again. Next year,
>> NIST standardises the SHA3 algorithm, in 256 (a.k.a. subtype A), 384
>> (a.k.a. subtype B) and 512-bit (a.k.a. subtype C) variants. Chrome
>> implements this as:
>> integrity="sha3_256-09F9..."
>> Firefox, however, checks in a patch on the same day, recognising:
>> integrity="sha3-09F9..."
>> with the variant chosen by the length of the hash given. And, by amazing
>> coincidence, Safari also publishes an implementation on the same day
>> going for:
>> integrity="sha3A-09F9..."
>> Who is right? In other words, who gets to mint new hash identifiers? The
>> first browser to publish? Or does Subresource Integrity have to wait for
>> the CSP people to publish a new version of their spec? Is that wise?
>> >     * The spec does not give guidance as to whether, if integrity
>> metadata
>> >     is loaded in a privileged context, and is applied to an unprivileged
>> >     document load, the user agent should consider the resulting context
>> >     still to be privileged ("secure"). I can see the case for both
>> answers.
>> >     There is no security loss in loading jQuery from a CDN using
>> >     integrity-protected HTTP. There may be a small privacy loss (e.g.
>> the
>> >
>> > There is no privacy in the transport, so there is a major security loss.
>> > I believe
>> >
>> https://w3c.github.io/webappsec/specs/subresourceintegrity/#unprivileged-contexts-remain-unprivileged-1
>> > does address this, making it explicit that unpriviliged contexts remain
>> > unprivileged.
>> I don't believe that section says what you claim it says. It's point is
>> "authors SHOULD deliver integrity metadata only to a privileged
>> document." That says nothing about whether a browser should show a
>> mixed-content warning when integrity metadata is delivered to a
>> privileged context, as suggested (loaded over HTTPS, a page starts as a
>> privileged context, and remains one until the first HTTP load, after
>> all), but the resource is then loaded over HTTP. Does it remain
>> privileged? If the suggestion is no, the spec should say so.
> You're right, my mistake. I'll upload a CL to make this more explicit.
>> > The idea is that if the hash function is broken, then it doesn't provide
>> > any security, so checking the integrity is completely unnecessary (since
>> > an attacker could just inject their own matching hash).
>> I think this view has an unnecessarily binary understanding of "broken".
>> I am not recommending continued use of known problematic algorithms, but
>> if we are in a state where checking Hash Algorithm A is proof against
>> everything including governments, and checking Hash Algorithm B is
>> suspected to be good against everyone except the NSA and GCHQ, there is
>> still value in checking Hash Algorithm B, if the alternative is simply
>> loading the content anyway. The attacker is not always a nation-state :-)
>> Checking even an MD5 would defeat _me_, if I was trying to attack you...
>> There's some value in that, surely? :-)
> I still disagree. We could also support ROT13, which would stop some
> number of attackers as well, but it wouldn't really fit a definition of
> "secure." It's important to keep a clear set of security guarantees of what
> an integrity check gets you. I actually do believe that it is as binary as
> I initially presented. I don't buy the "Gov't can break it but no one else
> can" theory. If the Govn't can break it... then it's broken, and we should
> assume that a large criminal org can also break it.
> Additionally, the discretion of what hash functions are used is still up
> to the user agent. For example, we explicitly allow the user agent to
> define its own algorithm for "strong" hash functions (see
> https://w3c.github.io/webappsec/specs/subresourceintegrity/#get-the-strongest-metadata-from-set.x).
> So perhaps our disagreement is all academic, because I think this section
> ought to satisfy your concerns. If a user agent decides that a hash
> function is "strong enough," it can keep it, but just put it low in the
> priority list.
> Also, to Dan's point that we shouldn't "fail closed," I agree, and I
> believe the spec is clear on that (
> https://w3c.github.io/webappsec/specs/subresourceintegrity/#agility-1).
> If the hash function is unsupported, the user agent must ignore the
> integrity value. Chrome will, of course, also give a console message, and I
> suspect other implementations will do similarly.
>> >     * The ABNF for the integrity attribute allows option names without
>> >     values, but it seems that it requires the = sign even when there's
>> no
>> >     value. That seems like a bug? It also doesn't seem to allow multiple
>> >     options for a hash - the option-expression is not allowed multiple
>> >     times, and no option separator is defined. In fact, & is an
>> >     option-value-char so it could not be used as things stand. Perhaps
>> >     semicolon? In general, it would be good to have a non-normative
>> >     fictional example of option use, even if none are defined in the
>> spec.
>> >
>> > I believe the production does allow for multiple options, using "?" as a
>> > separator.
>> Yes, now I look again, you are right. I don't like this, because it
>> seems like you are using URL parameter style syntax ("?" and "=") but
>> not actually using it, which is a footgun. I think you would be better
>> off supporting:
>> sha256-09F911029D74E35BD84156C5635688C0?foo=bar&baz=quux
>> but using semicolon as the separator is also URL-like.
>> > I don't think it's a bug to leave in the '=' sign; we wanted
>> > it to be explicit that the value should be empty.
>> I think you are going to hit exactly the same issue they had with HTML
>> and non-valued attributes ("checked='checked', anyone?); I predict you
>> will end up supporting the bare attribute name anyway. :-)
>> Gerv
Received on Tuesday, 12 May 2015 03:34:33 UTC

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