W3C home > Mailing lists > Public > public-webappsec@w3.org > May 2015

Re: [SRI] Comments on Subresource Integrity spec

From: Gervase Markham <gerv@mozilla.org>
Date: Mon, 11 May 2015 14:34:49 +0100
To: Joel Weinberger <jww@chromium.org>, public-webappsec@w3.org
Message-ID: <5550AFF9.2020405@mozilla.org>
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.

>     * 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.

> 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? :-)

>     * 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 Monday, 11 May 2015 13:35:19 UTC

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